G F B l a c k l e d g e BScTech, CEng, MIStructE, MIHT

Revised by R A Binns MCIPD, MICT

This publication Safety on site
This publication is for the guidance of those concerned with the Many construction activities are potentially dangerous so care is
construction and day-to-day supervision of concrete work in the needed at all times. Current legislation requires all persons to
UK. It gives useful advice, which will also provide students with an consider the effects of their actions or lack of action on the health
insight into the many and varied practical aspects relating to and safety of themselves and others. Advice on safety legislation
concrete and its uses. It is not intended to take the place of can be obtained from any of the area offices of the Health & Safety
regulations, codes of practice or specifications. Where it is Executive.
inappropriate to deal in detail with specialist topics, sources of
further information are referred to. Cement burns: health hazard
Dry cement powders in normal use have no harmful effect on dry
The scope is generally related to British Standard BS 5328 Concrete, skin. As with any dusty material there may be ill effects from the
current at the time of publishing this handbook. European inhalation or ingestion of cement dust and suitable precautions
Standard BS EN 206-1, Concrete - specification, performance, should be taken.
production and conformity, dated 2000, was followed in 2002 by its
When cement is mixed with water, alkali is released.
complementary British Standard BS 8500 for additional UK
provisions. Reference is made to BS EN 206-1 /BS 8500 where Precautions should therefore be taken to prevent dry cement
appropriate to the specification, production and general use of entering the eyes, mouth or nose and to avoid skin contact with
concrete. wet concrete and mortar.
Repeated skin contact with wet cement over a period of time may
Much of the technology and site practice described in this
cause irritant contact dermatitis. The abrasiveness of the concrete
handbook also applies to the production of ready-mixed and
or mortar constituents can aggravate the effect.
precast concrete. Contractors have a choice in their procurement
of concrete and expert advice is available in technical literature Some skins are sensitive to the small amount of chromate that may
describing the benefits and production methods for specialised be present in cements and can develop allergic contact dermatitis,
applications such as ready-mixed, precast or sprayed concrete that but this is rare.
are not included in this handbook. Continued contact with the skin can result in cement burns with
ulceration.

Handling precautions
Protection for the eyes, mouth and nose should be worn in
circumstances when dry cement may become airborne.
When working with wet concrete or mortar, suitable protective
clothing should be worn such as long-sleeved shirts, full-length
trousers, waterproof gloves with cotton liners and Wellington
boots.
Clothing contaminated with wet cement, mortar or concrete
should be removed and washed before further use. Should
concrete or mortar get into boots, remove them IMMEDIATELY and
thoroughly wash the skin and the inside of the boots before
proceeding with the job.
If cement enters the eye it should be washed immediately and
thoroughly with clean water and medical advice sought.
Concrete or mortar elsewhere on the skin should also be washed
off immediately.
Whenever there is persistent or severe irritation or pain a doctor
should be consulted.

48.037 First published 1975 British Cement Association

Second edition 1987 Century House, Telford Avenue,
Reprinted 1990 with minor amendments
Crowthorne, Berkshire RC45 6YS
Reprinted 1992 (with insert)
Third edition 2002 Telephone (01344) 762676
ISBN 0 7210 1 358 9 Fax (01344) 761214
Price Group G www. bca.org.uk
© British Cement Association 2002
www. concretebookshop.co.uk

All advice or information from the British Cement Association is intended for those who will evaluate the significance and limitations of its contents
and take responsibility for its use and application. No liability (including that for negligence) for any loss resulting from such advice or information
is accepted. Readers should note that all BCA publications are subject to revision from time to time and should therefore ensure that they are in
possession of the latest version.
CONCRETE PRACTICE
Contents
INTRODUCTION 3 CONCRETE SPECIFICATION 24

PORTLAND CEMENTS 4 Designed concretes 24

Prescribed concretes 25
Portland cement CEM 1........................... 4
Standardized prescribed concretes 25
Sulfate-resisting Portland cement SRPC 5
Designated concretes 26
White Portland cement 5
Proprietary concretes 26
Cements and mixer combinations incorporating mineral
constituents or additions 5 Strength 26
A guide to the notation of cements and combinations... 8 Effect of concrete constituents 27
Delivery and storage of cement 9 Trial mixes 27
Sampling and testing of cement 9 References/further reading 27
References/further reading 9 READY-R/MIXED CONCRETE 28
AGGREGATES 10 Batching plants 28
Sizes of aggregate 10 Exchange of information 28
Quality requirements 10 Delivery 29
Grading of aggregates 11 Discharge 29
Marine-dredged aggregates 12 References/further reading 29
Lightweight aggregates 12 SITE BATCHING AND MIXING 30
Delivery of aggregates 12
Storage of materials 30
Storage of aggregates 13
Concrete mixers 30
References/further reading 13
Batching 30
WATER 14 Operation of site mixers 31
Quality 14 TRANSPORTING CONCRETE 32
Reclaimed and recycled water 14
Pumping 32
Measuring the quantity of water 14
Crane and skip 33
References/further reading 14
Dumpers 33
ADMIXTURES 15 Other methods 33
Normal water-reducing admixtures (plasticizers, References/further reading 33
workability aids) 15
PLACING AND COMPACTION 34
Accelerating water-reducing admixtures 15
Retarding water-reducing admixtures 15 Placing 34

Air-entraining admixtures 16 Compaction 34

Superplasticizing/high-range water-reducing admixtures 16 References/further reading 35

Other admixtures 17 CONSTRUCTION JOINTS 36

Storage of admixtures 17
Location of construction joints 36
Dispensing 17
Preparation of construction joints 36
Trial mixes 17
Concreting at construction joints 37
References/further reading 17
References/further reading 37
CONCRETE PROPERTIES 18
CONCRETING ON COLD WEATHER 38
Fresh concrete 18
Raising the temperature 38
Hardening concrete 18
Strength development 38
Hardened concrete 20
Plant and equipment 39
References/further reading 24
Weather records 39
References/further reading 39

1
CONCRETING IN HOT WEATHER 39 TESTING CONCRETE AND CONCRETING MATERIALS 52
Loss of consistence 39
Sampling materials 52
Moisture loss 39
Testing materials 53
References/further reading 40
Testing fresh concrete 55
REINFORCEMENT 40 Testing hardened concrete 57

Bar types and identification 40 Non-destructive testing 59

Bar sizes and bending 40 Analysis of fresh concrete 60

References/further reading 61
Fabric 41
Prefabricated reinforcement 41 APPENDICES
Handling, storage and cleanness 41
Appendix 1 - Schedules for specifying concrete 62
Cover to reinforcement 41
Appendix 2 - Certificate of sampling fresh concrete . . . . 66
Fixing reinforcement 42
Appendix 3 - Certificate of slump test 67
References/further reading 42
Appendix 4 - Certificate of air content test 68
FORMWORK 43 Appendix 5 - Certificate of cube making 69

Types of formwork 43 Appendix 6 - Certificate of standard curing of test cubes. . 70

Design of formwork 43 ACKNOWLEDGEMENTS 71

Surface treatment 43
Striking of formwork 44
Care of formwork 45
References/further reading 45

CURING 45

Purpose of curing 45
Methods of curing 46
Uniformity of colour 47
White and coloured concrete 48
References/further reading 48

CONCRETE SURFACE FINISHE................................ 48

Range of finishes 48
Standard of finish 49
Plain smooth finishes 49
Textured and profiled finishes 50
Exposed-aggregate finishes 50
Tooled concrete finishes 50
Remedial work 50
References/further reading 50

FLOOR FINISHES ........................................................................51

Choice of finish 51
Curing 51
References/further reading 51

2
 INTRODUCTION
Concrete is a construction material composed of crushed rock or
Thes e developments will be complemented by the adoption of
gravel and sand, bound together with a hardened paste of cement construction techniques that will cut out waste and reduce time
and water. A range of different cements and aggregates, chemical taken on site, so shortening the period before a building or
admixtures and additions can be used to make an array of structure can be brought into use and begin to earn its keep.
concretes that have the required properties in both the fresh and
hardened states for a wide range of applications. The success of all these endeavours, both now and in the future,
will depend very much on sound concrete practice on site. This
publication combines the authors' many years of practical
History experience gained on site with information about the latest
techniques and developments in standards. It is aimed both at
Concrete was known to the Romans, Egyptians and to even earlier
those starting out on a career in construction as well as those who
Neolithic civilisations. After the collapse of the Roman Empire its
may wish to refresh their knowledge on a particular aspect of site
secrets were almost lost, to be rediscovered in more recent times.
practice.
Its modern development spans less than 200 years - 1824 is the
date on the patent for the manufacture of the first Portland
cement, one of the important milestones in concrete's history.
References/further reading
Since the middle of the 19th century, open sea has been spanned, BCA. Concrete through the ages - from 7000 BC to AD 2000.
huge buildings erected, mighty rivers dammed and extensive 1999, British Cement Association, Crowthorne. Ref. M26. 36 pp.
networks of roads constructed. In these and a thousand other ways
the face of the world has been changed as a result of the discovery Glass, J. Ecoconcrete: the contribution of cement and concrete to a
of concrete. Concrete has also been instrumental in improving the more sustainable built environment. 2001, RCC/ British Cement
health of the world's inhabitants, through its use for sewage Association, Crowthorne. Ref. 97.381. 20 pp.
disposal and treatment, and for dams and pipes providing clean
water for drinking and washing.

Uses of concrete
Concrete plays a major role, often unseen, in every aspect of our
daily lives. Its strength and durability are exploited to the full by
North Sea oil platforms and sea defences, while its thermal and
acoustic insulation properties help make houses and flats more
comfortable places to live.

Concrete bases to motorways and runways provide a solid

transport infrastructure, and the material's ability to span large
rivers makes a useful and often striking addition to our landscapes.
Dams, ring mains and water towers use concrete's ability to
contain water, and its resistance to chemicals make it an ideal
choice for sewerage works, slurry pits and even wine vats.

Not surprisingly, artists make full use of concrete, as its potential to

take any shape, colour or texture is limited only by their
imaginations.

The future
The way forward for concrete construction will be largely The Pantheon in Rome: built in AD 127, using early lightweight
influenced by the need to conserve the earth's resources, be they concrete, it is still a major tourist attraction.
materials, land or energy. Concrete has a major role to play in
sustainable construction, as it can be recycled after use, requires
relatively little energy in its manufacture and provides thermal mass
in buildings, thus reducing the need for air conditioning.

Skilled site labour is another resource that is likely to become

scarce in the future. Innovative construction techniques can help
overcome this. Self-compacting concrete is easier to place and
unmanned equipment could be used to finish concrete floors.
Transferring the construction process to a controlled operation in a
factory is another way of coping with a skills shortage. For
example, whole bathroom pods can be assembled off-site, even
down to the installation of the plumbing, and then slotted into
place at the site.

Accompanying all these advances will be the development of

concrete as a material. Continued improvements in cement and
concrete production, alternative reinforcing materials and the use
of computer-aided design will all have parts to play.
Kumata - one of the many sculptures by artist Carol Vincent who
works in coloured concrete.

3
 PORTLAND CEMENTS
Portland cements are made by burning together limestone and generally a 42,5N cement whereas CEM I for bulk supply tends to
clay, or other chemically similar suitable raw materials, in a rotary have a higher strength classification such as 42,5R or 52,5N.
kiln to form a clinker rich in calcium silicates. This clinker is ground
to a fine powder with a small proportion of gypsum (calcium CEM I can include up to 5% of minor additional constituents such
sulfate), which regulates the rate of setting when the cement is as limestone fines.
mixed with water. Over the years several types of Portland cement Within a 30-year period from around 1960 to 1990, the 28-day
have been developed. As well as cement for general use (which strength of this cement can be seen to have shifted from just
used to be known as ordinary Portland cement), there are cements below the bottom of what is now termed strength class 42,5N
for rapid hardening, for protection against attack from freezing and towards strength class 52,5N. This was a consequence of
thawing or by chemicals, and white cement for architectural continued improvements in the production process and quality
finishes. All Portland cements are produced to provide special control. In addition, early strength and the heat of hydration also
performance and properties that are of value in appropriate increased as a result of the higher reactivity of the product. Since
applications. They all contain the same active compounds - only around 1990, however, the introduction of the strength
the proportion of each is different. The main compounds in classification system into British Standards has led to stability in the
Portland cements are given in Table 1.

Table 1: Main compounds of Portland cements (typical percentage composition).

Compound Rate of hardening CEM I 42,5N SRPC W h i t e Portland cement

Tricalcium silicate C3S Rapid 56 64 65

Dicalcium silicate C2S Slow 16 10 22
Tricalcium aluminate C3A Rapid 8 2 5
Tetracalcium aluminoferrite C4AF Extremely slow 9 14 1

NOTES
1. The abbreviated chemical notation given for the above compounds is based on C = CaO S = SiO2 A = AI2O3 F = Fe2O3
2. Only main compounds are listed; therefore they do not total 100%.

By incorporating other materials during manufacture, an even strength of Portland cements. This stable situation should continue
wider range of cements is produced, including air-entraining into the long term, providing users with a more predictable
cement and combinations of Portland cement with mineral product regardless of their location in the UK.
additions. Cements are also made, for special purposes, from
materials other than those used for Portland cements, but the use Cements are now classified in terms of both their standard
of these non-Portland cements is outside the scope of this strength, derived from their performance at 28 days and at an
publication. early age, normally 2 days, using a specific laboratory test based on
a standard mortar prism. This is termed their strength class; for
The setting and hardening of cement results from a chemical example CEM I 42,5N where 42,5 denotes the standard strength
reaction between cement and water, not from a drying process. and N indicates a normal early strength.
This reaction is called hydration. It produces heat and is
It is important to recognize that cement strength classes do not
irreversible. Setting is the gradual stiffening whereby the cement
limit the strength of concrete that may be produced using these
paste changes from a workable to a hardened state. Subsequently
cements: they simply represent a cement classification system
the strength of the hardened mass increases, rapidly at first but
based on prisms of mortar tested in a laboratory.
becoming progressively less rapid. This gain of strength will
continue indefinitely provided moisture is present. The most common standard strength classes for manufactured
cements are 42,5 and 52,5 with 32,5 used less often. These can
All British cement manufacturers declare that their products
take either N (normal) or R (rapid) identifiers depending on the
conform to the appropriate British and European Standard by
early strength characteristics of the product. Another standard
marking their cement test reports and either the bags or delivery
strength class (22,5) also exists but this tends to be associated with
notes with the name, number and date of the relevant Standard.
particular special cements.
In addition, cement is currently manufactured and supplied to the
nationally-recognized third-party product certification scheme - If cement clinker is ground more finely, the greater surface area of
the BSI Kitemark Scheme for Cement. In the course of replacing the finer cement produces a faster rate of early strength
British Standards for cement by harmonized European standards, development. This is often used to advantage by precast concrete
these principles of declaration and certification will be verified by manufacturers in order to achieve a more rapid turn round of
affixing the European CE marking. moulds, or on site where it may be desirable to reduce the time for
which formwork must remain in position. Cements that have these
rapid-hardening properties, formerly known as rapid-hardening
Portland cement CEM I Portland cement (RHPC), are now produced in the UK within the
The cement most commonly used was formerly known as OPC in 52,5 strength classes of CEM I. They generate more early heat than
British Standards and, more recently, PC to BS 12. It is now known CEM I 42,5 and can often be useful in cold weather.
as Portland cement CEM I manufactured to conform to
CEM I 52,5 can also be used as an alternative to CEM I 42.5 when
BS EN 197-1.
high strength may be an advantage, particularly at early ages.
CEM I 42,5 and CEM I 52,5 strength class products account for all The term 'rapid-hardening' should not be confused with 'quick-
UK CEM I cement production and their main active chemical setting'. Concrete made, for example, with CEM I 52,5N stiffens
compounds are proportioned so that they have medium to high and initially hardens at a similar rate to a CEM I 42,5N; it is after
strength development and heat evolution. CEM I in bags is the initial hardening that the strength increases more rapidly.

4
 PORTLAND CEMENTS
A low early strength identifier (L) also exists in British Standards, clean It is equally important to make sure that the finished
but is reserved for blastfurnace cements with strength properties concrete is protected, because it gets dirty very easily in the early
outside the scope of BS EN 197-1, and is not applied to CEM 1. stages of its life and is almost impossible to clean later

It is worth noting that the setting times specified in standards Careful selection is required of the type of release agent and, if
relate to the performance of a cement paste of standard used, the sprayed-on curing membrane The use of damp hessian
consistence in a particular test under closely controlled is not recommended as it may stain the concrete
temperature and humidity conditions; the stiffening and setting of
concrete on site are not directly related to these standard setting
regimes, and are more dependent on workability, the cement
Cements and mixer
content, any admixture used, the temperature of the concrete and combinations incorporating
ambient conditions. mineral constituents or
Sulfate-resisting Portland additions CEM II and CII, CEM III
and CIII, CEM IV and CIV
cement SRPC
Sulfate-resisting Portland cement (SRPC) is a form of Portland
These are cements that are either interground or blended with
cement with a low tricalcium aluminate (C3A) content. BS 4027,
mineral materials at the cement factory or combined in the
the British Standard for SRPC, limits the C3A content to 3.5%. This
concrete mixer with additions The mineral materials and additions
limitation is achieved by adding iron oxide, thereby decreasing the
most frequently used in the UK and to which British Standards
proportion of alumina in the raw feed material; this favours the
apply are pulvenzed-fuel ash (pfa) to BS 3892, fly ash to
formation of calcium aluminoferrite (C4AF) over C3A in the cement
BS EN 450, ground granulated blastfurnace slag (ggbs) to BS 6699
kiln. This higher iron content tends to give SRPC a darker colour.
and limestone fines to BS 7979
When concrete made with CEM I cement is exposed to sulfate
Other additions include condensed silica fume, extracted during
solutions that are found in some soils and groundwaters, a reaction
the smelting process of ferrosilicon alloy, and metakaolin, produced
may occur between the sulfate and the hydrates from the C3A in
from China clay (kaolin) These are intended for specialised uses of
the cement, causing deterioration of the concrete. By limiting the
concrete beyond the scope of this publication
C3A content in SRPC, a cement with superior resistance to
conventional sulfate attack is produced. However, resistance to The two methods of incorporating the mineral additions make little
sulfate attack depends on the cement content and impermeability or no difference to the properties of concrete and, until recently, it
of the concrete as well as on the composition of the cement. was considered unnecessary to distinguish between them In 2000
Details of requirements can be found in BS 5328, BS EN 206-1, a new notation system for cements was introduced with
BS 8500 and BRE Special Digest 1. BS EN 197-1 and for mixer combinations with BS 8500 in 2002,
giving a unique code identifying both composition and method of
SRPC is normally a low-alkali cement, but otherwise is similar to
production
other Portland cements in that it is not resistant to strong acids.
Further details about durability and sulfate resistance are given on It is convenient to be able to identify cements by their notation
page 21 under Durability of concrete. The strength properties of and to consider them separately either as manufactured cement or
SRPC are similar to those of CEM I 42,5N and it should be stored mixer combinations It should be emphasised, however, that the
and used in the same way. SRPC normally produces slightly less controlled ways by which mineral additions have to be introduced
early heat than CEM I 42,5N. This may be an advantage in massive ensure that the quality of concrete is unaffected by differences in
concrete and in thick sections. their production methods

It is not normal practice to combine SRPC with pulverized-fuel ash

Technical benefits
or ground granulated blastfurnace slag. See Cements and mixer
combinations incorporating mineral constituents or additions (below) The incorporation of pfa, fly ash or ggbs with CEM I has been
for further information on these additional cementitious materials. particularly useful in massive sections of concrete where they have
been used primarily to reduce the temperature rise of the concrete,
and thus to reduce temperature differentials and peak
White Portland cement temperatures The risk of early thermal contraction cracking is
thereby also reduced
White Portland cement is made from specially selected raw
materials, usually pure chalk and white clay, containing only a very One of the options available for minimizing the risk of damage due
small quantity of iron. In addition, manufacturing processes are to alkali-silica reaction, which can occur with certain aggregates,
modified so that discolouring materials are not included during and for increasing the resistance of concrete to sulfate attack, is to
firing or grinding. use additions with Portland cement or CEM II or CEM III cements
White Portland cement generally available in the UK is a 52,5 Most additions do not react very quickly at early ages at normal
strength class product, which means it has a higher early strength temperature, and at reduced temperature the reaction -
and higher standard 28-day strength than a CEM I 42,5N but with particularly in the case of ggbs - can be considerably retarded and
similar setting properties. make little contribution to the early strength of concrete Provided
It is made to satisfy the requirements of CEM I to BS EN 197-1, so that the concrete is not allowed to dry out they can increase the
there is no separate British Standard. It is used for concrete where a long-term strength and impermeability of concrete
white or light colour finish is desired, often in conjunction with
special aggregates. Extra care must be taken in handling white
cement to avoid contamination, and in the batching, mixing and
transportation of the concrete to ensure that all equipment is kept

5
 PORTLAND CEMENTS
Table 2: Early-age properties of concrete incorporating pfa or ggbs - summary of comparisons with Portland cement CEM I

Property Pfa Ggbs Comment

Workability/ Increased for same w/c ratio. Small differences

consistence Possible to keep same
consistence but reduce w/c ratio
Setting times Increased Increased. Still within BS limits
Substantially increased with
high ggbs content
Formwork pressures Increased by about 1 0 - 2 0 kN/m 2 May be increased See Concrete Society Report CS030
and CIRIA Report 108
Bleeding Generally reduced (some Small differences
exceptions)
Quality of finish Improved quality with lean Much the same. May give
mixes. Not much difference temporary blue/green colour.
with rich mixes. Darker colour.
Time interval to Increased Substantial increase if ggbs Increased time until finishing can be a
finishing content is high and concrete disadvantage in cold conditions and
temperature is low an advantage in hot weather
Plastic settlement Generally reduced where Greater risk, which increases as Re-vibration at the correct time will
cracking bleeding is reduced the ggbs content increases remove plastic settlement cracking.
An alternative is to reduce bleeding
Plastic shrinkage Increased May be increased Prompt curing will prevent plastic
shrinkage cracking
Early age strength Substantially reduced. Lower
a) Equal binder Reduced strengths with increasing % ggbs
content
Significantly reduced e.g. after 3 Particular problems with ggbs
b) Equal 28-day Small reduction (about 10%) days at 20°C, a 40% ggbs mix will based cements in thin sections
strength
have about half the strength of in cold weather
CEM I
Formwork striking
times
a) Equal binder Increased Increased See CIRIA Report 1 36
content Other methods such as pull-out testing
or temperature-matched curing can be
b) Equal 28-day Small increase in thin sections; Increased in thin and medium used
strength much the same in large sections sections

Early-age thermal Risk reduced in sections between 500 mm and 2.5 m thick. Using aggregate with low coefficients
cracking of thermal expansion is more effective
Curing Increased sensitivity to poor Increased sensitivity to poor Views differ on this subject.
curing but larger potential for curing. Longer curing See BS 8110 for curing periods
recovery. Longer curing periods needed
periods needed
Air-entrainment Considerable increase in admixture Small differences Special admixture may be required
dosage likely to be required where pfa is used

When the terms 'water/cement ratio' or 'cement content' are used n Or, more commonly, blastfurnace cement CEM IIIA, which
in British Standards, these are understood to include combinations. contains 36 - 65% ggbs, conforming to BS EN 197-1.
Sometimes the word 'binder' is used which is interchangeable with
the words 'cement' or 'combination'. Alternatively, the granules may be ground down separately to a
white powder with a fineness similar to that of cement and
Tables 2 and 3 indicate the differences that can be expected combined in the concrete mixer with CEM I cement to produce a
between concrete made with CEM I and concrete incorporating blastfurnace cement.
pfa or ggbs.
Mixer combinations of typically 40 - 50% ggbs with CEM I have
the notation CIIIA and, at this level of addition, 28-day strengths
Blastfurnace slag cements are similar to those obtained with CEM I 42,5N.
Granulated blastfurnace slag (ggbs) is a by-product of iron
smelting. It is made by quenching selected molten blastfurnace As ggbs has little hydraulic activity of its own but is activated by
slag to form granules. The granulated slag may be interground or the calcium hydroxide and other alkaline solutions produced by
blended with Portland cement clinker at certain cement works to the hydration of Portland cement, it is referred to as 'a latent
produce: hydraulic binder'. Cements incorporating ggbs generate less heat
and gain strength more slowly. The strengths at early ages are
n Portland-slag cement CEM II/A-S with a slag content of 6 - 35%
lower than those obtained with CEM I.
conforming to BS EN 197-1

6
 PORTLAND CEMENTS
Table 3: Properties of hardened concrete incorporating pfa or ggbs - summary of comparisons with Portland cement CEM I

Property Pfa Ggbs Comment

Long-term strength (as a Greater with good curing Greater with good curing Depends on materials used and
proportion of 28-day curing
strength)

General physical properties Similar properties Similar properties Primarily depends on concrete
of hardened concrete strength at loading
(modulus, creep)

Resistance to carbonation- Similar resistance Similar resistance up to 50% Depends on concrete strength class,
induced corrosion exposure and curing conditions

Resistance to chloride- Greater resistance Greater resistance For equal w/c and well cured
induced corrosion

Seawater attack Similar performance Similar performance Primarily depends on concrete quality

Sulfate resistance Greater resistance with Greater resistance with over See BRE Special Digest 1
25 - 40% 60%

Freeze-thaw resistance Similar performance except Similar performance except at Depends on strength at time of
at early age early age exposure to freezing

Abrasion resistance Similar performance except Similar performance except at Depends on strength at time of
at early age early age exposure to abrasion

Alkali-silica reaction Often used for minimizing Often used for minimizing risk Requires selection of suitable proportion
risk of damage of damage of pfa or ggbs

NOTE
The durability of concrete depends on the correct proportions of pfa or ggbs incorporated. Guidance is given in BS 5328, BS EN 206-1,
BS 8500 and in BRE Special Digest 1.

Blastfurnace cement, either the manufactured CEM III/A or the The precipitated material is a fine powder of glassy spheres that
mixer combinations CIIIA, may be used for all purposes for which can have pozzolanic properties, i.e. when mixed into concrete it
CEM I is used but, because it has a lower early development of can react chemically with the calcium hydroxide (lime) that is
strength, particularly in cold weather, it may not be suitable where released during the hydration of Portland cement. The products of
early removal of formwork is required. It is a moderately low-heat this reaction are cementitious, and in certain circumstances pfa or
cement and can, therefore, be used to advantage to reduce early fly ash can be used to replace part of the Portland cement in
heat of hydration in thick sections. concrete.

When the proportion of ggbs is 66 - 80% the notation CEM III/B The properties of fly ash for use as a cementitious component in
applies for the manufactured cement and CIIIB for a mixer concrete are specified in BS EN 450 with additional UK provisions
combination. This was previously known as high slag blastfurnace for pfa made in BS 3892: Part 1. Pfa conforming to Part 2 of the
cement and is specified because of its lower heat characteristics or same BS (Part 2 ash) is more coarse and is generally regarded as an
to impart resistance to sulfate attack. inert addition used, for example, to modify properties of aggregate
such as their gradings.
Because the reaction between ggbs and lime released by the
Portland cement is dependent on the availability of moisture, extra Fly ash, in the context of BS EN 450, means 'coal fly ash' rather
care has to be taken in curing concrete containing these cements than ash produced from other combustible materials. Fly ash
or combinations in order to prevent premature drying out and to conforming to BS EN 450 can be coarser than that conforming to
permit the development of strength. BS 3892 : Part 1. However, fly ash to BS EN 450 can be used, in
accordance with BS EN 206-1 and BS 8500 as a 'Type II addition'
BS 146 continues in revised form to allow for UK provisions not (pozzolanic or latent hydraulic material) in order to improve certain
included in the European Standard EN 197-1 for blastfurnace slag properties or to achieve special properties.
cements. There is currently no equivalent EN relating to ggbs as an
addition and, accordingly, BS 6699 continues to apply. Substitution of these types of cement for Portland cement is not a
straightforward replacement of like for like, and the following
Pulverized-fuel ash cements and fly ash points have to be borne in mind when designing pfa concrete:
cements n Pfa reacts more slowly than Portland cement. At early age and
The ash resulting from the burning of pulverized coal in power particularly at low temperatures pfa contributes less strength; to
station furnaces is known in the concrete sector as pulverized-fuel achieve the same 28-day compressive strength the amount of
ash (pfa) or fly ash. cementitious material may need to be increased - typically by
about 10%. The potential strength after three months is likely
This ash is fine enough to be carried away in the flue gases and is to be greater than CEM I provided the concrete is
removed from the gases by electrostatic precipitators to prevent maintained in a moist environment, such as in underwater
atmospheric pollution. structures or concrete in the ground

7
 PORTLAND CEMENTS

A guide to the notation of cements and combinations

Manufactured cements are those made in a cement factory. The types of cement and combinations in most common usage
Where a mineral material is included, it is generally added to are shown with their notation in Table 4.
the cement clinker at the grinding stage. These manufactured
cements are all identified by the prefix letters CEM. Table 4: Cements and combinations in general use

Where a concrete producer adds an addition such as pfa or Cement/ Notation of Notation of
ggbs to CEM I Portland cement in the mixer, the resulting combination manufactured mixer
cement is known as a mixer combination identified by the type cement combination
prefix notation C. conforming to conforming to
BS EN 197-1 BS 8500 : Part 2
CEM I was described earlier. However, when the letters
CEM (or C) are followed by the numbers II, III or IV these Portland cement CEM I -
relate to an increasing proportion of addition:
Sulfate-resisting SRPC conforms to —
n CEM II (and CII) include up to 35% of mineral addition
Portland cement BS 4027.
n CEM III (and CIII) include higher proportions of
blastfurnace slag Portland-fly ash CEM II/B-V CIIB-V
n CEM IV (and CIV) include higher proportions of cement incorporating
21 - 35% pfa
pozzolana (such as pfa).

Cement type CEM I consists almost entirely of Portland cement Portland-slag cement CEM II/A-S CIIA-S
clinker and gypsum set regulator. Notation for the other types incorporating
includes a letter that in a simple way identifies the relative 6 - 20% ggbs
proportion of clinker. High proportions are represented by A,
moderate proportions by B whilst C means a lower proportion Portland-slag cement CEM II/B-S CIIB-S
of clinker. Of course, where the proportion of Portland cement incorporating
21 - 35% ggbs
clinker is high, the proportion of the second constituent, the
addition, is relatively low.
Blastfurnace cement CEM III/A CIIIA
Cement type CEM II (or CII) is the only one in which the type incorporating May also conform
of addition needs to be identified and the following letters are 36 - 65% ggbs to BIII/A of
used for this group: BS146

V - siliceous fly ash (such as pfa) Blastfurnace CEM III/B CIIIB

cement May also
S - blastfurnace slag incorporating conform to
D - silica fume 66 - 80% ggbs BIII/B of BS 146

L and LL - limestone Portland-limestone CEM II/A-L CEM IIA-L

cement CEM II/A-LL CEM IIA-LL
M - more than one of the above
incorporating
The strength class and strength development characteristics, 6 - 20% limestone
explained earlier, have their identifiers added to the end of the
notation so that the full title, for example, of a manufactured n The water demand of pfa for equal consistence may be less
cement with a relatively low proportion of ggbs would be than that of Portland cement
Portland-slag cement BS EN 197-1 CEM II/A-S 42,5N as shown
n The density of pfa is about three-quarters that of Portland
below. ;
cement
n The reactivity of pfa and its effect on water demand, and hence
CEM II/ A/S 42, 5N strength, depend on the particular pfa and the Portland cement
Sub-class: with which it is used. A change in the source of either material
rapid early strength R may result in a change in the replacement level required
normal early strength N n Where pfa concrete is to be air-entrained, the admixture
low early strength L dosage rate may have to be increased, or an alternative
formulation that produces a more stable air bubble structure
Standard strength class
should be used
n Portland-fly ash cement comprises, in effect, a mixture of
Sub-type: in this case,
CEM I and pulverized-fuel ash
blastfurnace slag
n When the ash is interground/blended with Portland cement
Proportion of cement clinker: clinker at an addition rate of 20 - 35% the manufactured cement
high A is known as Portland-fly ash cement CEM II/B-V conforming to
medium B
BSEN 197-1
low C
n Where this combination is produced in a concrete mixer it has
Main cement type the notation CIIB-V conforming to BS 8500 : Part 2.

8
 PORTLAND CEMENTS
Typical proportions are 25 - 30% ash and these cements can be covers with generous overlaps (Figure 1). The bags should be used
used in concrete for most purposes. It is likely to have a lower rate in the order in which they are received; thus each delivery should
of strength development compared with CEM I. When the cement be kept separate to avoid confusion. To avoid 'warehouse set',
contains 25 - 40% pfa or fly ash it may be used to impart which results from the compaction of cement, bags should not be
resistance to sulfate attack and can also be beneficial in reducing stacked higher than about 1.5 m. The paper bags used for packing
the harmful effects of alkali-silica reaction. cement are not vapour proof, so undue exposure should be
avoided. Even when stored under good conditions, bagged cement
Where higher replacement levels of ash are used for improved low- may lose 20% of its strength after two months' storage. To avoid
heat characteristics, the resulting product is pozzolanic (pulverized- risk of accidental confusion, cements of different types should be
fuel ash) cement CEM IV/B manufactured to conform to stored separately.
BS EN 197-1 or, if combined in the concrete mixer, CIVB-V
conforming to BS 8500 : Part 2.

Because the pozzolanic reaction between pfa or fly ash and free
Sampling and testing of
lime is dependent on the availability of moisture, extra care has to cement
be taken in curing concrete containing mineral additions in order
The testing of cement requires the resources of a well-equipped
to prevent premature drying out and to permit the development of
laboratory with strictly controlled temperature and humidity, which
strength.
are seldom achieved on site. Manufacturers in the UK produce
cement whose conformity is certificated by a third party in a
Portland-limestone cement CEM II/A/L scheme based on a strict regime of inspection and independent
and CEM II/A-LL audit testing. Cement test reports showing results of physical and
chemical tests are forwarded to users of the cement and it is
Cement that incorporates 6 - 35% of carefully selected fine
general practice for concrete producers to monitor cement quality
limestone powder is known as Portland-limestone cement
by continuously assessing the data and thereby avoiding
conforming to BS EN 197-1. Where a 42,5N product is
unnecessary duplication of costly tests on cement.
manufactured, the typical proportion of limestone is 10 - 20%,
with the notation CEM II/A-L or CEM II/A-LL. It is most popular in
continental Europe and its usage is growing in UK. Decorative
precast and reconstituted stone concretes benefit from its lighter
colouring and it is also used for general-purpose concrete in non-
aggressive and moderately aggressive environments.

Delivery and storage of

cement
Cement may be delivered in bulk or in bags. Bulk cement is
delivered by tanker, usually in loads of more than 25 tonnes and
blown into storage silos by compressed air. Bagged cement is
usually supplied in bags containing 25 kg or, very rarely 50 kg,
whilst 1 tonne bags are also available from some suppliers for
special purposes. It is often convenient to use bags on a smaller
site, but cement is cheaper in bulk.

Cement should be kept dry during storage as moist air leads to the
phenomenon of air-setting, which results in the formation of lumps
of hydrated cement. Air-set cement should not be used, as Figure 1: Correct storage of bagged cement on site: note raised
concrete made from it could have a much reduced strength. timber platform and plastic sheeting.

Silos have to be weatherproof but, during prolonged periods of

storage, some air setting may occur due to condensation in the References/further reading
silo. This is minimized by aeration, which should be done
BS 12 : 1996, Specification for Portland cement. British Standards
frequently in periods of prevailing damp weather. In addition, the
Institution, London. (Withdrawn in April 2002.)
weatherproofness of the silo should be checked if there is any
evidence of the formation of lumps in the cement. BS 146 : 2002, Specification for blastfurnace cements with strength
properties outside the scope for BS EN 197-1. British Standards
Regular maintenance of cement silos is essential. All moving parts Institution, London.
should be kept free from coatings of cement by cleaning at least at
BS 3892 : 1997 : Pulverized-fuel ash. Part 1 -Specification for
the end of every day. Weigh hoppers should also be cleaned every
pulverized-fuel ash for use with Portland cement. British Standards
day, both inside and out, since a build-up of cement can result in
Institution, London.
top little cement being dispensed, and weigh gear checks should
be carried out at least once a month. Silo air filters must be BS 4246 : 1996, Specification for high slag blastfurnace cement.
cleaned after every cement delivery to prevent them from British Standards Institution, London. (Withdrawn in 2002.)
becoming choked; this is done by giving them a thorough shaking
BS 5328 : 1997, Concrete. British Standards Institution, London.
or, preferably, by replacing old filters with reverse-air jet units that
prevent contamination of the environment. BS 6588 : 1996, Specification for Portland pulverized-fuel ash cements.
British Standards Institution, London. (Withdrawn in April 2002.)
Bagged cement should be stored on a raised floor in a weather-
BS 6610 : 1996, Specification for Pozzolanic pulverized-fuel ash
tight shed in order to prevent deterioration. Failing this, it should
cement. British Standards Institution, London.
be, stacked on a raised timber platform and covered by waterproof

9
 PORTLAND CEMENTS AGGREGATES
BS 6699 : 1992, Specification for ground granulated blastfurnace slag The term 'aggregates' is used to describe the gravels, crushed rocks
for use with Portland cement. British Standards Institution, London. and sands that are mixed with cement and water to make concrete.
As aggregates form the bulk of the volume of concrete and can
BS 7583 : 1996, Specification for Portland limestone cement. British
affect its performance, the selection of suitable material is important.
Standards Institution, London. (Withdrawn in April 2002.)
BS 8110 : 1997, The structural use of concrete. British Standards Sand includes natural sand, crushed rock or crushed gravel which is
Institution, London. fine enough to pass through a sieve with 5 mm apertures. Coarse
aggregate comprises larger particles of gravel, crushed gravel or
BS 8500 : 2002, Complementary British Standard to BS EN 206-1.
crushed rock. The size of the sieve used to distinguish between sand
British Standards Institution, London.
and coarse aggregate is expected to be changed to 4 mm
BS EN 197-1: 2000, Cement - composition, specifications and throughout Europe.
conformity criteria for common cements. British Standards Institution,
London. Most concrete is made from natural aggregates that are usually
specified to conform to the requirements of BS 882 and
BS EN 206-1: 2000, Concrete - specification, performance, BS EN 12620 together with the UK National Annex. Manufactured
production and conformity. British Standards Institution, London. lightweight aggregates are sometimes used (see page 12).
BS EN 450 : 1995, Fly ash for concrete. Definitions, requirements and
quality control. British Standards Institution, London.
Sizes of aggregate
CIRIA Report 108 : 1985, Concrete pressure on formwork.
The maximum size of coarse aggregate, Dmax is governed by the
Construction Industry Research and Information Association,
type of work to be done. For reinforced concrete it should be such
London.
that the concrete can be placed without difficulty, surrounding all
CIRIA Report 136 : 1995, Formwork striking times. Criteria, prediction reinforcement thoroughly, particularly in the cover zone, and filling
and methods of assessment. Construction Industry Research and the corners of the formwork. It is usual in the UK for coarse
Information Association, London. aggregate for reinforced concrete to have a maximum size of
BRE Digest 330, Alkali silica reaction in concrete. Construction 20 mm.
Research Communications, London, 1999.
Aggregate of D max 40 mm can be used for foundations and mass
BRE Special Digest 1, Concrete in aggressive ground. Concrete concrete and similar sections where there are no restrictions to the
Research Communications, London, 2001. flow of concrete. It should be noted, however, that concrete with
Dmax 40 mm aggregate is not always available from producers of
CS030, Formwork - a guide to good practice, The Concrete Society,
ready-mixed concrete. The use of a larger aggregate results in a
Crowthorne, 1995.
slightly reduced water demand and hence a slightly reduced cement
content for a given strength and workability.

Smaller aggregate, usually Dmax 10 mm, may be needed for

concrete that is to be placed through congested reinforcement for
example. In this case the cement content may have to be increased
by 10 - 20% to achieve the same strength and workability as with a
20 mm maximum-sized aggregate concrete because the sand
content and water content normally have to be increased to
produce a cohesive mix.

Quality requirements
Durability
Aggregates should be hard and should not contain materials that are
likely to decompose or change in volume when exposed to the
weather. Examples of undesirable materials are lignite, coal, pyrite
and lumps of clay. Coal and lignite may swell and decompose
leaving small holes on the surface of the concrete; lumps of clay may
soften and form weak pockets; and pyrite may decompose, causing
iron oxide stains to appear on the concrete surface. When exposed
to oxygen, pyrite has been known to contribute to sulfate attack.
High-strength concretes may call for additional special properties.
The mechanical properties of aggregates for heavy-duty concrete
floors and for pavement wearing surfaces may have to be specially
selected. Most producers of aggregate are able to provide
information about these properties, and reference, when necessary,
should be made to BS 882 / BS EN 12620.

There are no simple tests for durability or freeze/thaw resistance, and

assessment of particular aggregates may be based on experience of
the properties of concrete made with the type of aggregate in
question with a knowledge of its source. Some flint gravels with a
white porous cortex may be frost-susceptible because of the high
water absorption of the cortex, which results in pop-outs on the
surface of the concrete when subjected to freezing and thawing.

10
 AGGREGATES
Cleanness
Aggregates should be clean and free from organic impurities;
100 0
aggregate containing organic material makes poor concrete. The
particles should be free from coatings of dust or clay, as these
90 10
prevent the proper bonding of the material. An excessive amount
of fine dust or stone 'flour' may prevent the particles of stone from Limits for
80 20
being properly coated with cement and thus lower the strength of grading
the concrete. Gravels and sand are usually washed by the suppliers M sand
70 30
to remove excess fines (clay and silt, for example) and other
impurities, which, if present in excessive amounts, result in a poor-

Percentage retained
60 40

Percentage passing
quality concrete. However, excessive washing can remove all fine
material passing the 300 μm sieve. This may result in a concrete
50 50
mix lacking in cohesion and, in particular, being unsuitable for
placing by pump. Sands deficient in fines also tend to increase the
40 60
bleeding characteristics of the concrete, which can result in poor
vertical finishes due to water scour.
30 70
Limits on the amount of fines are given in BS 882 when
determined in accordance with the wet sieving method specified in 20 Limits for 80
graded
BS812. 20 mm
10 coarse 90
An approximate guide to the fines content of gravel sand can be aggregate
obtained from the field settling test. Results of this test cannot be
0 100
used as the basis for accepting or rejecting material but they are 75 150 300 600 1.18 2.36 5 10 20 37.5
µm µm µm μm mm mm mm mm mm mm
nevertheless useful by detecting changes in the cleanness of sand.
Sieve sizes (BS410: 1986)
More details are given on page 54 under Testing materials,
aggregates.

Where colour of surface finish is important, supplies of aggregate Figure 2: Grading envelopes for grading M sand and 20 mm
should be obtained from one source throughout the job whenever graded coarse aggregate (as given in BS 882 : 1992).
practicable. This is particularly important for the sand - and for the aggregate, should not be used for structural reinforced concrete
coarse aggregate if an exposed-aggregate finish is required. work because the grading will vary considerably from time to time
and hence from batch to batch, resulting in excessive variation in
Grading of aggregates the consistence and the strength. To ensure that the proper
amount of sand is present, the separate delivery, storage and
The proportions of the different sizes of particles making up the batching of coarse and fine aggregates is essential.
aggregate are found by sieving and are known as the 'grading' of
the aggregate: the grading is given in terms of the percentage by
mass passing the various sieves. Continuously graded aggregates
Coarse aggregates
for concrete contain particles ranging in size from the largest to For a high degree of control over the production of concrete, and
the smallest; in gap-graded aggregates some of the intermediate particularly where high-quality surface finishes are required, it is
sizes are absent. Gap-grading may be necessary in order to achieve necessary for the coarse aggregate to be delivered, stored, and
certain surface finishes. The sieves used for making a sieve analysis batched using separate single sizes rather than a graded coarse
should conform to BS 410 or BS EN 933-2. The tests should be aggregate.
carried out in accordance with the procedure given in BS 812 or
The sieve sizes in general use are 50, 37.5, 20, 14, 10, 5 and
BS EN 933-1
2.36 mm for coarse aggregate. The grading limits are shown in
An aggregate containing a high proportion of large particles is Table 5.
referred to as being 'coarsely' graded and one containing a high
proportion of small particles as 'finely' graded. Grading envelopes Graded coarse aggregates which have been produced by layer
for sand of grading M and for 20 mm graded coarse aggregate are loading - (filling a lorry with, say, two grabs of 20 - 10 mm and
shown in Figure 2. one grab of 10 - 5 mm) - are seldom satisfactory because the
materials are unmixed and will not be uniformly graded. Graded
Grading limits for 'all-in' aggregates are also given in BS 882/ coarse aggregate should be mixed efficiently by the producer
BS EN 12620. All-in aggregate, composed of both fine and coarse before loading lorries.

Table 5: Grading limits for coarse aggregate (from BS 882 : 1992).

Sieve Percentage by mass passing BS sieves for nominal sizes

size Graded aggregate Single-sized aggregate
(mm)
40 mm to 5 mm 20 mm to 5 mm 14 mm to 5 mm 40 mm 20 mm 14 mm 10 mm
- - - - -
50 100 100
37.5 90 - 100 100 - 85 - 100 100 - -
20 35- 70 90 - 100 100 0 - 25 85- 100 100 -
14 2 5 - 55 4 0 - 80 90 - 100 - 0 - 70 8 5 - 100 100
10 10- 40 3 0 - 60 50 - 85 0- 5 0 - 25 0 - 50 85 - 100
5 0- 5 0 - 10 0 - 10 - 0- 5 0 - 10 0 - 25
2.36 - - - - - - 0- 5

11
 AGGREGATES
Sand the salt content. Chloride contents should be checked frequently
throughout aggregate production in accordance with the method
The sieve sizes in general use are 10, 5, 2.36, 1.18 mm and 600,
given in BS 812 : Part 117.
300 and 150 μm. The fines content is determined by wet sieving
through a 75 μm sieve. Appendix C in BS 882 gives maximum chloride contents for the
combined aggregates; these limits have been derived from those
The C-M-F system of classification for sand in Table 6 is useful for
given in BS 5328 but do not necessarily ensure conformity to
selecting appropriate proportions of fine and coarse aggregates in
BS 5328 for all concrete mixes, particularly those with a low
a mix, because the optimum proportion of sand is partly related to
cement content. For reinforced concrete made with CEM I, in tests
its fineness. Good concrete can be made with sand within the
for the total maximum chloride content, expressed as percentage
overall limits shown in Table 6. Where the variability of grading
chloride ion by mass of combined aggregate, no result should be
needs to be restricted further for the design of particular mixes or
greater than 0.08% and 95% of the test results should not be
for the adjustment of sand content of prescribed concrete mixes,
more than 0.05%. For concrete made with sulfate-resisting
this can be achieved by reference to one or more of the three
Portland cement the maximum chloride content of the combined
additional grading limits C, M or F. Sand with grading F should
aggregate should not be more than 0.03%. For prestressed
normally be used only after trial mixes have been made with the
concrete and steam-cured structural concrete BS 882 recommends
proposed combination of sand and coarse aggregates and cement
that the maximum total chloride content expressed as percentage
to determine their suitability for the particular purpose. There may
of chloride ion by mass of combined aggregate should not
be occasions, such as when a high degree of control is required,
exceed 0.01 %.
and when high-quality surface finishes have to be achieved, when
it is necessary to specify sand gradings to closer limits than those Some sea-dredged sands tend to have a preponderance of one size
permitted in BS 882/BS EN 12620 and shown in Table 6. On the of particle and a deficiency in the amount passing the 300 μm
other hand, sand whose gradings fall outside the Standard limits sieve. This can lead to mixes prone to bleeding unless mix
may produce perfectly satisfactory concrete. It is not so much the proportions are adjusted to overcome the problem. An increase in
grading limits themselves that are important, but that the grading the cement content by 5 - 10% will often offset the lack of fine
is maintained reasonably uniform. particles in the sand.

Table 6: Grading limits for sand (from BS 882 : 1992). Beach sands are generally unsuitable for good-quality concrete,
since they are likely to have high concentrations of chloride
Percentage by mass passing BS sieve because of the accumulation of salt crystals above the high-tide
Sieve
Overall Additional limits for grading mark. They are also often single-sized, which can make the mix
size
limits C M F design difficult.
- - -
10 mm 100
5 mm 89 - 100 - - - Lightweight aggregates
2.36 mm 60 - 100 60-100 65 - 100 80 - 100
30-100 30- 90 45 - 100 70 - 100 In addition to natural gravels and crushed rocks, a number of
1.18 mm
600 μm 15 - 100 15- 54 2 5 - 80 55-100 manufactured aggregates are available for use in concrete.
300 μm 5 - 70 5 - 40 5 - 48 5 - 70 Lightweight aggregates such as sintered pfa are required to
150 μm 0 - 15* - - - conform to BS 3797/BS EN 13055-1.
*Increased to 20% for crushed rock sand except when they
are used for heavy-duty floors Lightweight aggregates have been used in concrete for many years
- the Romans made use of pumice in some of their construction
NOTE
work. Small quantities of pumice are imported and still used in the
Sand not conforming to this table may also be used provided
that the supplier can satisfy the purchaser that such materials UK, mainly in lightweight concrete blocks, but most lightweight
can produce concrete of the required quality. For heavy-duty aggregate concrete is made using manufactured aggregates. All
concrete floor finishes, the sand should conform to gradings lightweight materials are relatively weak because of their higher
CorM. porosity, which gives them reduced weight. This imposes a
limitation on strength, though this is not often a serious problem
because the strength that can be obtained is comfortably in excess
Marine-dredged aggregates of most structural requirements. Lightweight aggregates are used
to reduce weight in structural elements or to give improved
Large quantities of aggregates are obtained by dredging marine
thermal insulation.
deposits, and they have been widely and satisfactorily used for
making concrete for many years.

If present in sufficient quantities, hollow and/or flat shells may Delivery of aggregates
affect the properties of the fresh and hardened concrete. Limits on Quality control of concrete should start with a visual inspection of
shell content are given in BS 882: the aggregates as they are delivered, combined with some quick,
n For 20 mm and larger coarse aggregates (single-sized, graded simple testing if there is any doubt about their quality or grading.
or all-in) the limit is 8%
The cleanness of sands can be checked quickly by hand. If a
n For 10 mm to 5 mm coarse aggregate the limit is 20% sample of sand is rubbed between the palms of the hands, staining
n For sand there is no limit. of the palms may be an indication that an excessive amount of clay
and silt are present, due to inadequate washing.
In order to reduce the risk of corrosion of embedded metal, limits
are specified in BS 5328, BS EN 206-1 and BS 8500 for the Confirmation or denial of this indication can be determined by the
chloride content of the concrete. To conform to these limits it is field settling test described under Cleanness on page 54. Coarse
necessary for marine-dredged aggregates to be carefully and aggregates should be inspected visually for clay lumps and clay
efficiently washed with frequently changed fresh water to reduce coatings, grading and particle shape. Clay lumps are not always

12
 AGGREGATES
obvious and careful inspection of deliveries is advised. Loads BS 812, Testing aggregates. British Standards Institution, London.
containing such lumps should be rejected before discharge. Part 2 : 1995, Methods for determination of density.
Part 103 : Method for determination of particle size distribution.
As previously mentioned on page 11, layer loading of lorries Section 1: 1985, Sieve tests. Section 2 : 1989, Sedimentation
produces an aggregate that is unmixed, and problems will occur in test.
obtaining a uniform concrete. Such loads should be rejected. Part 117:1988, Method for determination of water-soluble
A further problem with gravel coarse aggregates may occur when chloride salts.
oversized material is crushed. Such material tends to be of an BS 882 : 1992, Specification for aggregates from natural sources for
angular particle shape, rather than rounded or irregular, and a load concrete. British Standards Institution, London.
of all crushed material or a load containing a large part of crushed BS 3797 : 1990, Specification for lightweight aggregates for masonry
as well as uncrushed, can lead to variations in the water demand, units and structural concrete. British Standards Institution, London.
consistence and strength unless adjustments are made to the mix. (BS EN 1 3055-1, Lightweight aggregates. Part 1 : Lightweight
Coarse aggregate should have a uniform particle shape for aggregates for concrete and mortars.)
production of high quality concrete.
BS 5328 : 1997, Concrete. British Standards Institution, London.
A useful means of detecting changes in grading or shape is by the BS 8500 : 2002, Complementary British Standard to BS EN 206-1.
loose bulk density test in accordance with BS 812 : Part 108. British Standards Institution, London.
BS EN 206-1: 2000, Concrete - specification, performance,
production and conformity. British Standards Institution, London
Storage of aggregates
BS EN 933 : Tests for geometrical properties of aggregates. British
Aggregates should be stored so that they are kept as uniform as
Standards Institution, London.
possible in grading and moisture content, and protected from
Part 1 : 1997, Determination of particle size - sieve method.
intermingling and contamination by other materials.
Part 2 : 1996, Test sieves, nominal size of apertures.
It is best to put down a layer of concrete over the areas where the pr EN 12620 : 2000, Aggregates for concrete. British Standards
aggregates will be stored. The concrete should be laid to fall away Institution, London.
from the mixer to allow free drainage of water from the aggregate, BRE Digest 330, Alkali silica reaction in concrete. 1999, Construction
and should extend well out from the mixer set-up so that all Research Communications, London.
deliveries can be tipped onto it. If a clean, hard base is not
BRE Special Digest 1, Concrete in aggressive ground. 2001,
provided, the bottom 300 mm of each aggregate pile should not
Construction Research Communications, London.
be used, since dirt and water can accumulate there.

It is essential to provide substantial partitions to separate the

different aggregate sizes and to prevent spillage from one bay to
another. Such partitions can be made using concrete, brick or
block retaining walls, or by driving H-section steel members into
the ground and laying heavy timber sections between them.

Stockpiles should be as large as possible, as this helps to ensure

uniformity of moisture content. Variations in the moisture content
of coarse aggregates as delivered, or in the stockpiles, are usually
not sufficient to have much effect on the control of free
water/cement ratio. However, the variations that commonly occur
in the moisture content of sand will require adjustment to be made
in order to control the free water/cement ratio.

Ideally, stockpiled sand should be allowed to stand for 12 hours

before use so that, apart from the lower part of the stockpile, the
moisture content will be reasonably uniform at about 5 - 7%.
When sand is very wet (as sometimes happens with fresh deliveries,
or after it has been raining) the moisture content can be as high as
1 2 - 1 5 % . Unless adjustments are made to the water added at the
mixer, excessive variations in workability, strength and durability
will result.

For large batching plants the aggregate would probably be lifted

by a conveyor system to covered overhead storage hoppers
discharging directly into weigh-batchers.

References/further reading
BS 410 : 2000, Test sieves. British Standards Institution, London.
Part 1 : Technical requirements and testing. Test sieves of metal
, wire cloth, (ISO 3310-1: 2000).
Part 2 : Technical requirements and testing. Test sieves of
perforated metal plate, (ISO 3310-2: 2000).

13
 WATER
The important, and most difficult, issue is the correct assessment of
Quality how much water is required. In the sections dealing with
The water used for mixing concrete should be free from any aggregates and batching, the variability of moisture content is
impurities that could adversely affect the process of hydration and, discussed and in the quest to control free water/cement ratio it is
consequently, the properties of concrete. essential to allow for water contained in aggregates. Because such
a large proportion of concrete consists of aggregates, a small shift
For example, some organic matter can cause retardation whilst
in the moisture content makes a big change in the quantity of
chlorides may accelerate the stiffening process and also cause
water to be added.
embedded steel such as reinforcing bars to corrode. Other
chemicals such as sulfate solutions and acids may have harmful Devices based on advanced electronics technology exist for
long-term effects by weakening the cement paste by dissolving it. measuring the moisture content of a batch of aggregates and for
calculating the free water/cement ratio of concrete during mixing,
It is important, therefore, to be sure of the quality of water. If it
but most producers rely on the experience of the batcher to judge
comes from an unknown source such as a pond or borehole it
the point at which the amount of water is correct from the way in
should be checked by making trial mixes. The British Standard
which the concrete moves and the sound it makes in the mixer.
3148 specifies the quality of water and gives the procedures for
Any equipment that is capable of indicating the consistence of
checking its suitability for use in concrete. BS EN 1008, which
concrete while it is being mixed assists the batcher in gauging the
supersedes BS 3148, had not been finalised at the time this
mixing water with greater speed and accuracy. Where the mixer is
handbook was published.
powered by an electric motor an ammeter or kilowatt meter
Drinking water is suitable, of course, and it is usual simply to accurately indicates the power consumed in mixing the concrete -
obtain a supply from the local water utility. But some recycled less power is demanded as concrete becomes more workable.
water is being increasingly used in the interests of reducing the Similarly, truckmixer drums that are turned by hydraulic drive can
environmental impact of concrete production, and seawater has have the consistence of concrete indicated by a pressure gauge
been used successfully in mass concrete with no embedded steel. (Figure 3).
The use of seawater does not normally affect the strength of plain
Portland cement concrete, but it must not be used for concrete
containing embedded metal because of the danger of corrosion of
the steel from the chloride content of the water, nor should it be
used where white efflorescence could mar the appearance of the
work.

Reclaimed and recycled

water
Recycled water systems are usually found at large-scale permanent
mixing plants such as precast concrete factories or ready-mixed
concrete depots where water used for cleaning the plant and
washing out mixers after use can be collected, filtered and stored
for re-use. Some processes are able to reclaim up to a half of the
mixing water in this way.
Figure 3: Pressure gauge indicating the consistence of the
When reclaiming water for use as recycled mixing water care needs concrete in a truckmixer.
to be taken to avoid impurities including harmful chemicals, oil or
organic matter, and any traces of powerful admixtures such as air- Regardless of the method of gauging the mixing water, it is
entraining agents, retarders or pigments must be diluted to such recommended that the concrete is finally checked by a batcher
an extent that they will have no effect. Any polypropylene or steel and/or driver to see that it has the specified consistence and a
fibres need to be filtered out and a careful check kept on the uniform appearance. This is the most effective way of ensuring that
amount of suspended fines carried in the water: after all the effort the concrete is thoroughly mixed and has the designed free water
and cost of obtaining clean aggregates, it is not sensible to put an content. When the free water content is closely monitored and
excessive quantity of fines back in the form of dirty water. cement content accurately weighed, the free water/cement ratio is
controlled and, therefore, strength, durability and many other
Large-volume settlement tanks are normally required. They do not essential properties of the concrete are assured.
need to be particularly deep but should have a large surface area
and, ideally, the water should pass through a series of tanks,
becoming progressively cleaner at each stage. Alternatively, the References/further reading
water may be chemically treated, particularly where space is BS 3148 : 1980, Methods of test for water for making concrete
limited, in order to make it suitable for re-use or for discharge into (including notes on the suitability of the water) (to be superseded by
drains in a condition that conforms to statutory requirements. BS EN 1008, currently prEN 1008 : 1997 Mixing water for concrete
- specifications for sampling, testing and assesing the suitability of
Measuring the quantity of water, including waste water from recycling installations in the
water concrete industry as mixing water for concrete). British Standards
Institution, London.
Mixing water is usually measured by volume but, in some plants, it BS 5328 : 1997, Concrete. British Standards Institution, London.
may be more convenient for it to be batched by weight. One litre
of clean water weighs exactly one kilogram and so the quantity of BS 8500 : 2002, Complementary British Standard to BS EN 206-1.
water remains numerically the same regardless of whether it is British Standards Institution, London.
measured by volume or by mass, but corrections should be applied BS EN 206-1: 2000, Concrete - specification, performance, production
when water contains fines. and conformity. British Standards Institution, London.

14
 ADMIXTURES
An admixture is a material, usually a liquid, which is added to a and develops strength more quickly. They have a negligible effect
batch of concrete during mixing in order to modify the properties on consistence, and 28-day strengths are seldom affected.
of the fresh or hardened concrete in some way.
Accelerating admixtures have been used mainly during cold
Most admixtures benefit concrete by reducing the amount of free weather when the slowing down of the chemical reaction between
water required for a given level of consistence, often in addition to cement and water due to low temperature could be offset by the
some other specific improvement. Permeability is thereby reduced increased speed of reaction resulting from the use of the
and durability increased. There are occasions when the use of an accelerator. The most widely used accelerator for some time was
admixture is not only desirable but also essential. calcium chloride but, because the presence of chlorides, even in
small amounts, increases the risk of corrosion, the use of
Because admixtures are added to concrete mixes in small admixtures containing chlorides is now prohibited in all concrete
quantities, they should be used only when a high degree of control containing embedded metal.
can be exercised. Incorrect dosage of an admixture - either too
much or too little - may adversely affect the strength and other Accelerators are sometimes marketed under other names such as
properties of the concrete. hardeners, anti-freezes on frost-proofers, but no accelerator is a
true anti-freeze and the use of an accelerator does not avoid the
BS EN 934-2 replaces BS 5075 in specifying the requirements for
need to protect the concrete from the cold by keeping it warm
the main types of admixture: (with insulation) after it has been placed.
n Normal water-reducing
NOTE: Accelerators are ineffective in mortars because the
n Accelerating water-reducing
thickness of mortar, either in a joint or on a rendering, is such that
n Retarding water-reducing any heat generated by the faster reaction is quickly dissipated.
n Air-entraining
n Superplasticizing / high range water reducing. Retarding water - reducing
admixtures
Normal water-reducing
admixtures (plasticizers, These are chemicals that slow down the initial reaction between
cement and water by reducing the rate of water penetration to the
workability aids) cement and slowing down the growth of the hydration products.
The concrete therefore stays workable longer than it would
Water-reducing admixtures act by reducing the inter-particle otherwise.
attraction between cement particles and produce a more uniform
dispersion of the cement grains. The cement paste is better The length of time during which a concrete remains workable
'lubricated', and hence the amount of water needed to obtain a depends on its temperature, consistence class, and water/cement
given consistence can be reduced. ratio, and on the amount of retarder used. Although the occasions
justifying the use of retarders in the UK are limited, these
This effect may be beneficial in one of three ways: admixtures may be helpful when one or more of the following
1. Added to a normal concrete at normal dosage, they produce an conditions apply:
increase in slump of about 50 mm. This can be useful with n In warm weather, when the ambient temperature is higher than
high-strength concrete, rich in cement, which would otherwise about 20°C, to prevent early stiffening ('going-off') and loss of
be too stiff to place. workability, which would otherwise make placing and finishing
2. The water content can be reduced while maintaining the same difficult
cement content and consistence; this results in a reduced n When a large pour of concrete will take several hours and must
water/cement ratio (by about 10%), and therefore increased be constructed without already placed concrete hardening
strength and improved durability. This can also be useful for before subsequent concrete is merged with it (i.e. without a
reducing bleeding in concrete prone to this problem, or for cold joint)
increasing the cohesion and thereby reducing segregation in
n When the complexity of slipforming demands a slow rate
concrete of high consistence class, or in harsh mixes sometimes
of rise
arising with angular aggregates, or low sand contents, or when
the sand is deficient in fines. n When there is a delay of half an hour or more between mixing
and placing - for example, when ready-mixed concrete is being
3. A given strength and consistence class can be achieved with a
used and when there may be traffic delays and/or long hauls.
reduced cement content. The water/cement ratio is kept
This can be seriously aggravated during hot weather, especially
constant, and with a lower water content the cement content
if the concrete has a high cement content.
can be reduced accordingly. This property should never be used
if the cement content would thereby be reduced below the The amount of retardation can be varied - usually up to about four
minimum specified. to six hours - by altering the dosage, but longer delays can be
obtained for special purposes.
Overdosing may result in retardation and/or a degree of air-
ehtrainment, but does not necessarily increase the workability and While the early strength of concrete is reduced by using a retarder,
therefore may not be of any benefit in fresh concrete. which may affect formwork striking times, the 7- and 28-day
strengths are not likely to be significantly affected.

Accelerating water-reducing Retarded concrete needs careful proportioning to minimise

bleeding due to the longer period during which the concrete
admixtures remains fresh.
Accelerators increase the initial rate of chemical reaction between
the cement and the water so that the concrete stiffens, hardens,

15
 ADMIXTURES
Table 7: Air contents of air-entrained concrete in accordance with
Air-entraining admixtures BS EN 206-1.
These may be organic vinsol resins or synthetic surfactants that
entrain a controlled amount of air in concrete in the form of small Nominal
air bubbles. The bubbles need to be very small, about 0.05 mm maximum Minimum volume Maximum volume of
(50 microns) in diameter and well dispersed. of entrained air
aggregate entrained air
The main reason for using an air-entraining admixture is that the size, Dmax
presence of tiny air bubbles in the hardened concrete increases its 40 mm 2.5% 6.5%
resistance to the action of freezing and thawing, especially when
20 mm 3.5% 7.5%
aggravated by the application of de-icing salts and fluids.
Saturated concrete - as most external paving concrete will be - can 10 mm 5.5% 9.5%
be seriously affected by the freezing of water in the capillary voids,
which will expand and tend to burst it. But if the concrete is air- One factor which has to be taken into account when using air-
entrained, the small air bubbles, which intersect the capillaries and entrained concrete is that the strength of the concrete is reduced,
remain unfilled with water even when the concrete is saturated, by about 5% for every 1% of air entrained. However, the
act as pressure relief valves and cushion the expansive effect by plasticizing effect of the admixture means that the water content
providing voids into which the water can expand as it freezes, of the concrete can be reduced, which will offset most of the
without disrupting the concrete. When the ice melts, surface strength loss which would otherwise occur, but even so some
tension effects draw the water back out of the bubbles. increase in cement content is likely to be required as well.

Air-entrained concrete should be specified and used for all forms of The amount of air entrained for a given dosage can be affected by
external paving, from major roads and airfield runways down to several factors: changes in sand grading, variation in temperature
garage drives and footpaths, which are likely to be subjected to and mixing time. These may call for adjustments to the dosage for
severe freezing and to de-icing salts. These may either be applied uniformity of air content to be maintained. When pfa is present in
directly or come from the spray of passing traffic or by dripping the concrete, a considerably increased dosage of air-entraining
from the underside of vehicles. Figure 4 shows the difference in admixture may be required. The measurement of air content in the
freeze/thaw resistance between air-entrained and non air-entrained fresh concrete is described in BS 1881 : Part 106 and
concrete. BS EN 12350-7. Brief details are given on page 57 under Air
content test.

Superplasticizing/high-range
water-reducing admixtures
These are chemicals that have a very great plasticizing effect on
concrete. They are used for one of two reasons:
1 To increase greatly the consistence of a mix so that 'flowing'
concrete is produced that is easy both to place and to compact;
some are completely self-compacting and free from segregation.
2 To produce high-strength concrete by reducing the water
content to a much greater extent than can be achieved by
using a normal plasticizer (water-reducing admixture).

Flowing concrete is usually obtained by first producing a concrete

in S2 consistence class (50 mm - 90 mm slump) and then adding
the superplasticizer, which will increase the slump to over 200 mm
(see Figures 5 and 6). This high consistence lasts for only a limited
Figure 4: Adjacent slabs of plain (left) and air-entrained concrete period of time; stiffening and hardening then proceed normally.
that have been subjected to freeze/thaw action. Because of this limited duration of increased consistence, when
ready-mixed concrete is used it is usual for the superplasticizer to
The volume of air entrainment relates to maximum aggregate size be added to the concrete on site rather than at the batching or
Dmax (see Table 7). With a reduction in Dmax, the specified air mixing plant.
content increases. Air content should be specified by minimum
value. Thus most heavy-duty concrete pavements, including roads Flowing concrete can be more susceptible to segregation and
and airport runways with their Dmax of 40 mm, require a minimum bleeding, so it is essential for the mix design and proportions to
air content of 2.5%. The specified minimum air content increases take account of the use of a superplasticizer.
to 3.5% for 20 mm and 5.5% for 10 mm Dmax sizes. The
As a general guide, if a conventionally designed mix is modified
maximum permitted air content is generally 4% greater than the
by increasing the sand content by about 5%, satisfactory flowing
minimum.
concrete can be produced by the addition of a superplasticizer. A
Air-entrainment also affects the properties of the fresh concrete. high degree of control over the batching of all the proportions is
The minute air bubbles act like ball bearings and have a essential, especially the water, because if the consistence is not
plasticizing effect, resulting in higher consistence. Concrete that is correct at the time of adding the superplasticizer, excessive flow
lacking in cohesion, are harsh, or which tends to bleed excessively, and segregation will occur.
is greatly improved by air-entrainment. Air-entrainment also
reduces the risk of plastic settlement and plastic shrinkage cracks. The fluidity of flowing concrete is such that little or no vibration is
There is also evidence that uniformity of colour is improved and required. Beams, walls and columns can be compacted manually
surface blemishes reduced. by rodding, although it is desirable to have an immersion vibrator

16
 ADMIXTURES
(poker) available. For slabs, the concrete is more easily moved
using rakes or pushers than by conventional shovels, and the Other admixtures
surface can be finished with a skip float drawn across it. Excessive There are a number of other admixtures that may occasionally be
vibration may cause segregation and bleeding and, accordingly, used for special purposes. These include bonding aids, pumping
some formulations of superplasticizer contain a viscosity modifier aids, expanding agents, damp-proofing and integral waterproofers,
to produce a self-compacting concrete that, in the right mix, is fungicidal admixtures and corrosion inhibitors. For details of these
free from segregation. reference should be made to specialist literature.

Storage of admixtures
Most admixtures are stable, but they may require protection
against freezing, which can permanently damage them, and may
also require stirring. The manufacturer's instructions should be
followed.

Dispensing
Because admixtures are usually added in small quantities, generally
30 -1000 ml per 50 kg cement, accurate and uniform dispensing
is essential. This is best done using manual or automatic dispensers
so that the admixture is thoroughly dissolved in the mixing water
as it is added to the concrete.

Superplasticizers for flowing concrete, however, are usually added

Figure 5: A typical S2 consistence class concrete with a cement just before discharge and the concrete should then be mixed for a
content of 300 kg/m3. further one minute per m but not less than five minutes, in
accordance with BS EN 206-1.

Trail mixes
Preliminary trials are essential to check that the required
modification of the concrete property can be achieved. The use of
an admixture is likely to require some adjustment of the mix
proportions. For example, when using an air-entrained concrete,
the additional lubrication produced by the small air bubbles
permits a reduction in the water content, and it is usually
advantageous at the same time to reduce the sand content by
about 5%. The correct adjustments can be determined only by trial
mixes.

Although the admixture manufacturer's instructions will usually

include recommended dosages, the optimum dosage will often
Figure 6: A similar concrete to that shown in Figure 5, but after depend on the cement type, the mix proportions, the grading of
the addition of a superplasticizer. the fine aggregate and the temperature.

The programme for trial mixes should include some with deliberate
The use of flowing concrete is likely to be restricted to work where double and treble over-dosages to determine the effect on both
the advantages in ease and speed of placing offset the increased the fresh and hardened concrete so that the dangers arising from
cost of the concrete - considerably more than for other mistakes can be appreciated by all concerned
admixtures. Typical examples are where the reinforcement is
particularly congested, making both placing and vibration difficult,
and where large areas such as slabs, would benefit from a flowing, References/further reading
easily placed concrete. BS 1881, Testing concrete : Part 106: 1983, Methods for
determination of air content of fresh concrete. British Standards
The fluidity of flowing concrete increases the pressures on
Institution, London.
formwork, which should be designed to resist full hydrostatic
pressure. Guidance on design pressures is given in CIRIA BS 5328: 1997, Concrete. British Standards Institution, London.
Report 108. BS 8500 : 2002, Complementary British Standard to BS EN 206-1.
British Standards Institution, London.
When used to produce high-strength concrete, a reduction in
BS EN 206-1 : 2000, Concrete - specification, performance,
water content of as much as 30% can be obtained using a
production and conformity. British Standards Institution, London.
superplasticizer, compared with a water reduction of only about
10% when using a normal plasticizer; 1-day and 28-day strengths BS EN 934: 2001, Admixtures for concrete, mortar and grout: Part 2.
can be increased by as much as 50%. Concrete admixtures - definitions, requirements, conformity, making
and labelling. British Standards Institution, London.
High-strength water-reduced concrete containing a superplasticizer
CIRIA Report 108, Concrete pressure on formwork. 1985,
is used both for high performance in-situ concrete construction
Construction Industry Research and Information Association,
and for the manufacture of precast units where the increased early
London.
strength allows earlier demoulding.

17
 CONCRETE PROPERTIES
The properties of concrete are too many and varied to be dealt Table 8: Consistence classes in BS EN 206-1 for slump tests
with fully in this publication: further information is available in conforming to BS EN 12350-2.
specialist textbooks. Therefore, only the main properties of
concrete in the fresh, hardening and hardened states are
considered here. Fire resistance, elasticity and other properties, Slump class Range of slump (mm)
which may be essential in some circumstances, have been omitted.
S1 10- 40
For methods of testing concrete, refer to the section titled Testing
S2 50- 90
concrete and concreting materials on page 52 and to relevant
Standards, in particular BS 1881, BS EN 12350 for testing fresh S3 100-150
concrete and BS EN 12390 for hardened concrete.
S4 160-210

Fresh concrete Three further test methods are recognized in BS EN 206-1, all with
their unique consistence classes. They are the Vebe, degree of
It is essential that the correct level of workability is chosen to
compactability and flow tests conforming to BS EN 12350 : Parts
match the requirements of the construction process. The ease or
3, 4 and 5 respectively. It should be noted that the compactability
difficulty of placing concrete in sections of different sizes, the type
test to BS EN 12350 : Part 4 is totally different from the
of compaction equipment, the complexity of reinforcement, the
compacting factor test to BS 1881: Part 103.
size and skills of the workforce are amongst the items to be
considered. In general, the more difficult it is to work the concrete,
the greater should be the level of workability. But the concrete
must also have some cohesiveness in order to resist segregation
Hardening concrete
and bleeding. Concrete needs to be particularly cohesive if it is to Early thermal cracking
be pumped, for example, or allowed to fall from a great height. The reaction of cement with water - hydration - is a chemical
reaction that produces heat. If this development of heat exceeds
Workability and cohesion cannot be considered in isolation
the rate of heat loss, the temperature of the concrete will rise.
because they are affected by each other: in general, more
Subsequently the concrete will cool and contract. Typical
workable concrete requires extra care to be taken with the mix
temperature histories of some concrete sections are shown in
design if segregation is to be avoided.
Figure 7.
The workability of fresh concrete is increasingly referred to in
If the contraction were unrestrained there would be no cracking.
British and European standards as consistence. It is useful to think
However, in practice there is always some form of restraint
of consistence as a combination of workability with cohesion.
inducing tension and hence a risk of cracking. Restraint occurs due
Although cohesiveness cannot at present be measured, some of
to either external or internal influences.
the test methods indicate whether a concrete is likely to segregate.

The slump test is the best-known method for testing consistence

and the recognized slump classes are listed in Table 8.

(a) Section 1 m, CC 360 kg/m 3 (b) Section 2 m, CC 350 kg/m 3 (c) Section 1 m, CC 225 kg/m 3
80 80 80
FS
Temperature - °C

Temperature - °C

FS
Temperature - °C

60 Concrete 60 Concrete 60
temperature FS
temperature
Concrete
40 40 40 temperature

Ambient temperature Ambient temperature

20 20 20 Ambient temperature"

0 1 2 34 9 16 25 0 1 2 34 9 16 25 0 1 2 34 9 16 25
Time after placing - days (log) Time after placing - days (log) Time after placing - days (log)

(d) Section 0.4 m, CC 450 kg/m 3 (e) Section 0.45 m, CC 370 kg/m 3 (f) Section 0.5 m, CC 225 kg/m 3
80 80 80
Temperature - °C

Temperature - °C

Temperature - °C

60 60 60
Concrete
FS temperature Concrete
40 Concrete 40 40 temperature
temperature FS
FS
20 Ambient
20 20 temperature
Ambient
temperature Ambient temperature

0 1 2 34 9 16 25 0 1 2 34 9 16 25 0 1 2 34 9 16 25
Time after placing - days (log) Time after placing - days (log) Time after placing - days (log)
FS = formwork struck, CC = cement content

Figure 7: Early temperature history of various concrete walls showing section thickness and cement content.

18
 CONCRETE PROPERTIES
External restraint often dictated by the specification requirements for strength and
Concrete is externally restrained if, for example, it is cast onto a durability of the concrete itself.
previously hardened base, such as a wall kicker, or if it is cast
In practice, cracking due to external restraint is best controlled by
between two already hardened sections, such as an infill bay in a
the provision of crack control reinforcement and the spacing of
wall or slab, without the provision of a contraction joint.
contraction joints, which should be determined by the designer. It
Internal restraint should be noted that reinforcement does not prevent crack
The surfaces of an element of concrete will cool faster than the formation, although it does control the widths of cracks, and with
core, producing a temperature differential, and when this enough of the right reinforcement, cracks will be fine enough so as
differential is large, such as in thick sections, cracks may develop at not to cause leakage or affect durability. With very thick sections,
the surface. In general, it has been found that by restricting the with very little external restraint, the temperature differential can
temperature differential to around 20°C between the core and the usually be reduced by insulating, and thereby keeping warm, the
surface, little or no cracking will result. surfaces of the concrete for a few days.

Factors affecting temperature rise

Plastic cracking
The main factors that affect the rise in temperature are discussed
below. There are two types of plastic cracks: plastic settlement cracks,
which may develop in deep sections and often follow the pattern of
Dimensions Thicker sections retain the heat generated, and will the reinforcement; and plastic shrinkage cracks, which are more
have higher peak temperatures and cool down more slowly. While likely to develop on slabs. Both types form while the concrete is still
peak temperatures increase with increasing thickness, above in its plastic state, before it has set or hardened and, depending on
thicknesses of about 1.5 m there is little further increase in the weather conditions, form within about one to six hours after
temperature. the concrete has been placed and compacted. They are often not
noticed until the following day. Both types of crack are related to
Cement or combination content The heat generated is directly the extent to which the fresh concrete bleeds.
related to the cement content. For Portland cement concretes in
sections of 1 m thickness and more, the temperature rise in the Bleeding of concrete
core is likely to be about 14°C for every 100 kg/m 3 of cement. Fresh concrete is a suspension of solids in water, and after it has
Thinner sections will exhibit lower temperature rises than this. been compacted there is a tendency for the solids (both the
aggregates and the cement) to settle. This sedimentation displaces
Cement type Different cement types generate heat at different the water, which is pushed upwards, and, if the process is
rates. The peak temperature and the total amount of heat excessive, the water appears as a layer on the surface. This bleed
produced by hydration depend upon both the fineness and the water may not always be seen, as it may evaporate on hot or windy
chemistry of the cement. As a guide, those cements whose days faster than it rises to the surface. The tendency of a concrete
strength develops most rapidly tend to produce most heat. Sulfate- to bleed is affected by the materials and their proportions. Bleeding
resisting cement generally gives off less heat than CEM I 42,5N can generally be reduced by increasing the cohesiveness of the
and cements that are interground or combined with mineral concrete by one or more of the following means:
additions such as pfa or ggbs are often chosen for massive
construction because they have the lowest heat of hydration. n Increasing the cement content
n Increasing the sand content
Initial temperature of the concrete A higher initial
n Using finer sand
temperature of the concrete results in a greater temperature rise:
for example, concrete placed at 10°C in a 500 mm thick section n Using less water
may have a temperature rise of 30°C, whereas the same concrete n Air-entrainment
placed at 20°C may have a temperature rise of 40°C. n Using a rounded natural sand rather than an angular
crushed one.
Ambient temperature In cooler weather there is likely to be a
greater differential between peak and ambient temperatures, i.e. The rate of bleeding will be influenced by drying conditions,
greater cooling and contraction. During hot weather concrete will especially wind, and bleeding will take place for longer on cold
develop a high peak temperature but the differential may be lower. days. Similarly, due to the slower stiffening rate of the concrete,
concrete containing a retarder has a tendency to bleed for a longer
Type of formwork Steel and CRP formwork will allow the heat period of time and their use will, in general, increase the risk of
generated to be dissipated more quickly than will timber plastic cracking.
formwork, which acts as an insulating layer. Timber formwork
and/or additional fnsulation will reduce the temperature differential Plastic settlement cracks
between the core and the surfaces. Plastic settlement cracks are caused by differential settlement and
are directly related to the amount of bleeding. They tend to occur
Admixtures Retarding water-reducers delay the onset of in deep sections, particularly deep beams, but they may also
hydration and heat generation but do not reduce the total heat develop in columns and walls. This is because the deeper the
generated. Accelerating water-reducers increase the rate of heat section the more sedimentation or settlement that can take place.
evolution and increase the temperature rise. However, cracks will form only where something prevents the
concrete 'solids' from settling freely. The most common cause of
The problem of early thermal cracking is usually confined to slabs
this is the reinforcing steel fixed at the top of deep sections; the
over about 500 mm thick and to walls of all thicknesses. Walls are
concrete will be seen to 'break its back' over this steel and the
particularly susceptible because they are often lightly reinforced in
pattern of cracks will directly reflect the layout of the steel below
the horizontal direction and the timber formwork tends to act as a
(Figure 8).
thermal insulator, thus encouraging a larger temperature rise. The
problem may be reduced by a lower cement content, the use of a Settlement cracks may also occur in trough and waffle slabs
cement with a lower heat of hydration or one containing ggbs or (Figure 9) or at any section where there is a significant change in
pfa. There is a practical and economic limit to these measures, the depth of concrete.

19
 CONCRETE PROPERTIES

Figure 8: Plastic settlement cracks mirror the reinforcement. Figure 10: Plastic shrinkage cracks in a concrete road
so the only alternative is to protect the concrete for the first few
hours with polythene sheeting (Figure 11). This is essential on hot
and/or windy days.

Figure 9: Plastic settlement cracks in trough and waffle floors, at

change of depth of section.

If alterations to the concrete, particularly the use of an air-

entraining or water-reducing admixture, cannot be made due to
contractual or economic reasons, the most effective way of
eliminating plastic settlement cracking is to re-vibrate the concrete
after the cracks have formed. Such re-vibration is acceptable
provided the concrete is still plastic enough to be capable of being Figure 1 1 : Polythene sheeting supported clear of a concrete slab
'fluidized' by a poker, and yet not so stiff that a hole is left when by means of blocks and timber. Note that all the edges of the
the poker is withdrawn. The timing will depend on the weather. polythene are held down to prevent a wind-tunnel effect.

Plastic shrinkage cracks Remedial measures

These cracks occur in horizontal slabs, such as floors and roads. The main danger resulting from plastic cracking is the possible
They usually take the form of one or more diagonal cracks at 0.5 ingress of moisture leading to the corrosion of reinforcement. With
to 2 m centres that do not extend to the slab edges, or they form both plastic settlement and plastic shrinkage cracks, if the affected
a very large pattern of map cracking. Plastic shrinkage cracks such surface will be protected subsequently either by more concrete or
as those shown in Figure 10 do not usually increase in length or by a screed, no treatment is usually necessary.
width with the passage of time and seldom have a detrimental
Often the best repair is simply to brush dry cement (dampened
effect on the load-bearing capability of suspended slabs or on the
down later) or wet grout into the cracks the day after they form
carrying capacity of roads. They may occur in both reinforced and
and while they are still clean; this encourages natural or
non-reinforced slabs.
autogenous healing.
Plastic shrinkage cracks are most common in concrete placed on
hot or windy days because they are caused by the rate of Hardened concrete
evaporation of moisture from the surface exceeding the rate of
bleeding. Compressive strength
The strength of concrete is normally specified by strength class,
It has been found that air-entrainment almost eliminates the risk of
that is the 28-day characteristic compressive strength of specimens
plastic shrinkage cracks developing.
made from the fresh concrete under standardised conditions. The
Clearly, plastic shrinkage cracks can be reduced by preventing the results of strength tests are used routinely for both control of
loss of moisture from the surface of the concrete in the critical first production and contractual conformity purposes.
few hours. While sprayed-on resin-based curing compounds are
Characteristic strength is defined as that level of strength below
very efficient at curing concrete that has already hardened, they
which a specified proportion of all valid test results is expected to
cannot be applied to fresh concrete until the free bleed water has
fall. Unless otherwise stated, this proportion is taken to be 5%.
evaporated. This is too late to prevent plastic shrinkage cracking,

20
 CONCRETE PROPERTIES
Test cubes - either 100 mm or 150 mm - are the specimens Durability of concrete
normally used in the UK and most other European countries, but
Concrete has to be durable and resistant to various environments
cylinders are used elsewhere. Because their shapes are different,
ranging from mild to most severe, including weathering, chemical
the strength test results, even from the same concretes of the same
attack, abrasion, freeze/thaw attack and fire. In addition, for
ages, are also different, cylinders being weaker than cubes. For
reinforced and prestressed concrete, the cover concrete must
normal-weight aggregates, cylinders are about 80% as strong as
provide protection against the ingress of moisture and air, which
cubes, whereas cylinders made from lightweight aggregates have
would eventually cause corrosion of the embedded steel.
90% of the corresponding cube strength.
The strength of the concrete alone is not necessarily a reliable
Accordingly the strength classes recognized in BS EN 206-1 /
guide to the durability of concrete; many other factors also have to
BS 8500 are classified in terms of both values, with the cylinder
be taken into account. Of all the factors influencing the durability
strength followed by the cube strength. The standard compressive
of concrete the most important is that of impermeability. The
strength classes are listed in Table 9.
degree of impermeability is mainly dependent on:

Table 9: Concrete compressive strength classes taken from n Constituents of the concrete, and in particular the free
BS EN 206-1. water/cement ratio
n Compaction, to eliminate air voids
Concrete compressive strength classes n Curing, to ensure continuing hydration.
Concrete made with Concrete made with Constituents
normal-weight aggregates lightweight aggregates Concrete has a tendency to be permeable due to the presence of
capillary voids in the cement paste matrix. In order to obtain
C8/10 LC8/9
workable concrete it is usually necessary to use far more water than
C12/15 LC12/13
is actually necessary for hydration of the cement; this excess water
C16/20 LC16/18
occupies space and when later the concrete dries out capillary
C20/25 LC20/22 voids are left behind. Provided the concrete has been fully
C25/30 LC25/28 compacted and properly cured these voids are extremely small and
C28/35 LC30/33 their number and size decrease as the free water/cement ratio is
C30/37 LC35/38 reduced. At high free water/cement ratio the particles of cement
C32/40 LC40/44 along with their hydration products will tend to be spaced widely
C35/45 LC45/50 apart (Figure 12c) and the capillaries will be greater compared
C40/50 LC50/55 with a mix at a lower free water/cement ratio (Figures 12b and
12a). The more open the structure of the paste, the more easily it
C45/55 LC55/60
will permit the ingress of air, moisture and harmful chemicals. It
C50/60 LC60/66
will also be very sensitive to the drying regime, both at early ages
C55/ 67 LC70/77
and in the more mature hardened state.
C60/75 LC80/88
C70/85
C80/95
C90/105
C100/115

In principle, the compressive strength may be determined from

cores cut from the hardened concrete. Core tests are normally
made only when there is some doubt about the quality of concrete
placed, for example, if cube strengths have been unsatisfactory - (a) Free water/cement ratio 0.30
or to assist in determining the strength and quality of an existing
structure for which records are not available.

Great care needs to be taken in the interpretation of the results of

core testing: core samples drilled from the in-situ concrete are
expected to be lower in strength than the cubes made, cured and
tested under standard laboratory conditions.

For more information see Test cores on page 58. The standard
reference for core testing is BS EN 12504-1 and a useful guide is
(b) Free water/cement ratio 0.50
given in Concrete Society Digest No. 9

Flexural and indirect tensile strength

The tensile strength of concrete is generally taken to be about one-
tenth of its compressive strength, but different aggregates cause
this proportion to vary and a compressive test is therefore only a
very general guide to the tensile strength

The indirect tensile strength (cylinder splitting) is seldom specified

nowadays. Flexural testing of specimens, to measure the modulus
(c) Free water/cement ratio 0.80
of rupture, may be used on some airfield runway contracts where
the method of design is based on the modulus of rupture, and for Figure 12: The effect of initial cement particle spacing upon the
some precast products such as flags and kerbs. permeability of concrete.

21
 CONCRETE PROPERTIES
Table 10: Typical relationships between free water/cement ratio, aggregate type, consistence class and Portland cement content.

Free w / c Type of Consistence/slump class

ratio aggregate
Low Medium High
S 1 ( 1 0 - 40 mm) S2 (50 - 90 mm) S3 ( 1 0 0 - 1 5 0 mm)

Free water Cement Free water Cement Free water Cement

demand content demand content demand content
(litres/m 3 ) (kg/m 3 ) (litres/m 3 ) (kg/m 3 ) (litres/m 3 ) (kg/m 3 )

Uncrushed 160 230 180 260 195 280

0.7
Crushed 190 270 210 300 225 325

Uncrushed 160 265 180 300 195 325

0.6
Crushed 190 315 210 350 225 375

Uncrushed 160 320 180 360 195 390

0.5
Crushed 190 380 210 420 225 450

Uncrushed 160 400 180 450 195 490

0.4
Crushed 190 475 210 525 225 565

NOTES
1. 20 mm maximum aggregate size.
2. Uncrushed - natural gravels and natural sands.
Crushed - crushed gravel or rock and crushed sand.
. . water demand
3. For a given consistence class, cement content = free w/c ratio
4. Where concrete contains a water-reducing admixture the relationship will be different.
S. Actual free water demands may vary from the above values by ±10 litres/m3 and corresponding adjustments to the cement
contents may be required.

Table 1 1 : Exposure classes.

Class Class description

designation

XO Concrete without reinforcement or embedded metal

Concrete with reinforcement or embedded metal in very dry conditions
All exposures with no freeze/thaw, abrasion or chemical attack
xc Corrosion induced by carboration
XC1 Dry or permanetly wet
XC2 Wet, rarely dry
XC3 Moderate humidity or cyclic wet and dry
xs Corrosion induced by chloride from seawater
XS1 Exposure to airborne salt but not in direct contact with seawater
XS2 Permanently submerged
XS3 Tidal, splash and spray zones
XD Corrosion induced by chloride other than seawater
XD1 Moderate humidity
XD2 Wet, rarely dry
XD3 Cyclic wet and dry
XF Freeze/thaw attack
XF1 Moderate water saturation without de-icing agent
XF2 Moderate water saturation with de-icing agent
XF3 High water saturation without de-icing agent
XF4 High water saturation with de-icing agent
XA Chemical attack. Sulfate classification is given in BS 8500

Although free water/cement ratio is the main factor affecting Exposure Classes
impermeability, and hence durability, it cannot easily be measured It is recommended that exposure classes be given 'X' codes,
either in the fresh or hardened concrete. However, for a particular ranging from XO for mild exposure through the following codes for
aggregate type and grading, the water demand for the same exposure to different causes of deterioration:
consistence class is more or less constant and is independent of n XA for exposure to chemical attack
the cement content. Therefore, by knowing the water demand for
n XC for risk of corrosion induced by carbonation
a particular consistence class the cement content can be evaluated
for the required and specified free water/cement ratio. This is n XS for exposure to the sea and sea spray
illustrated in Table 10. n XD for exposure to chlorides from sources other than the sea
n XF for risk of freeze/thaw attack (with and without salt present.

22
 CONCRETE PROPERTIES
Each group (apart from X0, mild exposure class) has a ranking Resistance to freezing and thawing
system from 1 to 3 or 4 depending on the severity of the The freeze/thaw resistance of concrete depends on its
exposure. The exposure classes and their descriptions are listed in impermeability and the degree of saturation when exposed to
Table 11. Guidance on limiting values recommended as being frost; concrete with a higher degree of saturation is more liable to
suitable for resisting these exposure classes is given in BS 8500. damage. The use of salt for de-icing roads greatly increases the risk
of freeze/thaw damage.
With increasing severity of exposure the free water/cement ratio
needs to be decreased since durability is related to the concrete's The benefits of air-entrained concrete have been referred to on
impermeability. It should also be noted that requirements for page 16 under Air-entraining admixtures where it was
exposure classes tend to include requirements for lowest strengths recommended that all exposed horizontal paved areas, from
of concrete. In the past, specified strengths tended to be lower motorways to garage drives, footpaths and marine structures,
than the minimum recommended for durability because the earlier should be air-entrained. Alternatively, the strength of concrete
specifications were largely related to structural rather than should be SO N/mm2 or more. Whilst C50 concrete is suitable for
durability requirements. many situations, it does not have the same freeze/thaw resistance
as air-entrained concrete. Similarly, those parts of structures
Compaction adjacent to highways and in car parks, likely to be splashed or
In addition to the capillary voids (pores), which are dependent on come into contact with salt or salt solution used for de-icing,
the water/cement ratio, air pockets or voids and even large cavities should also be air-entrained.
or 'honeycombing' may also be present if the concrete has not
been fully compacted. Concrete that has not been properly Particular care needs to be taken to ensure that the concrete is
compacted because of bad workmanship or because the mix properly cured (see the section on Curing on page 45).
design made compaction difficult can result in a porous concrete,
which may, for example, allow water seepage as well as easy
Resistance to chemical attack
Portland cement concrete is attacked by acids and by acid fumes,
ingress of air and chemicals harmful to concrete. Well-compacted
including organic acids, which are often produced when foodstuffs
concrete should not contain more than 1 % of entrapped air.
are being processed. Vinegar, fruit juices, silage effluent, sour milk
This subject is considered in more detail in the section entitled and sugar solutions all attack concrete. In general, concrete made
Placing and compaction on page 34. with Portland cement is not recommended for use in acidic
conditions where the pH is 5.5 or less without careful consideration
Curing of the type of exposure and the intended construction. Alkalis have
The importance of curing in relation to durability is seldom fully little effect on concrete.
appreciated. It is essential that proper curing techniques are used
to reduce the permeability of concrete by ensuring the continued For construction exposed to made-up ground, including
hydration process. The formation of the reaction products, which contaminated and/or industrial material, specialist advice should be
fill up the capillary voids, ceases when the concrete dries to below sought so that the Design Chemical (DC) class can be correctly
80% relative humidity. determined and a suitable concrete specified.

Detailed information is given in Table 17 on page 46, from which The most common form of chemical attack that concrete has to
it should be noted that longer curing periods are required when resist is the effect of solutions of sulfates that may be present in
cements containing additions are used. some soils and groundwaters.

Cover In all cases where concrete is subject to chemical attack, resistance

Many defects in reinforced concrete are the result of insufficient is related to the free water/cement ratio, cement content, the type
cover, leading to reinforcement corrosion. Too often, not enough of cement and the degree of compaction. Well-compacted
care is given to the fixing of reinforcement to ensure that the concrete will always be more resistant to sulfate attack than one
specified minimum cover is achieved. The position of the which is less well compacted, regardless of cement type.
reinforcement, and its cover, should be checked before and during
BS 5328 and BS 8500 incorporate a primary set of
concreting, and may need to be checked after the concrete has
recommendations specific to concrete exposed to sulfate-
hardened. Further information about cover is given in the section
containing groundwater and chemically-contaminated brownfield
titled Reinforcement on page 4 1 .
sites.
Carbonation
Alkali-silica reaction
Reinforcement embedded in good concrete with an adequate
Alkali-silica reaction (ASR) in concrete is a reaction between certain
depth of cover is protected against corrosion by the highly alkaline
siliceous constituents in the aggregate and the alkalis - sodium and
pore water in the hardened cement paste. Loss of alkalinity of the
potassium hydroxide - that are released during the hydration of
concrete can be caused by the carbon dioxide in the air reacting
cement. A gelatinous product is formed, which imbibes pore fluid
with and neutralising the free lime. This reaction is called
and in so doing expands, inducing an internal stress within the
carbonation and if it reaches the reinforcement, corrosion will
concrete. The reaction will cause damage to the concrete only
occur in moist environments.
when the following three conditions occur simultaneously:
Carbonation is a slow process progressing from the surface and n A reactive form of silica is present in the aggregate in critical
dependent on the permeability of the concrete and the humidity quantities
of the environment. Provided that the depth of cover and quality
n The pore solution contains sodium, potassium and hydroxyl
of concrete are correctly specified and achieved to suit the
ions and is of a sufficiently high alkalinity
exposure conditions, corrosion due to carbonation should not
occur during the lifetime of the structure. n Water is available.
If any one of these factors is absent, then damage from ASR will
not occur and no precautions need be taken.

23
CONCRETE PROPERTIES CONCRETE SPECIFICATION
It is possible for the reaction to take place in the concrete without Two essential properties of hardened concrete are durability and
inducing expansion. Damage may not occur, even when the strength. Both properties are affected by the voids and capillaries
reaction product is spread throughout the concrete, and the gel in the concrete, which are caused by excessive water or by
may fill cracks induced by some other mechanism. incomplete compaction.

Recommendations are available for minimizing the risk of damage In principle the lower the free water/cement ratio the stronger and
from ASR in new concrete construction, based on ensuring that at more durable the concrete will be. The concrete should be fully
least one of the three factors listed above is absent. compacted if it is to retain or exclude water and provide corrosion
protection to reinforcement.

Reference/further reading Within the UK, the producer is normally required to take action to
prevent damaging alkali-silica reaction and therefore provisions in
BRE Digest 330, Alkali-silica reaction in concrete. 1999, Construction
the specification are not normally required.
Research Communications, London.
CSTR30, Alkali-silica reaction: minimising the risk of damage to The required consistence needs to be known at the time of
concrete. 1999, The Concrete Society, Crowthorne. 72 pp. specification so that the concrete can be proportioned to give the
BRE Special Digest 1, Concrete in aggressive ground. 2000, required strength and durability. High-strength concretes can be
Construction Research Communications, London. designed and proportioned to a very high or self-compacting
consistence so overcoming conditions that make placing or
CS020, Digest No. 9. Concrete core testing for strength. 1988, The
vibration difficult.
Concrete Society, Crowthorne. 8 pp.
CSTR 11, Core testing for strength. 1987, The Concrete Society, The methods of specification and what to specify are given in
Crowthorne. 44 pp. BS 8500-1. Three types of concrete - designed, prescribed and
standardized prescribed concretes - are recognized by BS EN
CSTR 22, Non-structural cracks in concrete. 1992, The Concrete
206-1, but BS 8500 adds two more: designated and proprietary
Society, Crowthorne. 48 pp.
concretes.
CSTR 44, The relevance of cracking in concrete to corrosion of
reinforcement. 1995, The Concrete Society, Crowthorne. 32 pp.
Designed concretes
Plastic cracking of concrete. 1991, British Cement Association
These are concretes for which the producer is responsible for
Crowthorne. Ref. 45.038. 4 pp.
selecting the mix proportions to meet the required performance as
Minimum requirements for durable concrete. Edited by D W Hobbs. communicated by the specifier. Therefore it is essential that the
1998, British Cement Association, Crowthorne. Ref. 45.034. 172 pp. specifier, in compiling the specification, takes account of:
BS 5328: 1997, Concrete. British Standards Institution, London. n The uses of the fresh and hardened concrete
BS 8500 : 2002, Complementary British Standard to BS EN 206-1. n The curing conditions
British Standards Institution, London.
n The dimensions of the structure; this affects heat development
BS EN 12504, Testing concrete in structures. British Standards
n The environmental exposure conditions
Institution, London.
Part 1 : 2000, Cored specimens - Taking, examining and testing in n Surface finish
compression. n Maximum nominal aggregate size
n Restrictions on suitability of materials.
The most common form of designed concrete is that defined by
the characteristic compressive strength at 28 days and identified by
the strength class. For example, strength class C25/30 concrete is
one having a characteristic compressive cube strength of
30 N/mm2 at 28 days. (The same concrete would have a
characteristic cylinder strength of 25 N/mm2 at 28 days if cylinders
were used for testing, as in certain European countries.) To
understand the meaning of the term 'characteristic' see Strength on
page 26.

However, strength alone does not necessarily define the required

durability, and for structural concrete BS 8500 indicates minimum
strength class, the maximum free water/cement ratio and
minimum cement content that are required for different degrees of
exposure. The maximum free water/cement ratio, minimum
cement content and types of constituent materials are the main
factors influencing durability.

If a specification for designed concrete is to be compiled correctly

the following details need to be included:

n A requirement to conform to BS 5328, or BS EN 206-1 and

BS 8500-2
n The compressive strength class
n The limiting values of composition e.g. maximum free
water/cement ratio, minimum cement content or the design
chemical class where appropriate

24
 CONCRETE SPECIFICATION
n Type of cement or combination Standardized prescribed
n The maximum aggregate size concretes
n The chloride class
Standard mixes conforming to BS 5328 are now called stand-
n The consistence class.
ardized prescribed concretes and are described in BS 8500 : Part 2.
Optional items may be included such as the target density of
lightweight concrete, heat development or other technical Whilst strength testing is not intended to be used to judge
requirements listed in BS 8500 : Part 1. conformity for standard or standardized prescribed concrete, the
characteristic compressive strength, as shown in Table 12, may be
At the time of publication, the use of Form A in BS 5328 : Part 2 is assumed for the purposes of design.
recommended when specifying designed concretes. A copy is
reproduced in this publication in Appendix 1 a, which can be used For these concretes it is necessary to specify:
for this purpose by ringing the appropriate items. n The mix title (ST1, ST2, ST3, ST4 or ST5)

Conformity of designed concretes is usually determined by strength n The class of concrete as reinforced or unreinforced
testing of 100 mm or 150 mm cubes and in BS 8500 this is the n The maximum aggregate size
responsibilty of the producer. Recommendations about the required n The consistence class.
rate of sampling are given in BS 5328 and BS EN 2 0 6 - 1 .
Admixtures are not permitted in standard mixes but are permitted
The producer will respond to the specification by producing a mix in standardized prescribed concretes and, whilst numerous cement
design that satisfies all of the specified requirements. Mix design types are permitted, it is not intended that properties normally
methods are described in several publications and the subject will associated with some of those cements - such as low heat or
not be dealt with in any great detail here. sulfate resistance, for example - will be produced in the concrete.

Prescribed concretes The concrete producer is responsible for ensuring that the
materials used conform to those specified and that the batched
These are concretes where the specification gives the mix
weights are based on the proportions given in the appropriate
proportions in kilograms of each constituent in order to satisfy
standard. A guide to the correct selection of standard/standardized
particular performance requirements. Such concretes seldom need
prescribed concretes is reproduced in Table 12.
to be used but may be required for special surface finishes or where
particular properties are required. The specifier should include Conformity with the specification for standard mixes and
details of the cement content, the type and strength class of standardized prescribed concretes is judged against supply of
cement and either the free water/cement ratio or consistence class. concrete with the correct materials and proportions as defined in
BS 5328 : Part 2, or BS 8500 : Part 2. Strength testing does not
At the time of publication, the use of Form B in BS 5328 : Part 2 is
form part of the assessment of standard mixes, or standardized
recommended when specifying prescribed concretes. A copy is
prescribed concretes. Because the proportions of standardized
reproduced in this publication in Appendix 1 b, which can be used
prescribed concretes have been selected to take into account
for this purpose by ringing the appropriate items. Methods for
different types of aggregate and variations in cement strengths,
checking conformity of prescribed concretes are given under
cube compressive strengths would be likely to exceed by as much
Checking conformity on page 26.
as 12 N/mm2 the assumed characteristic strengths associated with

Table 12: Guide to the selection of standard/standardized prescribed concrete in housing and general applications.

Standard/ Assumed Application Recommended

standardized characteristic in conditions where Design Chemical class 1 concrete is appropriate consistence
prescribed cube strength class
concrete title (N/mm 2 )

ST1 8 Kerb bedding and backing S1

ST2 10 Pipe bedding and drainage works to give immediate support S1

Other drainage works S3
Strip footings S3
Mass concrete foundations S3
Trench fill foundations S4
Blinding and mass concrete fill S3
Oversite below suspended slabs S3
House floors with no embedded metal - permanent finish (e.g. screed) to
be added S2

ST3 15 House floors with no embedded metal - direct finish S2

ST4 20 Garage floors with no embedded metal S2

Wearing surfaces - light foot and trolley traffic S2

ST5 25 House floors with embedded metal S2

NOTES
1. See Resistance to chemical attack on page 23.
2. See BS 8500 for details of Design Chemical classes.
3. Concrete containing embedded metal should be regarded as reinforced.

25
 CONCRETE SPECIFICATION
the respective strength classes. This compensates for the lack of At the time of publication, the use of Form D in BS 5328 : Part 2 is
strength testing and the fact that standardized prescribed concretes recommended when specifying designated concretes. A copy is
are intended for site production with basic equipment and control. reproduced in Appendix 1d, which can be used for this purpose by
ringing the appropriate items.
At the time of publication, the use of Form C in BS 5328 : Part 2 is
recommended when specifying standard concretes. A copy is
reproduced in Appendix 1c, which can be used for this purpose by Proprietary concretes
ringing the appropriate items.
A new sub-group of concrete is proposed for UK practice to
Checking conformity provide for those instances when a concrete producer would give
BS 5328 and BS EN 206-1 / BS 8500 give options for checking the assurance of the performance of concrete without being required
conformity of prescribed, standard and standardized prescribed to declare its composition. This class of concrete is termed
concretes, indicating that they may be assessed by one of the proprietary concrete. However, because the producer is
following methods: nominated, specification of these concretes may be unsuitable for
use by public authorities.
n Observation of the batching
n Examination of the records of batch weights used
n Analysis of the fresh concrete in accordance with procedures Strength
defined in British Standards. The strength of concrete is usually defined by the crushing
strength of 100 mm or 150 mm cubes at an age of 28 days.
However, other types and ages of test and other sizes and shapes
Designated concretes of specimen are sometimes used.
This group of wide-ranging concretes provides for almost every
type of concrete construction. They have developed from the Test procedures are described under Testing of hardened concrete
original designated mixes introduced in 1991 to BS 5328 and, on pages 57 - 59. The strength of a concrete will usually be
being specific to the UK, are perpetuated in BS 8500. Divided into specified as a characteristic strength. This is the strength below
four sub-groups, designated concretes are deemed to be fit for the which not more than a stated proportion of the concrete falls. In
following specific purposes: BS 5328 and BS EN 206-1 this proportion is defined as 5% (1 in
20). To protect the user, an absolute minimum strength of any
1. General purpose low grade applications (GEN concretes)
batch is specified.
2. For use as foundations in sulfate-bearing ground conditions
(FND concretes) The variability in results needs to be considered statistically, and a
detailed discussion on this subject is outside the scope of this
3. Air-entrained concretes for pavement quality concrete (PAV
publication. However, it is briefly mentioned to clarify the
concretes)
consideration of concrete strengths.
4. Normal structural classes for reinforced concrete applications
(RC concretes). Because of the variability of test results, and the inherent variability
of the constituent materials, the concrete must be designed to
Full specifications for all designated concretes are given in BS 8500 : have a mean strength high enough above the characteristic
Part 2. The use of the designation e.g. RC35, is an instruction to the strength to ensure that not more than the expected percentage of
producer to conform to the specification in BS 8500 : Part 2. results fall below the characteristic strength. The difference
between this 'target mean' and characteristic strength is known as
The producer of designated concretes must operate a recognized
the 'margin'. The spread of results from concrete strength tests has
accredited, third-party certification system, and ensure that the
been found to follow what is known, in statistics, as a 'normal'
concrete conforms to the specification given in BS 8500 : Part 2,
distribution, which enables it to be defined by the 'standard
including:
deviation1 of the results. The standard deviation is a measure of the
n Characteristic strength control that has been exercised over the production of the
n Minimum cement content concrete. Where the spread of results and the standard deviation
n Maximum free water/cement ratio. are large, the margin also must be large, but where control over
materials, mixing and testing procedures is good, the standard
For these concretes it is assumed that the nominal maximum deviation will be smaller and the margin may be reduced, leading
aggregate size will be 20 mm; it is necessary simply to state that to economies in materials. In practice, the margin will usually be
the concrete is required to conform to BS EN 206-1 / BS 8500 : about 7 - 1 2 N/mm2.
Part 2 and to specify the designation. The consistence class is
selected by the user of the concrete and this information is passed To use this statistical method reliably for judging conformity to the
to the specifier for inclusion in the specification. specification, a large number of test results is needed. Yet
conformity is commonly judged by examining the results of
Aggregate sizes other than 20 mm may be specified, of course, but smaller numbers of results as outlined below.
this detail would be given along with any further additional
requirements such as the use of fibres or a higher than normal air
BS 5328 conformity rules
content to allow for any loss of air during pumping, for example.
BS 8500 : Part 1 lists the options that may be exercised by specifiers In BS 5328, groups of four test results are used, each result being
for these special cases. the average of two results of cube tests on concrete from the
same batch.
Because designated concretes are quality-assured, there is no
necessity for the purchaser of the concrete to make test cubes. For the highest classes of concrete (C20 and higher) to meet the
Product conformity is ensured through accredited third-party BS 5328 specification requirements the average strength of a
inspection of the quality procedures. group of four consecutive test results must exceed the
characteristic strength by 3 N/mm2, and the strength of any

26
 CONCRETE SPECIFICATION
individual result must not be less than the characteristic strength considerations applicable to air-entrained concrete are discussed on
minus 3 N/mm2. page 16, under Air-entraining admixtures.

In the case of the lowest classes (C15 and below) the BS 5328 Aggregate particles that have an angular shape or a rough texture,
specification requirements are deemed to have been met when the such as crushed stone, give greater strength for a given free
average strength of a group of four consecutive test results exceeds water/cement ratio but need more water than smooth and rounded
the specified characteristic strength by 2 N/mm2 and the strength particles to produce concrete of the same consistence. With smaller
of any individual test result is not less than the characteristic sized aggregates, the amount of sand needed to fill the voids
strength minus 2 N/mm2. increases with a corresponding increase in water demand. To
maintain the free water/cement ratio necessary for strength and
Additionally, for all strength classes, the rules for the very first sets durability, at the specified consistence, more cement and/or
of test results for a particular concrete on a new project permit the admixture is necessary.
average of the first two and the first three test results to be lower
than the requirements for the mean of four by 2 N/mm2 and The sand and coarse aggregates need to be proportioned to produce
1 N/mm2 respectively. a stable, cohesive mix at the required consistence with the minimum
amount of water. Badly proportioned constituents require an
EN 206-1 conformity rules excessive amount of water to achieve the required slump, and this
will result in concrete of lower strength and durability, as well as
During the initial stages of production, that is, until at least 35 test
resulting in a mix prone to segregation.
results have been obtained, the results are assessed in overlapping
or non-overlapping groups of three results. The mean strength of
each group of three test results must be not less than 4 N/mm2 Water
greater than the specified characteristic strength whilst the Water quality is the most consistent of the constituents of concrete
occasional individual test result is permitted to be 4 N/mm2 less but water quantity, as it affects the free water/cement ratio, is most
than the specified characteristic strength. important for control of consistence, strength and durability. The
amount of water used should be the minimum necessary to ensure
After 35 test results have been generated within a period of not thorough compaction of the concrete. When deciding how much
more than 12 months the initial production period is over and water is required, allowance must be made for absorption by dry or
continuous production is achieved. The standard deviation is porous aggregates and for the free surface moisture of wet
calculated, the test results are assessed in groups of at least 15 and aggregates, as explained under Storage of aggregates on page 1 3
the minimum requirement is that the mean strength of each group and Water on page 14.
of results must be not less than the specified characteristic strength
plus 1.48 x standard deviation. As with the initial production
period, the occasional individual test result is permitted to be
Admixtures
4 N/mm2 less than the specified characteristic strength. Admixtures have been described in the section on Admixtures. All
admixtures are batched in small quantities and need great care in
Conformity may be established using individual concretes or dispensing and mixing to ensure dispersion through the mix.
defined concrete families. The 'members' of each family would
typically be concretes that use the same type and strength class of
cement from a single source, their aggregates would be
Trail mixes
demonstrably similar and they would all either contain an It may be necessary to establish that the proposed mix proportions,
admixture or not contain one. Test results are collected over the full including cement content, will produce concrete of the required fresh
range of consistence classes and a limited range of strength classes, and hardened properties, or satisfy a requirement to meet a
enabling statistical evaluation to be made in determining whether a maximum free water/cement ratio. This can be achieved either from
concrete remains within its family or must be removed from it. examination of previous data or by the use of trial mixes.

During any contract the materials will vary, and by keeping It should be noted that, when ready-mixed concrete is supplied with
continuous records of test results it is possible to vary the margin so third-party certification, trial mixes by the producer are not needed.
as to make the best use of the materials while conforming to the Purchasers normally receive certificates for the intended mix designs.
specification. Any changes that are made must not conflict with the
specific limiting values. The cement content, for examples, must
not be reduced below the specified minimum figure.
References/further reading
BS 5328, Concrete. British Standards Institution, London.
Part 2 : 1997, Methods for specifying concrete mixes.
Effect of concrete constituents Part 4: 1990, Specification for the procedures to be used in
Cement sampling, testing and assessing compliance of concrete.
The effects of different types of cement have already been BS 8500 : 2002, Concrete - complementary British Standard to
described in the section on Cements. Within one type the properties BS EN 206-1 : 2000, British Standards Institution, London
will vary, but if the supply is derived from one works only, this Part 1 : Method of specification and guidance for the specifier.
variation will be small. Part 2 : Specification for constituent materials and concrete.

Aggregates BS EN 206-1 : 2000, Concrete - specification, performance, production

and conformity. British Standards Institution, London.
The overall grading of the aggregate affects the amount of water
that must be added because, in simplified terms, 'fine' gradings BRE Report 331, Design of normal concrete mixes. 2nd edn. 1997.
require more water than 'coarse' gradings to obtain the same Construction Research Communications, London.
degree of consistence. The aim is to combine the different sizes of
aggregate in such a way as to achieve the optimum packing of the
particles and so reduce voids to a minimum. The special

27
 READY-MIXED CONCRETE
About three-quarters of all concrete placed on site in the UK is enquiry. If it is not known, it is recommended that a high
supplied ready-mixed. To ensure ready-mixed concrete is used consistence class should be specified (S3), see Table 8, page 1 7.
successfully, it is essential that there is close liaison and co-
Many specifications will also state maximum free water/cement
operation between the main contractor and the concrete supplier
ratios and minimum cement contents required for durability
at all stages, from quotation and ordering to discharging the
purposes, and it is essential that the supplier is notified about these
concrete. The use of ready-mixed rather than site-mixed concrete
requirements.
allows for wide variations in demand.
The benefits of using designated concretes include a simple
It is recommended that ready-mixed concrete should be supplied
specification process and an assurance that the concrete conforms
from a plant that holds current accredited third-party certification,
to British Standard requirements.
ensuring that sound practices are followed and systems are in
place to maintain high standards of quality and production Additional requirements should also be given to the supplier when
control. the concrete is for a high-quality surface finish or has to be
pumped, because modifications to the concrete proportions may
be needed for what might otherwise be a satisfactory concrete for
Batching plants general purposes.
There are two basic types of batching plant:
Quotations submitted will usually be accompanied by the supplier's
mix design form. This should be carefully checked to ensure
'Dry batch' plants compliance with the contract specification and that the
The cement and aggregates are weighed and discharged into the proportions are suitable for the intended use and placing
waiting truck-mixer along with most, if not all, of the mixing conditions. When high quality finishes are required, particular care
water, plus any admixture. The concrete is mixed in the truck- needs to be taken in assessing the proposed mix design to ensure
mixer drum and any additional water required to obtain the that the cement content and aggregate proportions and gradings
specified consistence may be added either at the plant or, in the are in accordance with the basic requirements as indicated under
case of high consistence concrete, on site. Concrete for high quality finishes (page 49).
Thorough mixing is essential to ensure concrete of uniform quality. When a maximum free water/cement ratio is specified, it is
In transit the mixer drum may rotate slowly at about one or two essential to check that the amount of water in the design
revolutions per minute to keep the concrete turning over. When represents a realistic amount appropriate to the consistence
the truck-mixer arrives on site, the drum should always be rotated required.
at between 10 and 15 revolutions per minute for at least three
minutes and sometimes longer, to ensure thorough mixing before On all jobs the contractor and the supplier need to establish a
discharge. formal communication system and to discuss the planning and
ordering procedures in good time before delivery of concrete. This
Central mixing plants is best done by the contractor nominating one person to be
directly responsible for ordering the concrete on a day-to-day basis
The cement, aggregates and water plus any admixture are mixed
and for making sure that all is ready on site for the delivery.
in a central mixing plant before discharge into the truck mixer,
which is then used as an agitator. In transit the mixer drum may
rotate slowly at about one or two revolutions per minute to keep Day-to-day ordering
the concrete turning over. When the truck-mixer arrives on the site Details of all the concrete to be used on site should always be
the drum should always be rotated at between 10 and 15 given to the supplier well in advance. This will help to ensure that
revolutions per minute for at least three minutes and sometimes when individual loads are ordered, the supplier's dispatch clerk
longer, to ensure thorough mixing before discharge. (shipper) will know precisely what is wanted. Orders should be
placed at least 24 hours before delivery is required; large pours
involving several hundred cubic metres require much longer notice
Exchange of information so that the supplier can organize and plan accordingly. When
Full details of the concrete specification must be submitted by the making an order by 'phone, the information given should include
contractor at the earliest stage, i.e. when quotations are being the following items:
sought from the supplier. n Name of the purchaser

When several different concrete mixes are used on one contract n Name and location of site and order reference number if there
the essential items to specify for the different types of concrete is one
(designated, designed, prescribed, standard or standardized n Mix reference: each concrete should be given an unambiguous
prescribed) are outlined on pages 24 - 26 and are fully described in reference that is linked to the full set of specified requirements,
BS 5328 / BS 8500. designated, designed, prescribed, standard or standardized
prescribed concretes are simply referred to by their BS titles.
In addition, the contractor should specify the consistence required Where prescribed concretes are required, this should be clearly
for each concrete to suit the proposed placing and compacting indicated
techniques. Some concretes of the same strength may need to be
n Consistence class
supplied at different consistence classes to suit the particular
construction. For example, RC30 concrete placed in a sloping n The total amount of concrete of each type required to be
ramp may be required at a slump of 40 mm (consistence class S1) delivered
whereas the same strength class in a narrow wall may need a n The time at which deliveries are required
slump of 120 mm (consistence class S3). As the consistence will
n The rate at which deliveries are required and particular
affect the cement content of designated concretes it is essential
requirements for continuity of any pour.
that the consistence required on site is given at the time of the

28
 READY- MIXED CONCRETE
Provision of access Discharge
The route(s) from the site entrance to the point(s) of discharge The truck-mixer can discharge at a rate of about 0.5 m3 per
need to be planned in advance. A fully loaded six-wheeled truck- minute. While it may not always be possible to handle the concrete
mixer weighs 26 tonnes and eight-wheelers weigh 32 tonnes, so as fast as this due to limitations of placing and compaction rates, it
access roads must be strong enough to carry the load, even in wet is to the advantage of the site and the supplier for the concrete to
conditions. be discharged as quickly as possible - delays longer than
30 minutes from arrival on site to completion of discharge may be
In many cases truck-mixers have to reverse into position to charged for.
discharge so an adequate turning space on firm ground may be
needed near to the discharge point; a turning circle of about 1 8 m For construction at or below ground level, the quickest and most
is necessary for a typical truck. To avoid contamination of the site, efficient way to discharge concrete is directly from the truck. The
an area should be designated for hosing down chutes and cleaning maximum discharge height for the chute is about 1.5 m above the
wheels. ground and, with extensions, chutes can cover a radius of about
3 m from the back of the truck. If discharging into trenches or pits,
it is essential that the excavation sides are properly shored to
Delivery prevent collapse from the weight of the vehicle. If the concrete is
Before discharging any batch of concrete the delivery ticket should to be placed by crane and skips, a lot of time can be saved by
be checked to confirm that the concrete is of the correct class and using two skips; the empty one can be filled while the other is
conforms to what was ordered. This checking is best done by the in use.
contractor's authorised and nominated representative who also is
Samples of ready-mixed concrete for compressive strength tests
responsible for the ordering.
should be representative of the whole load, with increments being
If all the details are correct the driver should be instructed to remix taken from different parts of the discharge. Cubes must be made
the load to ensure uniformity - for at least three minutes when the from incremental samples, not from soot samples.
concrete has been plant-mixed, or for at least 4 - 5 minutes
(40 - 60 drum revolutions) when any water is added on site.

Concrete mixed at a depot, either in a central mixer or in a truck-

mixer, should arrive on site with the ordered consistence, and no
extra water should need to be added. Some suppliers using dry
batching plants add a quantity of water when the truck arrives on
site. It is then the driver's responsibility to add only the amount of
water as instructed, to achieve the specified consistence as shown
on the delivery ticket.

When the site asks for additional water to make the concrete more
workable, this will have to be signed for on the driver's copy of the
delivery ticket and, in such a case, the supplier cannot be held
responsible for the concrete failing to meet the specified strength.

The slump of concrete delivered in a truck can be measured using

a spot sample obtained from the initial discharge, but note that
such a sample is not representative for cube making. After allowing
a discharge of about 0.3 m3, six standard scoopsful should be
collected from the moving stream to provide a sample of about
20 kg, the scoopsful being taken as quickly as possible and
preferably from the next 0.2 m3 of the discharge. The discharge
should then be stopped, the slump measured, and if it is within
the specified consistence class, the remaining part of the load may
then be discharged. Figure 13: Ready-mixed concrete being placed by pump into a
ground supported slab.
If the concrete does not conform to the requirements either by
slump, ticket details or visual inspection, it must be refused and
may be returned to the depot. The reasons for return should be References/further reading
written on the delivery ticket and the truck number and time of The essential ingredient - Quality. 1992, British Cement Association,
rejection be recorded. However, under certain circumstances, it Crowthorne. Ref. 97.323. 23 pp.
may be permissible for water to be added to a load of stiff ready-
mixed concrete in order to achieve the specified target consistence The essential ingredient - Production and transport. 1993, British
class. This is in accordance with BS 5328 and BS 8500 and Cement Association, Crowthorne. Ref. 97.326. 20 pp.
applies when:
Dewar, J D and Anderson, R. Manual of ready-mixed concrete.
n The slump is less than the lower limit of the consistence class
(2nd edn). 1992, London, Blackie. 245 pp.
n The quantity of added water is controlled by being measured
accurately and recorded
• The stiffness is not due to an excessive delay since batching.

29
 SITE BATCHING & MIXING
Although most concrete nowadays is delivered ready-mixed, there Batching
may be occasions when it is more economic and practicable for
the concrete to be batched and mixed on site. There are many For all but the smallest of jobs and for all strength classes of
different types and sizes of batching plants and mixers; the concrete over 20 N/mm2, all materials should be weigh-batched.
following general recommendations apply to all mixer set-ups, and Provided the weighing mechanisms are carefully maintained and
may also be relevant to ready-mixed concrete. regularly calibrated, reasonable accuracy should be achieved in the
material proportioning.
The main objective is to produce every batch with the required
consistence, strength and other specification requirements. There are many different types and sizes of batching plant and the
choice for a particular job will usually depend on the amount of
concrete to be produced, daily and in total.
Storage of materials
For large quantities of concrete the aggregate weigh hoppers are
Materials must be stored so that they are not harmed in any way
likely to be fed from overhead storage bins and then discharged
during storage.
directly into the mixer, or onto a conveyor belt feeding the mixer.
Cement must be kept dry either in silos or, if in bags, kept under The cement from a silo with its own weigh hopper is usually fed
cover and off the ground as described on page 9 under Delivery directly into the mixer.
and storage of cement.
On smaller sites, where the output may be 20 - 50m3 a day, the
Aggregates should be handled and stored so as to avoid materials are often weigh-batched into a loading hopper that is
segregation and contamination by other aggregates, or by fuel, integral with the mixer (Figure 14). When in the lowered position
mud, etc. Each site will impose its own conditions and will have to the hopper rests on a load cell or hydraulic capsule connected to a
be considered individually, but factors to be taken into account weighing dial, which should be in a position such that it can easily
include: be seen by both the mixer driver and the drag-line skip operator.
The cement will be weighed in the cement silo dispenser and fed
n Access for delivery
directly into the mixer or into the mixer hopper.
n Adequate storage area available in relation to the quantity
to be stored
n Drainage
n Avoidance of double handling
n Convenience in relation to subsequent use.

Further information on aggregate storage is given on page 1 3

under Storage of aggregates.

Storage of water is not usually a problem if a normal mains supply

is used. If water is taken from a stream, well or lake, some storage
may be needed in addition to the tank provided on the batching
and mixing plant. Water-heating facilities may be required if
concreting is to continue during cold weather.

Special arrangements should be in place to prevent contamination

of the site when cleaning down plant and equipment.

Concrete mixers
Concrete mixers are designated by a number representing the Figure 14: Typical site batching and mixer set-up with drag-line
nominal batch capacity in litres and a letter indicating the type of skip for loading the hopper.
mixer, as follows:
The method and order in which the materials are fed into the
n Tilting drum, type T mixer can affect the uniformity of the concrete; this applies
n Non-tilting drum, type NT particularly to the water. Ideally, the cement, sand and coarse
n Reversing drum, type R aggregate should be fed into the mixer simultaneously; this
produces a more uniform concrete than when the materials are
n Forced action, type P (commonly known as a pan mixer).
introduced one after another. Similarly, the water and admixtures
Thus a 200 litre tilting drum mixer is designated as 200T. should enter the mixer at the same time and over the same period
as the other materials. This is not always possible, in which case it
A concrete mixer must be accurately levelled, and checked is advisable to start the flow of water a little in advance of the other
regularly to see that it stays so; inaccurate levelling results in poor ingredients. If all the water is added before or after the other
mixing and increases mechanical wear as well as affecting ingredients, the batch of concrete is liable to vary in consistence
weighing accuracy. from part to part.

Mixers should not be overloaded beyond their rated capacities, Where the loading hopper turns upside down to discharge into the
otherwise spillage of materials will occur and the mixing will be mixer, it is best if the coarse aggregate goes into the hopper first
less efficient, leading to lack of uniformity within the batches of so that it pushes the sand and cement out in front of it and gives a
concrete; mechanical wear will also be increased. clean discharge of the hopper. When the cement is fed into the
hopper from the silo dispenser, it is best for it to be sandwiched
When bagged cement is to be used, the selection of the size of
between the coarse and fine aggregates.
mixer should be related to the number of whole bags required for
each batch.

30
 SITE BATCHING & MIXING
If bagged cement is used, the weights of sand and coarse When the concrete is mixed it should be discharged in one
aggregate should be adjusted to suit a whole number of bags. operation before loading the next batch.
Attempting to judge half or quarter bags by splitting them leads 1
With a clean mixer - at the beginning of a day's concreting, for
large errors and variability between batches.
example - some of the finer material from the first batch will stick
On the rare occasions when volume batching of aggregates is to the mixer sides and blades and the batch will be discharged
unavoidable it should be done with buckets or gauge boxes, and harsh and stony, short of sand and cement. To make up for this
on no account should batching by the 'shovelful' be permitted. loss of fine material, the amount of coarse aggregate in the first
Allowance should also be made for bulking of the sand; sand batch should be reduced by about half and the water addition
increases in volume, up to 20 - 30%, as the moisture content rise reduced to maintain the required consistence.
to about 5 - 6%. Further increases in moisture content result in a
decrease in bulking until, when the sand is completely saturated, The mixer drum must be thoroughly cleaned out after the end of
its volume is almost the same as it was in a dry condition. Unless concrete mixing for the day, and before long stoppages such as
tests are made, it is usual to assume an average value of 20% for meal breaks, by filling the mixer with coarse aggregate and water
the bulking of damp sand. and allowing it to rotate for about five minutes before emptying;
this will remove any build-up of hardened mortar on the blades or
sides of the mixer.
Operation of site mixers Weigh hoppers should be cleaned daily to prevent any build-up of
It is the mixer operator's responsibility to ensure that the concrete material, especially if the cement is being put into the hopper,
is properly mixed, uniform throughout each batch and at the otherwise dial gauge readings may be inaccurate. There is also a
required consistence. He must see that the materials are being risk of aggregate spillage building up around the weighing
accurately batched and, when a loading hopper is in use, that they mechanism under the hopper, resulting in inaccurate weighing,
are loaded in the right order and that the hopper is uniformly and any such build-up should be removed daily.
loaded (non-uniform loading can lead to weighing inaccuracies).

Adding the right amount of water to each batch is the mixer

driver's main responsibility, so that consistence is maintained from
batch to batch. The free water content should be the same for
each batch. However, as described on page 1 3 under Storage of
aggregates, the moisture contents of the aggregates are likely to
vary so that the actual amount of water to be added may also have
to be varied in order to keep the free water in each batch the
same. This is best dealt with by adding most of the water,
estimated from the average moisture contents of the aggregates,
but keeping a little back to add later if it is needed. A skilled mixer
driver can tell by looking at the concrete in the mixer as it gets to
the end of mixing whether enough water has been added to give
it the right consistence. An ammeter or kilowatt meter connected
to the mixer motor also give a good indication. Normally, the
amount of water to be added from batch to batch will not vary
much, only by about 5 - 1 0 litres/m3.

Mix proportions are usually based on the saturated surface-dry

weights of aggregates. For weigh-batching purposes an allowance
has to be added for the moisture contents.

Batch weights of aggregate need to be adjusted to allow for

variations in their moisture contents in order to reduce variations in
consistence and strength, and if aggregate deliveries can be seen
to have widely different moisture contents and they are to be used
immediately, batch weights may require adjustment; similarly, after
rain has fallen on exposed stockpiles, adjustments may be
necessary. If, on the other hand, the weather is settled and
stockpiles and deliveries are known not to have widely varying
moisture contents, such adjustments are not necessary because of
their smallness in comparison with the total batch weights.

Thorough mixing of concrete is essential. Mixing times will vary

according to the mix, the mixer and whether or not it is being
filled to capacity. A uniform colour is usually the best guide to
whether the mixing has been efficient. For rotating drums up to
about 1 m3 capacity, the mixing time needs to be 1½ - 2 minutes
after all the materials have been fed in. Very small mixers used on
building sites and some large free-fall mixers require longer times.
For pan mixers, because of the forced action, 30 - 45 seconds is
usually enough.

31
 TRANSPORTING CONCRETE
A number of methods for transporting concrete on site are
available, ranging from hand wheelbarrows to concrete pumps.
The choice of method will depend on the size and complexity of
the project, and factors such as ground conditions and distance to
be covered and the availability of cranes or other plant. On many
jobs several different methods, or even a combination of methods
may be required. In all cases the concrete must be moved to the
point of placing as quickly and economically as possible without
allowing segregation, loss of any constituents, contamination with
water or any other material after it has left the mixer.

Pumping
Pumps were first used to transport and place concrete in this
country in the 1930s, and their use has grown to the extent that
some 20% of concrete is now placed in this way. Many pumps are
capable of moving up to 100 m3 per hour, depending on the
pump type, the horizontal and vertical length of the pipeline, the
number of bends, and the consistence of the concrete. In practice,
the output is usually about 30 m3 per hour due to supply and
organisational limitations. Most pumps can transport concrete
more than 60 m vertically or 300 m horizontally (shorter distances
when pumping both horizontally and vertically). Some high-
pressure pumps have achieved heights in excess of 400 m and
horizontal distances greater than 1000 m.

Most mobile pumps can place concrete directly to where it is

required (Figure 15), removing the need for other forms of
transport and pumping is particularly beneficial when access is
difficult or restricted. Standard boom sizes are 16 m, 22 m, 24 m,
32 m, 40 m and 52 m whilst greater distances require a static
pipeline which bypasses the boom altogether. There is usually little
or no waste. Labour costs are generally minimized since only one
person is usually needed to place the concrete, with others
compacting and finishing, although the workforce must be
adequate to cope with the fast rate of placing. For high-rise
construction, pumping permits placing rates to be maintained
regardless of the height, without any increase in labour costs.

In order to make best use of the pump, the concrete mix design
may have to be adjusted to make it suitable for pumping without Figure 15: Placing by pump.
using excess pressure. Essentially the concrete should not be prone
In order to make the most economical and efficient use of
to segregation or excessive bleeding and should have a low
pumping, it is important to understand that the decision to utilize
enough frictional resistance for the pump to be able to push the
a concrete pump ought to be made at the planning stage of a
concrete along the delivery line. It is sometimes necessary to
project. Bringing the pump onto site to solve placement problems,
increase the sand content by a few per cent, along with an
when other alternatives have proved unsuccessful, is likely to result
increase in the cement content, in order to provide sufficient fine
in additional costs.
material and to achieve an overall aggregate grading that is
continuous and without gaps. Each site where pumping is proposed will require careful individual
planning but a number of considerations, likely to be common to
In this context the consistence of the concrete is an important all sites, can be identified. The concrete placing gangs must be
factor, since a very low consistence class may result in increased able to cope with the pump output for the duration of the pour,
resistance to pumping. A target slump of 70 - 90 mm is generally without skimping on compaction, finishing or curing. The pump
considered to be about the right level and the addition of a should not be allowed to stand idle waiting for concrete to be
plasticizing admixture avoids a higher free water/cement ratio. delivered and a steady supply of concrete to the pump should be
Discussion at an early stage with the ready-mixed concrete supplier planned, consistent with the rate of placing which can be
and/or the concrete pumping subcontractor is therefore accommodated. For large or important pours, standby pumps
recommended in order to ensure that a satisfactory pumpable should be arranged. The siting of the pump should be such that
concrete is obtained. delivery vehicles have easy access and two vehicles can be
accommodated at the pumps so that the second one can start
For trouble-free pumping the concrete must be consistent, since discharging as soon as the first one finishes, maintaining a
minor variations in the mix proportions are sufficient to make an continuous flow of concrete. The choice of pump location should
otherwise pumpable concrete difficult to pump or even completely also take into account the need to keep pipelines as short and as
unpumpable. If only a small part of a load proves to be straight as possible. Good communication between the pump
unpumpable the pump may become blocked, leading to a time- operator and the placing gang is essential.
consuming and expensive delay while the pump and/or line is
stripped down and the blockage removed.

32
 TRANSPORTING CONCRETE
Most mobile concrete pumps are fitted with folding booms, which The openings of skips should be large enough to allow easy
are an advantage for placing concrete in difficult situations. discharge and the consistence class needs to be adequate to allow
However, a boom is not necessarily the best method of placing. for controlled discharge into difficult sections. When concrete with
There are circumstances where a static pipeline is better, this a low consistence class is placed by skip, poker vibrators may have
system having less resistance to flow than a delivery boom of the to be used to assist discharge.
same diameter.
Skips should be properly maintained if they are to function
The pour should be planned so that pipes may be removed as it efficiently. After a day's concreting the skip should be thoroughly
progresses - pipes should never be added. All couplings should be cleaned and washed down and the gate operating mechanism
completely free from leakage, otherwise loss of fine material from should be oiled and greased. A build-up of hardened concrete on
joints is likely to result in problems due to blockage. In hot, sunny the outside of the skip may be prevented by rubbing it over,
weather, it may be necessary to protect the pipeline from before it is used, with a light coating of oil or chemical release
overheating. In such conditions, concrete in the pipeline must be agent to prevent adhesion.
kept moving.

Concrete in pump pipelines is often under considerable pressure, Dumpers

so that the safety of site staff must be considered. The pump
Dumpers, generally of about 0.5 m3 capacity, are a common form
should be stopped, and if possible reversed, while pipes are being
of transport on many construction sites. They may be discharged
disconnected. Flexible end sections of pipes may move violently
either forward or sideways and are best when hydraulically
when a cleaning plug is passed through and operatives should be
operated so that the discharge can be controlled. The main
kept well clear. Falsework should be designed to accommodate the
disadvantage of gravity discharge is the sudden uncontrolled surge
vibration and additional loading caused by pipelines resting on it.
of concrete - the heavy impact can displace reinforcement and
other objects in the formwork. For small sections it may be
necessary to discharge onto a banker board first and then shovel in
Crane and skip the concrete by hand.
The crane is still probably the most common method of handling
concrete for combined vertical and horizontal movement. A crane If the haul routes are so long that segregation becomes a problem,
is frequently needed on site for handling formwork and agitator trucks, or lorry-mounted transporters fitted with screws or
reinforcement, and its further use in transporting concrete may be paddles to remix the concrete as it is discharged, may be
both economic and convenient. However, when concreting preferred.
requirements dictate the choice of crane capacity, or if the crane is
likely to be fully occupied with other tasks, it may be more
economic to transport the concrete by other means. Other methods
There are several other less common methods of transporting
Skips generally range in capacity from 0.2 to 1.0 m3 (but there are
concrete, including pneumatic placers, monorails and the railcars
many variations in detail), and are of two broad types (Figures 16
sometimes used in tunnel work, which are not covered in this
and 1 7):
publication.
n Lay-back or roll-over skips
n Constant attitude skips.
References/further reading
Concrete on site — 4: Moving concrete. 1993, British Cement
Association, Crowthorne. Ref. 45.204. 9 pp.

The essential ingredient - Site practice. 1994, British Cement

Association, Crowthorne. Ref. 97.341. 23 pp.

Figure 16: Roll-over skip with wheel-controlled discharge

mechanism.

33
 PLACING & COMPACTION
The successful placing and compaction of concrete can be In deep lifts of columns and walls, delays and interruptions should
achieved only if there has been careful forethought and planning. be avoided to prevent colour variations on the surfaces; the rate
should exceed 2 m height per hour. The correct rate of rise will
Because they are done almost simultaneously, placing and
have been calculated by the temporary works co-ordinator and
compaction are interdependent, and the two operations need to
must be observed in the interests of avoiding excessive formwork
be considered together. The rate of delivery of the concrete should
pressure and achieving satisfactory surface finish. On columns and
match the rate at which the concrete can be both placed and
walls, care should be taken so that the concrete does not strike the
compacted.
face of the formwork, otherwise the surface finish may be affected;
The consistence of the concrete at the point of placing needs to care also needs to be taken to avoid displacing reinforcement or
suit both the placing technique and the means of compaction ducts and to ensure that the correct cover is maintained.
available.

Compaction
Placing After concrete has been mixed, transported and placed, it contains
Before concrete placing begins, the insides of the forms should be entrapped air in the form of large voids. The object of compaction
inspected to make sure they are clean and have been treated with is to get rid of as much of this air as possible. Before compaction,
release agent. If the forms are deep, temporary openings and concrete of consistence class S2 may contain 5% entrapped air,
lighting may have to be provided for this inspection. Rubbish, such while concrete of S1 consistence class may contain as much as
as sawdust, shavings and reinforcement tying wire, should be 20%.
blown out with compressed air. Any rainwater at the bottom of the
If this entrapped air is not removed by proper compaction the
form should be removed. Similarly, the reinforcement should be
presence of these large voids will:
inspected to see that it complies with the drawings, and that the
correct spacers have been used and there are enough of them. n Reduce the strength of the concrete - more than 5% loss of
strength for every 1 % air
The main objective with placing is to deposit the concrete as close n Increase the permeability and hence reduce the durability and
as possible to its final position, quickly and efficiently and in such a protection to the reinforcement
way that segregation is avoided (Figure 17). Moving the concrete
n Reduce the bond between concrete and reinforcement
with poker vibrators should generally be avoided.
n Result in visual blemishes such as excessive blowholes and
Particular care is necessary when using a skip for placing in thin honeycombing on formed surfaces.
walls and other narrow sections in order to avoid heaps and
Fully compacted concrete will be dense, strong, impermeable
sloping layers. The skip discharge needs to be carefully controlled
and durable.
and the skip moved so that a ribbon of concrete is placed. The
concrete should be placed in uniform layers not more than
500 mm thick or less, depending on the length of the poker blade.
Vibration
Otherwise compaction may be impeded by the weight of concrete Most concrete is compacted by means of internal poker vibrators
on top. Provided that the concrete has been well designed and that 'fluidize' the concrete and permit the entrapped air to rise to
proportioned and is sufficiently cohesive, there is generally no the surface. Pokers vary in size, usually from 25 - 75 mm in
need to restrict the height from which the concrete is dropped. diameter. Table 13 gives a broad indication of poker sizes, and
This assumes that the concrete is unimpeded and does not their characteristics and typical applications.
ricochet off formwork or reinforcement, which may cause
The radius of action will determine the spacing and pattern of
segregation of the mix.
insertions. As a guide, a spacing up to 500 mm centres is about
right for a 60 mm diameter poker with concrete of medium
consistence (see Table 13).

The poker should be inserted vertically and quickly and should

penetrate some 100 mm into any previous layer; thereby stitching
the two layers together. It should remain in the concrete until the
air bubbles cease to come to the surface. Figures 18, 19 and 20
illustrate the process.

Being able to judge when the concrete has been fully compacted is
largely a matter of experience. Sometimes the sound can be a
useful indicator, in that the pitch (whine) becomes constant when
the concrete is compacted. In addition, a thin film of glistening
mortar on the surface is a sign that the concrete is compacted, as
is cement paste showing at the junction between the concrete and
the formwork. The poker should be withdrawn slowly so that the
concrete can flow back into the space occupied by the poker.

External vibrators are occasionally used, but their usefulness is

limited on site by the heavy formwork needed to resist the stresses
and shaking they produce. Their use is mainly confined to precast
concrete elements, but they may be necessary for heavily
reinforced walls and the webs of deep beams where it is difficult or
impossible to insert a poker.

Figure 17: Placing concrete from a constant attitude skip.

34
 PLACING & COMPACTION
Table 13: Characteristics and uses of internal poker vibrators.

Diameter of head Radius of action Approximate rate of Uses

(mm) (mm) compaction, assuming
rapid placing
(m3/h)
20-30 80 - 150 0.8-2 Concrete with class S3 consistence and above in very
(needle) thin sections and confined places. May be needed in
conjunction with larger vibrators where reinforcement,
ducts and other obstructions cause congestion

35-40 130-250 2-4 Concrete with S2 consistence and above in slender

columns and walls and confined places
50-75 180-350 3-8 Concrete with class S1 consistence and above in general
construction free from restrictions and congestion

Slabs are best consolidated by vibrating beam compactors. These Over-vibration

combine the action of a screed and a vibrator, but they are only The dangers and problems arising from under-vibration are far
effective for a limited depth. In general, a slab more than 150 mm greater than any supposedly arising from over-vibration, since it is
thick should be compacted with poker vibrators and finished with virtually impossible to over-vibrate a properly designed and
a vibrating beam. The edges of all slabs butting up to side forms proportioned concrete.
should always be poker vibrated. Construction joints need
particular attention (see the next section). Re-vibration
Provided that it is still workable, no harm will be done if concrete
that has already been compacted is re-vibrated. In fact, tests have
shown that the strength is likely to be slightly increased.

Re-vibration of the top 75 - 100 mm of deep sections can

minimize plastic settlement cracks or close them if they have been
seen to develop.

Similarly, the re-vibration of the tops of columns and walls can

often reduce the tendency of blow-holes to occur in the top
600 mm or so.

References/further reading
Figure 18: Inserting a poker in fresh (stiff) concrete in a beam. Concrete on site - 5: Placing and compacting. 1993, British Cement
Association, Crowthorne. Ref. 45.205. 16 pp.

The essential ingredient - Site practice. 1994, British Cement

Association, Crowthorne. Ref. 97.341. 23 pp.

Figure 19: The concrete is fully compacted in the immediate

vicinity of the poker

Figure 20: The poker has now been moved about 500 mm along
the mould.

35
 CONSTRUCTION JOINTS
A construction joint, or day-work joint, is one where fresh concrete one with harder bristles in case the concrete has stiffened more
has to be placed on or against concrete that has already hardened. than expected. Brushing should be done gently so that pieces
This type of joint is different from contraction and expansion of the coarse aggregate are not undercut or dislodged - just
joints, which are used to accommodate movement, and from the tips of the aggregate showing is correct
joints incorporating water bars. n If the laitance has hardened but the concrete is still 'green' -
say the following morning - a wire brush and some washing
Some construction joints do not need to be fully bonded, the
will usually be enough to remove it. The surface should be well
reinforcement across the joint being adequate to transmit tensile
washed afterwards to remove the dust
or shear stresses across the slight gap that may occur due to
contraction. But many construction joints may require the concrete n Pressure washing can be done up to about 48 hours after
to be bonded so that shear and tensile stresses are transmitted placing, but timing is again critical and will also depend on the
across the joint, in which case the risk of a shrinkage gap is to be pressure, otherwise the aggregate particles may be dislodged
avoided. n If the surface has been allowed to harden, then mechanical
scabbling must be used. Small hand-held percussion power
In both cases it needs to be recognized that joints always show, no
tools such as those used for tooling exposed-aggregate finishes,
matter how well they are made, so they should always be made to
or a needle gun, are the best to use. The danger with this
form a clean line on the surface. If appearance is important, such
method is that it can shatter and weaken coarse aggregate at
as with high-quality finishes, there are advantages in making a
the surface or loosen the larger particles, so it should not be
feature of the joints.
done until the concrete is more than three days old, and then
only carefully. It is a slow and expensive method
Location of construction n Wet or dry abrasive blasting is usually suitable only when large
areas have to be treated, such as in floor surfaces.
joints
The position of construction joints should be settled before any
concreting begins. As a general rule, joints in columns are made as
near as possible to the underside of beams; joints in beams and
slabs are normally made at the centre, or within the middle third,
of the span.

Preparation of construction
joints
The first requirement for a good bond is that the hardened
concrete surface must be clean, free from laitance and have an
exposed-aggregate appearance.

After concrete has been vibrated, bleeding occurs by surplus water

rising to the surface. The bleed water brings with it a small amount
of cement and fines, and these are left on the surface after the
water has evaporated. This layer of laitance is weak and, as well as
being porous and not watertight, will not give a good bond for
fresh concrete.
Figure 2 1 : Washing and brushing to remove laitance about two
Similarly, concrete cast against vertical formwork also has a skin of hours after placing.
cement paste on the surface, which, although not quite as weak as
that at the top of a horizontal joint, is still likely to affect the bond A method used occasionally is to spray a retarder onto the surface
when fresh concrete is placed against it. of the concrete to 'kill' the set, so that the laitance can be brushed
off the following day or later. This method is not recommended
Laitance from both horizontal and vertical surfaces must be because it is difficult to be sure that all the retarded concrete has
removed if and when a good bond or watertightness is required of been removed - if not, fresh concrete cast against it will have a
the concrete itself. However, if watertightness is to be achieved by poor bond. The bond with reinforcement may also be affected.
the incorporation of a water bar, removal of the laitance may not
be necessary.
Vertical surfaces
Vertical construction joints in walls, beams and slabs will usually
Horizontal surfaces have been formed against a stop-end. Stop-ends should be located
There are a number of ways of removing laitance from the top of where the reinforcement is least dense; they should be well made,
cast concrete to provide a surface with an exposed-aggregate easily strikable, and fixed to avoid grout loss. A typical stop-end
appearance: detail is shown in Figure 22.
n The easiest way is to brush off the laitance while the concrete is
Most vertical construction joints do not require any surface
still fresh but has stiffened slightly. The timing for this is critical
treatment, plain smooth surfaces being quite satisfactory. Only
because it depends on the weather and the concrete - in warm
when a joint is subject to high shear forces (and the engineer must
weather concrete stiffens faster than in cold weather and a rich
decide this) or when a monolithic watertight joint is required will
concrete stiffens faster than a lean one. The best time will
the surface need preparation. Suitable methods are as follows:
usually be about 1 - 2 hours after the surface water has
evaporated. A small brush is used to remove the laitance while n If the stop-end can be removed some 4 - 6 hours after
gently spraying the surface with water (see Figure 21). It is concreting without disturbing the main formwork and
worth having two brushes handy - one with soft bristles and

36
 CONSTRUCTION JOINTS
out should be done before the formwork is erected, but if there is still
some debris left after erection it should be removed by taking out one
of the stop-ends.

On prepared concrete surfaces the use of mortars or grouts or wetting

the face of a joint is not recommended for the following reasons:
n Tests have shown that the bond between the hardened and
fresh concrete is not significantly increased
n The restricted access to a horizontal joint at the bottom of a lift
Foamed polyurethane
strip (well compressed) for which the formwork has been erected makes it difficult to
ensure the grout or mortar has been uniformly applied; in any
case, it would need to be scrubbed into the surface to be effective
Fillet to form n It is virtually impossible to apply mortar or grout to a vertical
featured joint joint - especially when the formwork is in place
n There is a danger the grout or mortar will dry out before the
concrete is placed; any drying out puts back the laitance, which
had been carefully removed
n The appearance of the joint may be spoilt by a line of
different colour.
tie-bolt
Succesful construction joints are achieved simply by careful placing
and thorough compaction of concrete against a properly prepared
Figure 22: Stop-end detail at a vertical construction joint. The surface.
essential requirements are simplicity of construction and sealing to
prevent grout loss. Horizontal joints
reinforcement, a spray-and-brush method as described in The first layer of concrete must not be deficient in fine material. If
method 1 above for horizontal surfaces can be used. there is any danger of losing mortar from the concrete by leakage
while transporting or placing it, the first batch should ideally be made
n If the stop-end is removed the following morning, the concrete
richer than subsequent batches by reducing the coarse aggregate
will usually still be green enough for the cement skin to be
content (see Operation of site mixers on page 31). With ready-mixed
removed to a depth of about 2 mm using a wire brush. The
concrete there may be a tendency for the beginning of the discharge
surface should be brushed immediately after striking the
to be rather coarse, in which case the first barrow-full may be
stop-end
discarded.
n If the surface has hardened, then light scabbling, pressure
washing or abrasive blasting are likely to be necessary to obtain The first layer should be spread uniformly over the surface to a
the right texture thickness of only about 300 mm. For small columns it will probably be
n Expanded metal mesh, suitably framed, is particularly useful for necessary to use shovels to avoid putting in too thick a layer.
stop-ends, especially when the reinforcement is congested. In Discharging a 0.5 m3 skip into a 600 mm square column, for example,
some situations, such as where watertightness at the joint is not will result in honeycombing at the bottom. When casting columns
essential, expanded metal can be left in. If it is removed the and walls the poker should always be put in before the concrete goes
following day by pulling it off, the surface should then be in, and be drawn up slowly as the concrete is progressively placed;
sufficiently rough and laitance-free for no further treatment to this avoids compacting the surface layer, which would make it very
be required. Where the joint line and appearance are difficult for the air trapped lower down to be expelled upwards
important, the expanded metal should be kept about 40 mm subsequently. If using a skip for walls it should be moved along the
away from the face to avoid breaking off the corners or arrises top. Baffle boards are useful to make sure the concrete is discharged
along the face. cleanly to the bottom of the forms. The first layer of concrete must be
thoroughly compacted by poker vibrators inserted at close centres,
When horizontal or vertical joints are featured or a good clean line
depending on the size of the poker and the consistence class of the
is required, care must be taken, especially when tooling, to avoid
concrete (see Table 1 3).
chipping or breaking the arrises along the joint line. It is a good
idea to leave untreated a margin of about 25 - 40 mm. Lighting may be necessary for seeing the concrete at the bottom of
the pour to check that it has been properly placed and compacted.

Concreting at construction
Vertical joints
joints It is usually undesirable to make concrete flow horizontally using
It is essential for the fresh concrete to be placed and compacted s vibration, but at vertical joints some flow of the concrete towards the
that it bonds with the prepared surface. joints helps to avoid possible lack of compaction. As the layers of
concrete are placed in a wall they should be kept back about 150 -
Poorly compacted concrete or honeycombed concrete at the 300 mm from the vertical joint and the poker used to make the
bottom of a lift in a wall or column leaves a joint which is both concrete flow towards the joint; this needs particular care and a well-
weak and unsightly. designed concrete if segregation is to be avoided.

First, any dirt, dust or rubbish (e.g. sawdust, pieces of wood, nails
and bits of tie wire) must be removed from the surface of the References/further reading
concrete. This can best be done by blowing out all the dirt and
Concrete on site - 7: Construction joints. 1993, British Cement
rubbish with a compressed air hose. If compressed air is not
Association, Crowthorne. Ref. 45.207. 8 pp.
available, then thorough brushing out is necessary. This cleaning

37
 CONCRETING IN COLD WEATHER
It is well known that water expands when it freezes. This can cause noted that 19 mm of plywood has fairly good insulating properties
permanent damage and disruption if freezing is allowed to occur on its own) and slabs should be covered with insulating mats
within fresh concrete, or in hardened concrete that has not immediately after laying (Figure 23). The tops of walls and
developed much strength. For practical purposes it has been found columns are particularly vulnerable and should be covered with
that, provided the concrete has achieved a strength of 5 N/mm2, it insulating material.
can resist the expansive forces caused by the freezing of the water
in the concrete. For most concrete this critical strength is reached
within about 48 hours when the temperature of the concrete has Strength development
been kept above 5°C. Both the early and subsequent strength development in cold
weather can be accelerated using a water-reducing admixture or
The gain of strength is delayed at low temperatures so it is
more conveniently by increasing the strength class of concrete.
necessary to protect concrete against cold for some time after
placing. Thin sections normally require more protection and for a External in-situ paving is particularly vulnerable to the effect of low
longer period than thicker ones, corners and edges being temperatures because of the large surface area, which loses heat
particularly vulnerable. quickly. In addition, slabs are open to drying winds that can add a
chilling factor to the effects of low temperature. There is a variety
Many of the precautions that can be taken to protect concrete
of materials and methods that can be used for protecting and
from cold make use of the heat that concrete generates as it
providing insulation to exposed concrete surfaces, ranging from
hardens. However, this is effective only if the concrete temperature
plastic sheeting and tarpaulins to proprietary insulating mats. As an
is sufficiently high at the time of placing for the heat evolution to
indication of the relative merits of different methods, tarpaulin or
start rapidly. To this end the temperature of the concrete when
plastic sheeting enclosing a 50 mm dead air space has about the
placed in the form should never be less than 5°C, preferably not
same insulating value as 19 mm thick timber; but proprietary
below 10°C. To achieve this, the concrete temperature in the
insulation mats (Figure 23) are more effective.
mixer or ready-mixed concrete truck needs to be higher to allow
for heat losses during transportation and placing. Some ready-
mixed plants can deliver heated concrete.

It should be noted that it is important for prevent water loss from

newly laid concrete, see Curing on page 46.

Raising the temperature

The easiest way to raise the temperature of the fresh concrete is to
heat the mixing water. Aggregates should be free from ice and
snow because it requires as much heat to melt the ice as to heat
the same quantity of water from 0°C to 80°C. Aggregates should
be covered and kept as dry as possible. Heated water should be
added to the mixer before the cement so that its temperature will
be lowered by contact with the mixer and the aggregates. If this is
not done there could be a flash set when hot water comes into
Figure 23: Laying a thermal blanket on a freshly finished slab.
contact with the cement.
Even when concrete has been protected from freezing during its
Sometimes, in very cold weather, aggregates also must be heated early life, the subsequent slow gain of strength in cold weather
to achieve the desired concrete temperature. This can be done by needs to be allowed for. Longer periods than are necessary in
injecting live steam or using hot air blowers, closed steam coils or warmer weather will be required before forms are struck (see
electric heating mats. Live steam probably produces the most Table 14). The gain of strength of concrete in cold weather can be
uniform heating, but the increased moisture content needs to be assessed by tests on cubes cured, as far as possible, under the same
allowed for in determining batch weights. conditions as the concrete in the structure. Alternatively, guidance
If concrete with a sufficiently high initial temperature is prevented can be obtained from CIRIA Report 1 36.
from losing heat to its surroundings, the heat evolved during
setting and hardening will protect it from damage by freezing.
Thus, formwork should be insulated (in this respect it should be

Table 14: Minimum period before striking formwork (concrete made with CEM I or SRPC).

Mean air Sides to beams, walls Soffits to slabs, props Props to slabs Soffits to beams, props Props to
temperature and columns (hours) left under (days) (days) left under (days) beams (days)
(°C)
3 36 8 14 14 18
10 24 5 8 8 12
16 18 4 6 6 10

NOTES
These times are based on a well-managed concreting operation that includes effective curing.
It may be necessary in cold conditions to instruct the supplier to reduce or eliminate the proportion of any ggbs or pfa
For concretes containing pfa or ggbs recommended striking times should be increased. Full details are given in CIRIA Report 1 36.

38
 CONCRETING IN COLD CONCRETING IN HOT
WEATHER WEATHER
Minimum striking times In the UK when temperatures exceed about 20°C, there are two
main factors leading to problems with concreting:
The use of cements with pfa or ggbs in cold weather presents a
particular problem because these concretes are more adversely n When the temperature of the concrete itself exceeds 20°C, the
affected than ordinary Portland cement concretes due to their rate of reaction between the cement and the water is increased
slower gain of strength. It should be noted that Table 14 applies and this in turn leads to an increased rate of stiffening and loss
only to concrete made with CEM I or SRPC of consistence. There is also an increased risk of early-age
thermal cracking because the peak temperature will be
Cold weather delays the stiffening of concrete, and ground floor increased
slabs, for example, are likely to take considerably longer than
n High air temperatures, especially when accompanied by a low
normal before the trowelling operations can be started.
humidity, can increase the rate at which water evaporates from
the concrete. Evaporation of water due to delays between
Plant and equipment mixing and placing will cause loss of consistence. Evaporation
Preparations for winter working should be made well in advance of will also be increased from exposed horizontal surfaces after the
the onset of cold weather, and the necessary plant and equipment concrete has been placed.
made ready for use when required. Modifications in site
organisation to help keep work going in winter may not always be
applicable, but they should be considered because their cost is Loss of consistence
usually small in relation to the benefits of a smooth flow of work, a Stiffening due to high temperatures and/or water loss can cause
quicker end to the job and no idle labour. One technique that may problems by:
be considered is the total enclosure of the work area with, for n Making it difficult to place and compact the concrete.
instance, polythene sheeting fixed to the scaffolding, and the use
n Increasing the risk of 'cold' joints in large pours
of space heaters within this enclosure. Consideration should also
be given to the use of cements of higher strength class such as n Creating surface finishing problems with floors and paved areas.
42,5R or 52,5 (formerly known as rapid-hardening Portland
The existence of drying conditions makes it more important to
cement) or the use of a higher strength class of concrete, which
ensure that exposed surfaces of concrete after compaction and
will give an increased rate of strength gain leading to the ability to
finishing are protected against loss of moisture by efficient curing
strike forms earlier than would be possible with the concrete
methods.
originally specified.
Accelerated stiffening and loss of consistence can best be
minimized by placing the concrete as soon after mixing as
Weather records possible. It is essential that concrete should be of the required
Keeping weather records and planning with an eye to the weather consistence at the point of placing, and any delays due to
forecast is necessary for efficient winter working. Records of prolonged transportation should be allowed for by designing the
maximum and minimum temperatures, together with a more concrete so that the consistence of the concrete at the mixer is
continuous record during working hours, will help towards an higher than required at placing to allow for consistence loss.
assessment of maturity and formwork striking times. This
It should be noted that this higher consistence may require an
assessment should take account of wind and cloud cover because
increased cement content or the use of a plasticizing admixture in
the temperature of the concrete is the factor that matters and this
order to maintain the correct free water/cement ratio.
is not always the same as the air temperature. On a windy,
cloudless night concrete can be cooled below the air temperature. Over a delay period of 30 minutes the loss in slump can be 20 - 50
The weather forecast is available by telephone or via the internet, mm and will become progressively worse with cement contents
and is an invaluable guide to the planning of winter work. Freezing higher than 300 kg/m3. Rapid stiffening can be minimized by using
conditions can usually be predicted and precautions taken. a retarding admixture and/or cements containing pfa or ggbs,
Specifications frequently call for precautions to be taken at which reduce temperature rise and minimize the risk of early-age
particular temperatures, depending on whether the temperature is thermal cracking.
rising or falling.
Another factor which should be taken into account is that the
higher the temperature of the batch ingredients (and hence the
References/further reading concrete temperature) the greater will be the quantity of water
CIRIA Report 1 36. Formwork striking times - criteria, prediction and needed to produce any given consistence class. For example,
methods of assessment. 1995, Construction Industry Research and concrete with S2 consistence class at 20°C is likely to have a
Information Association, London. 71 pp. consistence class of only S1 at a temperature of 30°C when made
with the same free water content.
Concrete on site -11: Winter working. 1993, British Cement
Association, Crowthorne. Ref. 45.211. 9 pp.
Moisture loss
The rapid loss of moisture from the surface of exposed concrete
increases the risk of plastic cracking, and the concrete should be
cured thoroughly as soon as possible after finishing. As soon as the
surface has hardened sufficiently, polythene sheeting can be used,
or a sprayed-on curing membrane applied, (see Figure 24)
preferably using a pigmented type that reflects solar radiation (see
Curing on page 46).

Concrete that has lost workability due to early stiffening should not
be retempered by additional water.

39
 CONCRETING IN HOT
WEATHER REINFORCEMENT
NOTE: Low water content makes concrete more susceptible to the Reinforcement for concrete may consist of round or deformed steel
adverse effects of the moisture loss. bars or welded steel mesh fabric. These materials are covered by
BS 4449, 4482 and 4483. The requirements for scheduling,
It is generally considered inadvisable for concrete to be placed
dimensioning, bending and cutting of reinforcement are covered
when its temperature exceeds about 30°C unless special
by BS 8666.
procedures are followed, such as those that apply in very hot
climates.
Bar types and identification
The two grades of steel used for bar reinforcement are mild steel,
identified by 'R' on the reinforcement drawings and schedules, and
high-yield steel identified by T. The letter 'X' is used to denote
other steels, for instance stainless steel.

All plain smooth round bars produced in the UK are hot rolled mild
steel, which has a characteristic strength of 250 N/mm2.

High yield reinforcement is produced by hot rolling a low-alloy

steel. It has a characteristic strength of 460 N/mm2 and is known
as 'deformed' steel because of its pattern of raised ribs (Figure 25).

Figure 24: Applying a sprayed-on curing membrane to freshly

laid concrete.

References/further reading
Shirley, D E. Concreting in hot weather. 1980. British Cement
Association, Crowthorne. Ref. 45.013. 12 pp.

Figure 25: Types of reinforcement in general use: plain mild steel

bar (left); high yield bar (right).

Bar sizes and bending

The preferred nominal diameters of bars are 8, 10, 12, 16, 20, 25,
32 and 40 mm. Should a larger diameter bar be required, bars of
50 mm diameter are normally available by special arrangement
with the manufacturer. In the case of bars less than 8 mm
diameter, a 6 mm type is sometimes obtainable.

Minimum radii for bends are given in BS 8666 and these should be
used unless larger radii are detailed. The minimum inside radius for
different types of steel is given below.
n Mild steel - twice the bar diameter
n High-yield steel
- for bars up to and including 20 mm in diameter: three times
the bar diameter
- for bars 25 mm and greater in diameter: four times the bar
diameter.
All reinforcement should be bent on proper bar-bending machines
and should be to the specified dimensions and within the
allowable tolerances. It may be impossible to fit steel in the correct
position and with the correct cover if the bars have not been bent
accurately; reinforcement in the wrong position may reduce the
strength of the unit, and a reduction in the specified cover will
reduce durability due to an increased risk of the reinforcement
corroding.

40
 REINFORCEMENT
Reinforcement should not be bent or straightened in a way that
will fracture or damage the bars. All bars should preferably be bent
Handling, storage and
at ambient temperature, but when the temperature of the steel is cleanness
below 5°C special precautions such as a reduction in the speed of Whether the reinforcement is being delivered uncut (generally in
bending or increasing the radius of bending may be necessary. 12 m lengths) or already cut and bent it is essential to off-load it
Alternatively the steel can be warmed to a temperature not carefully. Pushing bundles of bars off lorries or throwing them onto
exceeding 100°C. Heated bars should never be cooled by stacks inevitably leads to bends or kinks.
quenching. Reinforcement should not be re-bent or straightened
without the approval of the engineer. The bars should be stacked off the ground and well supported to
ensure that they do not become covered with mud and dirt. Bars of
Standard shapes of bent bar are readily available from suppliers in different types and diameters for bending on site should be stacked
a range of bar sizes, and most suppliers will also work from the separately and well labelled so that the bar bender can identify them
reinforcement schedule to supply steel ready cut and bent to easily. Cut and bent steel should be delivered already bundled and
specified dimensions and tolerances. labelled with the bar schedule reference and the bar mark to enable
the fixers to find the bars they need.
Fabric Before concreting, the reinforcement needs to be free from mud, oil,
Factory-made sheets of mesh made from welded bars or wires are grease, release agents, paint, retarders, loose flaky rust, loose mill
known as fabric reinforcement. It is used extensively for ground scale, snow, ice or any substance that will affect the concrete or steel
and suspended slabs and for reinforced concrete roads. Fabric is chemically or reduce the bond between the two materials.
available in a British Standard range of preferred meshes in stock
The effect of loose rust and mill scale on the bond between steel
sheets 4.8 m long by 2.4 m wide, but other mesh arrangements
and concrete is often a cause of contention on sites.
and sizes of sheets are also available to order.
Tests carried out on rust-free and rusty bars have shown that,
The preferred types of fabric designated in BS 4483 are divided
provided the cross-sectional area of the bar has not been reduced,
into four categories, each classified by a letter, as shown in
the effect of a little rust is not harmful and normal handling will
Table 15.
remove loose rust and mill scale; the same effect can be achieved by
Table 15: Preferred types of designated steel fabric. dropping bars on the ground or giving them a sharp tap, preferably
on the end. Where it is suspected that the cross-sectional area of the
Prefix Type of fabric Size of mesh Typical bars has been reduced by corrosion, the most accurate way of
letter (mm x mm) applications checking, especially with deformed bars, is to weigh a known
length. Mortar or grout droppings on bars projecting from concrete
A Square mesh 200 x 200 Slabs - suspended do not need removing provided they are firmly bonded to the bars.
and ground

B Structural mesh 100 x 200 Suspended slabs,

walls Cover to reinforcement
C Long mesh 100 x 400 Roads, paved areas, The strength and durability of a reinforced concrete structure
ground floor slabs depend on, among other factors, the reinforcement being correctly
positioned, within allowable tolerances, in the hardened concrete.
D Wrapping 100 x100 Sprayed concrete The most common cause of corroding reinforcement is insufficient
work, concrete
cover. The position of reinforcement should be checked before and
encased steelwork.
during concreting to ensure that the correct cover is maintained;
further checks should be carried out using a covermeter after the
Each type is available in a limited number of weights, depending concrete has hardened, as discussed on page 60 under
on the wire diameter used. References on drawings and schedules Electromagnetic covermeter.
use the letter followed by a number denoting the cross-sectional
area, in mm2/per metre width, of the main wires. For example, The nominal cover should be given on the working drawings.
B503 is a structural mesh with a main wire area of 503 mm2 per Nominal cover is the depth of concrete cover shown on drawings to
metre width. An 'A' or square mesh has the same cross-sectional all reinforcement including links. The actual cover should nowhere
area in each direction. be less than the nominal cover minus a margin which, in British
Standards, is currently 5 mm. If the surface of the concrete is to be
Fabric should be cut and bent to the tolerances and dimensions tooled or the aggregate exposed, the depth of tooling or exposure
given in BS 8666. must be taken into account. The nominal cover will depend on the
conditions of exposure of the particular piece of concrete and the
cement content and free water/cement ratio of the specified
Prefabricated reinforcement concrete.
It is often more convenient to obtain cages and complex
reinforcement arrangements already assembled from the supplier's Spacers
factory. Delivery can be timed to fit in with the construction Reinforcement should be held off the formwork or blinding (for a
programme but the same requirements for storage on site apply a: slab on the ground) by suitable spacers which should be of the same
for conventional reinforcement. Some of the assemblies are heavy nominal size as the specified cover.
and will need suitable lifting equipment.
Spacers are generally made of concrete, fibre cement or plastic, in
several shapes and various sizes to give the correct cover (Figure 26).

Circular or wheel type spacers are more suitable for reinforcement in

vertical members, such as columns and walls, while the block or
trestle types are more suitable for reinforcement in horizontal

41
 REINFORCEMENT
members. They must be durable, and not cause corrosion of the The reinforcement must be fixed rigidly in the correct position
reinforcement or spalling of the concrete. Those made from (Figure 28) and with the correct cover in such a way that it is not
concrete should be comparable in strength, durability, porosity and displaced during concreting. Top layers of reinforcement in slabs
appearance if the finish of the surrounding concrete is important.
Site-made concrete blocks must not be used.

Spacers and chairs (Figure 27) should be placed at a maximum

spacing of 1 m, or less if necessary. Guidance on using spacers is
given in publication CS 101 and BS 7973.

Figure 28: Reinforcement fixed in place for a slab and column.

Figure 26: Typical spacers for reinforcement. should be well supported on bent reinforcement, or chairs so that
they are not displaced by operatives walking on them or by being
used as a storage area for plant or equipment - both practices
should be discouraged as far as possible. Special care should be
taken in fixing the top tension reinforcement in cantilevers.

References/further reading
BS 4449 : 1997, Specification for carbon steel bars for the
reinforcement of concrete. British Standards Institution, London.

BS 4482 : 1985, Specification for cold reduced wire for the

reinforcement of concrete. British Standards Institution, London.

BS 4483 : 1998, Steel fabric for the reinforcement of concrete.

British Standards Institution, London.

BS 7973 : 2001, Spacers and chairs for steel reinforcement and their
specification. British Standards Institution, London.
Part 1 : Product performance requirements
Part 2 : Fixing and application of spacers and chairs and tying of
reinforcement.

Figure 27: Stool, continuous chair and ring supports for top BS 8666 : 2000, Specification for scheduling, dimensioning, bending
layers of reinforcement. and cutting steel reinforcement for concrete. British Standards
Institution, London.

Fixing reinforcement CS101, Spacers for reinforced concrete. 1989, The Concrete Society,
Crowthorne. 30 pp.
The reinforcement bars should be securely tied together with steel
wire, tying devices or by welding, and care should be taken to Concrete on site - 2: Reinforcement. 1993, British Cement
ensure that projecting ends of ties or clips do not intrude into the Association, Crowthorne. Ref. 45.202. 9 pp.
concrete cover. Welding on site should be avoided if possible, but
provided that suitable safeguards and techniques in accordance
with the manufacturer's recommendations are adopted, it may be
undertaken with the engineer's approval.

Structural connections between two bars can be made by welding

or by the use of mechanical couplers if lapping is not feasible. If
there is any uncertainty about the arrangement of the
reinforcement, or any discrepancy between the bar schedules and
the drawings, the engineer should be consulted.

42
 FORMWORK
The purpose of formwork is to contain freshly placed and The system of falsework supporting the formwork must be
compacted concrete until it has gained enough strength to be self- designed to withstand the loads imposed on it. Tubular steel
supporting; to produce a concrete member of the required shape scaffolding and adjustable proprietary steel props are the most
and size; and to produce the desired finish to the concrete. To common forms of support, although heavy-duty shoring and
achieve this, the general design and construction requirements of specially designed supporting systems are often required. When
formwork are as follows: adjustable steel props are used, they must be installed so that they
n The formwork should be sufficiently rigid to prevent undue are vertical and loaded axially, and the hardened steel pins
deflection during the placing of the concrete. provided by the manufacturers must be used.

n It should be of sufficient strength to carry the working loads Slipforms are occasionally used for walls, lift shafts and building
and the weight or pressure of the wet concrete, and to cores, silos, towers, chimneys and shaft linings. This type of form is
withstand incidental loading and vibration of the concrete. moved almost continuously, usually by means of hydraulic jacks,
n It should be set to line and level within the specified tolerances leaving concrete of the required shape and dimensions behind.
and include any camber that may be required. Slipforming saves time by eliminating the task of striking and
resetting formwork and by allowing continuous concreting, but it
n Joints should be sufficiently tight to prevent loss of water or
is not normally an economic solution for vertical structures less
cement paste from the concrete, which can have a serious
than about 15 m high. The design and operation of slipforms
effect on the appearance of the finished concrete.
require considerable experience and are usually undertaken only by
n The size of panels or units should permit safe and easy handling specialist subcontractors.
using the equipment available on site. The design should
permit an orderly and simple method of erection and striking.
n The arrangements of panels should be such that they are not Design of formwork
'trapped' during striking, and it should be possible to strike side Formwork should be designed to withstand all expected loads.
forms from beams without disturbing the soffit formwork. These include the self-weight, weight of reinforcement, weight of
wet concrete, construction and wind loads, all incidental loads
Types of formwork caused by placing and compacting the concrete, and the
Over the years the number of materials used for formwork has horizontal pressure of the wet concrete against vertical formwork.
grown considerably, although traditional methods using materials Detailed information about these loadings is given in BS 5975,
such as timber are still used. Formwork facing materials include CIRIA Report 108 and the Concrete Society publication, CS030.
timber, plain and resin-faced plywood, steel, alloy, concrete, glass
Particular care is needed to provide an adequate number of form
fibre reinforced plastics (GRP), glass fibre reinforced cement (GRC),
ties where these are used to link together the opposite panels of a
hardboard and expanded polystyrene. In addition, form liners of
wall form. Whereas slightly inadequate design of other elements of
rubber, thermoplastics or other sheet materials, including
the formwork may lead to large deflections or leakage, the failure
permeable liners, to produce controlled permeability formwork
of form ties can more easily cause a dangerous collapse. Failure in
(CPF) systems may be used (see Influence of formwork on page 49).
ties may also occur when they are over-tightened or put into
Some liners are re-usable but others can only be used once.
bending rather than simple tension.
Many systems of proprietary formwork are available, and large jobs
The strength of formwork, although very important, is often
often make use of special formwork designed as a system for that
secondary to its stiffness, which must be sufficient to prevent it
job (Figure 29). Precast concrete, profiled steel decking units, GRC
deflecting significantly under load, otherwise the resulting concrete
panels or other materials may be used as permanent formwork; the
surface will show the deformation. When using plywood, it is
supplier's instructions regarding bearings, supports and ties must
important to recognize that stiffness parallel to the face grain is less
be carefully observed.
than the stiffness at right-angles to it.

Formwork must be watertight, because small leaks lead to

unsightly stains on the concrete surface and large leaks can cause
honeycombing. The use of foamed plastic sealing strip or moisture
curing gunned silicone rubber provides effective means of sealing
joints. Joints may also be sealed by adhesive tape, but it must be
accepted that such joints will be apparent on the finished surface.
Such patterning is generally acceptable in public spaces such as car
parks if it is formalized or set uniformly with the bays of the
structure.

Surface treatment
Where the appearance of the concrete is of importance, it is vital
that care is taken with the surface of the form. All marks on the
form, such as vibrating poker 'burns', as well as varying properties
in the form-face material, for example uneven water absorbency in
timber, will show on the finished concrete. Loose wire and other
debris should be cleaned out of forms prior to concreting; this is
usually done with a compressed air hose. It is particularly
important that all steel particles are removed as they will rust and
spoil the final appearance of the concrete.
Figure 29: Proprietary panel formwork system for a concrete
retaining wall.

43
 FORMWORK
To permit easy striking of the formwork and to reduce the slight movement of the form at a critical time during the
incidence of blowholes, the surface of the form must be coated hardening process; normally it occurs only with impervious form
with a release agent prior to concreting. There are various types of faces. Similar discolouration of concrete placed against a steel form
release agent, the merits of which are summarized in Table 16. The can usually be attributed to the presence of mill scale on the steel.
most useful are chemical release agents, neat oils with surfactants,
and mould cream emulsions. Release agents should be applied to Striking of formwork
give a very light film. A common fault is the use of too much. If it
The period which should elapse before the formwork is struck will
is thin, application by an airless spray is recommended. Thicker oils
vary from job to job and will depend on the concrete used, the
may be applied by brush or cloth and spread as far as possible, all
weather and the exposure of the site, any subsequent treatment to
excess being removed with a cloth. The use of barrier paints
be given to the concrete, the method of curing and other factors.
produces a hardwearing surface and may extend the life of timber
Formwork must not be removed until the concrete is strong
or plywood forms. If paint is not used, three coats of mould oil
enough to be self-supporting and able to carry imposed loads.
should be applied before the form is used for the first time. Some
Thus, the time of striking should be related to the strength of the
barrier paints are not suitable for use with certain tropical
concrete, and obviously soffit forms to beams and slabs must be
hardwoods, and the manufacturer's advice should be sought on
left in place longer than is necessary for the side forms.
this point. To avoid contamination of reinforcement, the release
agent should be applied before the forms are erected, but then it Subject to the requirements of the specification and where no
may be necessary to protect the forms from the weather. other information is available, the periods given in Table 14 under
Strength development may be taken as a general guide for the
Table 16: Types of release agent.
removal of formwork. It should be noted that these periods relate
to concrete made with Portland cement CEM I 42,5N; shorter
Release Comments periods may apply where a more rapid-hardening cement is used,
agent type
but considerably longer periods may be required where the
Chemical Recommended for all types of formwork. concrete contains ggbs or pfa.
release Suitable for high-quality finishes. Based on light,
When using Table 14, if the surface temperature cannot be
agent volatile oils that usually dry on the surface of the
obtained, air temperatures may be used. Alternatively, the tables of
form to leave a thin coating which is resistant to
striking times published in CIRIA Report 1 36 can be used. These
washing off by rain. The dried coating gives a
take into account the grade of the concrete, the cement type, the
safer surface to walk on than an oily film, and
dimensions of the section, the type of formwork, the temperature
the release agent does not then transfer from
of the concrete at the time of placing and the mean air
operatives' footwear onto reinforcement. Rate of
temperature. Shorter formwork striking times are achievable by
coverage is greater than for conventional oils.
measuring the strength development of the in-situ concrete.
More expensive for a given volume but can be
economical if used sparingly. Soffit formwork may be struck when the in-situ strength of the
concrete is 10 N/mm2 or twice the stress to which it will be
Mould Widely used release agent recommended for all subjected, whichever is the greater. The in-situ strength can be
cream types of formwork except steel. Especially assessed by pull-out tests (see page 60) or from cubes cured, as far
emulsion recommended for absorbent forms such as as possible, under the same conditions as the in-situ concrete or by
timber. Suitable for high-quality finishes. Mix temperature-matched curing by which test cubes are immersed in
thoroughly before application and use as water whose temperature is made to copy that of the structure.
supplied without further dilution. Avoid using Proprietary quick-strip systems permit the removal of soffit
emulsions when there is a risk of freezing. formwork without disturbing the propping.
Storage life may be limited. May be spray-
applied with care. When formwork to beam sides, walls and columns is struck at early
ages the concrete will still be 'green' and easily damaged, so extra
Neat oil with A useful general-purpose release agent for all care is required to avoid damage to arrises and other features; this
surfactant types of formwork, including steel. Over- is particularly important during cold weather. Striking must be
application may result in staining of the carried out with care, to avoid damage to arrises and projections,
concrete. Oil film may be affected by heavy rain. and it may be necessary to protect some of the work from damage
immediately after removing the forms.

Unpainted timber forms become progressively less absorbent as Curing should start as soon as the formwork has been removed
the pores of the wood become filled with cement paste during and, if necessary, the concrete should be insulated as a protection
use. This affects the appearance of the finished work - an abrupt against low temperatures. Timber formwork is a good insulator in
colour change will be seen on the concrete - so new and old its own right, so in winter it is particularly important to avoid
materials should not be used alongside each other. Similarly, thermal shock to the warm concrete when timber or insulated steel
patches of new material in old formwork will produce noticeable forms are removed and the concrete is exposed to the cold air. If
colour changes. the formwork is not required elsewhere, it may be convenient to
leave it in place until the concrete has cooled from its high early
In a similar way, forms made from painted timber and various temperature. The formwork must be removed slowly as the sudden
types of plastics having a glazed or glossy surface produce an removal of supports is equivalent to a shock load on the partly
appearance that changes with the number of uses. In this case the hardened concrete. Careful removal is also less likely to damage
surface glaze is reduced by the first use, and subsequent uses the formwork itself.
produce a less highly polished surface on the concrete. Some
plastic-faced plywoods give a similar effect. The first few uses of
these materials can sometimes produce a very hard, dense, and
almost black surface to the concrete. This is probably caused by

44
 FORMWORK CURING
Care of formwork Curing is the process of preventing the loss of moisture from the
concrete while maintaining a satisfactory temperature regime.
Formwork frequently accounts for over a third of the cost of the
finished concrete, so it should be handled with care. The life of Purpose of curing
forms can be extended considerably by careful treatment, thus
The hardening of concrete, the development of strength and
decreasing the overall cost of the job. Rough treatment may make
impermeability depend on the presence of water. At the time the
timber and plywood forms useless after one pour, whereas eight or
concrete is placed there is always an adequate quantity of water
more uses may be obtained by following good site practice.
present for full hydration but it is necessary to ensure that this
All formwork should be cleaned as soon as it has been struck. water is retained so that the chemical reaction continues until the
Timber and plywood forms are best cleaned with a stiff brush to concrete has developed the necessary degree of impermeability
remove dust and grout; stubborn bits of grout can be removed and strength.
using a wooden scraper. With GRP and other plastics, a brush and
The areas most affected by poor curing are the surface zones, and
wet cloth are all that should be needed. The use of steel scrapers
these are critical with respect to durability. In particular, the
should be restricted to steel formwork.
protection of reinforcement against corrosion depends on the
Steel forms should be lightly oiled to prevent rusting if they are not quality of the concrete in the cover. Similarly, abrasion resistance
going to be re-used immediately; similarly, timber and untreated depends on concrete quality in the top few millimetres. If the
plywood should be given a coat of release agent for protection. curing is inadequate the concrete may not be durable nor provide
adequate protection to the reinforcement despite conforming to
Any formwork surface defects such as depressions, splits, nail holes specification in all other aspects.
or other unwanted holes should be repaired and made good.
Curing should be carried out until the capillary voids are
Formwork should be properly stored and protected. Panels and discontinuous, but at present it is not possible to establish the
plywood sheets are best stored horizontally on a flat base, clear of precise times when this occurs. The figures in Table 1 7 (based on
the ground, so that they lie flat without twisting, and should be Table 6.5 of BS 8110) provide a useful guide but are minimum
stacked face-to-face to protect the surfaces. Large panels are values and may need to be exceeded. As indicated by the
usually best stored vertically in specially made racks. Stored minimum periods in Table 1 7, when some of the Portland cement
formwork should be protected from the sun and weather by in a concrete is replaced by ggbs or pfa, curing times are increased
tarpaulins or plastic sheeting. by about 50% to compensate for the lower rate of strength
development.

References/further reading The minimum time required for preventing loss of moisture from
BS 5975 : 1996, Code of practice for falsework. British Standards the surface of the concrete depends on a number of factors
Institution, London. including:
n The type of cement or combination
CIRIA Report 108. Concrete pressure on formwork. 1995,
Construction Industry Research and Information Association, n The cement content and the free water/cement ratio
London. n The temperature of the surface layer
n The ambient conditions
CIRIA Report 1 36. Formwork striking times - criteria, prediction and
methods of assessment. 1995, Construction Industry Research and n The intended use of the concrete.
Information Association, London.
In addition to these factors, ambient conditions after casting will
CS030, Formwork - a guide to good practice. 2nd edn. 1995, vary during the period of curing, so it is recommended that
The Concrete Society, Crowthorne. 306 pp. curing, either by preventing evaporation or by keeping the surface
of the concrete continually damp is carried out for at least:
Concrete on site - 2: Formwork. 1993, British Cement Association,
n 7 days for horizontal surfaces
Crowthorne. Ref. 45.203. 12 pp.
n 3 days for vertical surfaces.

Specific recommendations for particular elements are given on the

following pages.

Concrete also needs to be kept at a favourable temperature -

the lower the temperature, the slower is the rate at which
concrete hardens and develops strength. If the temperature
of the plastic concrete falls below freezing point before adequate
strength has developed, the freezing and resulting expansion
of the water may cause permanent damage. Refer to
pages 38 - 40 for further information relating to cold and hot
weather conditions.

Curing increases resistance to abrasion, so effective curing is

essential for floors and other surfaces subject to wear.
Continuous curing from the time the concrete is placed helps to
ensure a hard,dense surface which reduces the risk of crazing
and dusting, increases impermeability and improves
weathering characteristics. This is of vital importance for the
concrete cover to reinforcement.

45
 CURING
Table 17: Minimum periods of curing and protection.

Type of cement Curing conditions Minimum periods of curing and protection (days)
after casting
Average surface temperature of concrete

5°C to 10°C Above 1 0 ° C t (any temperature

between 5°C and 25 C)

CEM I 42,5N
Average 4 3 60
SRPC t+10

CEM I 42,5R
Poor
CEM I 52,5 6 4 80
t+10
CEM II and CII Average
CEM IIIA and CIIIA
Poor 10 7 140
CEM IV and CIV t+10
All cement types Good No special requirements
NOTE
Curing conditions are defined as:
Good - damp and protected (relative humidity greater than 80%, protected from sun and wind)
Average - intermediate between good and poor
Poor - dry or unprotected (relative humidity less than 50%, not protected from sun and wind)

Early curing will reduce evaporation of water from the surface of

fresh concrete in drying conditions; unless evaporation is checked,
'plastic' cracks may appear while the concrete is still setting. This
was discussed on page 19 under Plastic cracking.

Methods of curing
Methods of curing can conveniently be considered in two groups:
n Those which keep water or moisture in close contact with the
surface of the concrete, such as ponding, spraying/sprinkling,
damp sand and damp hessian
n Those which prevent the loss of moisture from the concrete,
such as plastic sheeting, building paper, leaving the formwork
in place, and sprayed-on curing membranes.

Although tests have shown that the methods in the first group are
the most efficient and may be appropriate for some work, they
tend to suffer from the practical disadvantages of being expensive
in both materials and labour and, perhaps more importantly, it is
difficult to ensure that they are done properly; damp hessian, for
example, is seldom kept continuously damp.

Curing menbranes
Curing membranes are liquids sprayed onto either fresh or
hardened concrete surfaces, which dry leaving a thin film of resin
to seal the surface and reduce the loss of moisture (Figure 30).
They can be used on both horizontal areas of fresh concrete and
vertical surfaces after the removal of formwork. Most sprayed-on
curing compounds should be applied immediately after the water
sheen which results from bleeding has evaporated. Figure 30: Spraying a reservoir wall with a curing membrane.

The resin film remains intact for about four weeks, but then Most proprietary makes are available in various grades, a standard
becomes brittle and peels off under the action of sun and weather. grade usually having what is termed a curing efficiency of 75%,
Curing membranes were developed for roads and airfield and a better grade one of 90%; both are pale amber/straw in
pavements which are difficult and impractical to cure satisfactorily colour. In addition, both grades are usually available with a white
by any other means, and although they are now used extensively or aluminized pigment, or containing a fugitive dye. The white and
for curing structural concrete there are some occasions when they aluminium-powder pigmented types are specifically for external
may not be suitable. paved areas, where the pigments reflect the sun's rays so reducing
the amount of heat absorbed by the concrete. Those containing a
fugitive dye make it easier to see where the membrane has been
applied and that it has been applied uniformly; the dye quickly

46
 CURING
disappears after application and will not stain the surface provided Cement-sand screeds
that it is not applied to a dry concrete surface. Special non-toxic Curing compounds are not recommended because of the need for
compounds are available for use on concrete that is to contain floor covering materials to bond to the screed. After laying, a
drinking water. Pigmented and aluminized varieties should be cement-sand screed should be kept continuously damp for at least
agitated frequently to ensure uniform dispersion of the solids. seven days, preferably by covering with polythene sheeting.

Generally, and certainly in the UK, the pigmented higher-efficiency

grades should be used for external paved areas, and the non-
Vertical surfaces
pigmented lower efficiency grades on structural concrete. In The curing of columns, walls and beam sides is more difficult than
tropical climates, the higher-efficiency grades should be used for all horizontal surfaces. Curing membranes can sometimes be used but
applications. are not generally suitable when any subsequent treatment or
rendering is to be applied.
On any job it is therefore essential to make sure that the right type
of curing membrane is used. Further information is available in the While in position, formwork protects the concrete against loss of
BCA publication Concrete on site - 6: Curing. moisture and it is only after striking that further curing may be
necessary.

Horizontal surfaces When formwork has been kept in position for four days there is
It is essential for most horizontal surfaces to be well cured. This is usually no need for further curing of concrete made with CEM I,
particularly important where the concrete will be trafficked, either even in drying conditions. Concretes containing pfa or ggbs may
by foot or by vehicles. require a longer moist-curing period, see Table 17 on page 46.

Curing should always start as soon as possible after the concrete For concrete surfaces not exposed to the weather, which are to
has been compacted and finished, generally within 30 minutes of receive an applied decorative treatment such as rendering, plaster
the water sheen (bleed water) disappearing. or paint, or will be tooled or abrasive blasted, no further curing of
CEM I concrete is usually necessary, however soon the formwork
Road slabs and other external concrete (paths and drives) is struck.
Major concrete roads are usually sprayed with a curing membrane
by a machine that is part of the paving train. This is not discussed In general, further curing of vertical surfaces of concrete is required
in this publication. For smaller paved areas where semi-manual in temperate climates only when the formwork is struck within four
methods of construction are used, a curing membrane can be days of placing the concrete and either:
applied using a hand-operated spray of the garden type. A white n The surface will be permanently exposed to the weather; or
pigmented or aluminized super grade of compound should be
n The surfaces have to be uniform in colour, e.g. a number of
used, taking particular care to ensure an even coat is applied,
similar columns or series of pours making up a wall.
especially in strong winds.

Although curing membranes can be used for small areas of

Exposed concrete
external concrete, it may be more convenient, and just as good, to All vertical surfaces of concrete, including white and coloured,
use polythene sheeting. The covering should be kept in place for at which will be permanently exposed to the weather need extra care
least seven days, paying particular care at the edges of sheets, as with curing. A well-cured surface will be more impermeable and
there is a tendency for the concrete to dry out here due to wind better able to withstand the action of freezing and thawing and
tunnelling effects. wetting and drying; surface crazing will also be reduced. Good
curing will also help the long-term appearance of the concrete by
Slabs to receive a screed reducing dirt collection.
A curing compound should not be used when a slab will later
receive a cement-sand levelling screed, a wearing screed or any All concrete surfaces that will be permanently exposed to the
other bonded covering, as the bond may be affected. weather, including those to be abrasive blasted or tooled, should
be cured for at least seven days. Although polythene sheeting can
Polythene sheeting will usually be the most convenient method of be used for this, it will usually be more convenient to use a
curing. Covering with damp hessian is not recommended because sprayed-on curing compound.
of the difficulty of keeping it moist in dry weather, especially
overnight and at weekends. The concrete should be covered as
soon as possible after finishing, especially in a drying wind. Uniformity of colour
The colour of concrete can be affected by the age at which
Direct-finished concrete and wearing screeds
formwork is removed and by the weather, both at the time of
Power-floated finishes and wearing screeds have to be hard-
striking and subsequently.
wearing and abrasion-resistant, and particular attention to curing is
essential. Where uniformity of colour is important, for example with as struck
'fair-faced' and board-marked surfaces, either:
After the final trowelling (either by hand or by power trowel) the
surface should be firm enough for immediately covering with n The formwork should be left in position for four days, in which
plastic sheeting or similar, or applying a curing compound. case no further curing is usually necessary; or
n When the formwork is removed in less than four days the
The surface should not be allowed to dry out before curing. concrete should be covered or wrapped in polythene sheeting
Polythene sheeting should be kept in place for at least seven days. for at least another three days. Alternatively, but only if the
Some loss of moisture may occur at the edges and at laps and it concrete will be permanently exposed to the weather, a curing
may be necessary every other day to turn back the sheeting and compound may be applied.
spray with water, and then replace the sheeting.
Damp hessian is not recommended because it contains a dye,
Resin-based hardeners also act as curing agents.
which can stain the surface. It is also ineffective if it dries out.

47
 CURING CONCRETE SURFACE FINISHES
Visual concrete, which is designed to be seen in a completed
White and coloured concrete building or structure, requires special consideration at an early
It is not advisable to use a curing compound on white or coloured stage, since its appearance will largely determine the quality of the
concrete if there is a risk of discolouration. whole job. To produce concrete with a good finish, the formwork,
the concrete itself and the way it is placed and compacted must all
Polythene sheeting is the preferred material because it cannot stain
be to a consistently high standard, following the guidelines
the concrete. Sheeting that is firmly fixed and left in place also
outlined in the preceding sections.
protects the surface from dust caused by other site work;
subsequent removal of dirt and stains is both time-consuming and
expensive.
Range of finishes
There are two main types of finish:
References/further reading n Those produced direct from the formwork, often called 'as
BS 8110 : 1997, The structural use of concrete. British Standards struck' finishes, comprising plain, smooth finishes, and textured
Institution, London. and profiled finishes such as board-marked concrete and ribbed
or striated and modelled surfaces
DD ENV 1 3670, Execution of concrete structures. British Standards
n Those produced indirectly by further treatment after the
Institution, London.
formwork is removed, including exposed-aggregate and tooled
Part 1 : 2000, Common rules.
finishes.
Concrete on site - 6: Curing. 1993, British Cement Association,
The two types are often combined to produce, for example, a
Crowthorne. Ref. 45.206. 16 pp.
striated and tooled finish, or a modelled exposed-aggregate finish
(Figures 31 and 32).

Figure 3 1 : An example of an in-situ exposed-aggregate finish

obtained with the aid of a surface retarder.

Figure 32: Examples of profiled direct finished and abrasive-

blasted finishes.

48
 CONCRETE SURFACE FINISHES
Standard of finish surfaces is drained away and blow holes of entrapped air are also
absorbed into the fabric. The resulting concrete surface benefits
Concrete can be produced within close dimensional tolerances, both from a blemish-free appearance and, by virtue of having been
but the inherent variations in colour and the presence of some effectively dewatered, the free water/cement ratio in the cover
small air bubbles or blow-holes on the surface produce a finish zone is reduced, making the concrete more durable.
which is seldom entirely blemish-free; it is unrealistic to expect
perfection in appearance. It is essential to have a full-size section of
Concrete for high quality finishes
part of the structure produced as a sample in order to avoid later
dispute about the quality of the work. Guidance on the standards In order to be able to control the quality of the concrete to the
of surface finish is given in the BCA series of publications standard required for visual concrete, it is essential to have the
Appearance Matters and Concrete Society Technical Report 52, coarse aggregate stocked and batched separately in two sizes, one
Plain formed concrete finishes. To judge the quality of the finish, the of 20 - 10 mm and the other 1 0 - 5 mm and to combine them in
sample should be viewed at the same distance from which the job the concrete in the proportions required, according to the
will be seen. following guidelines:
n A minimum cement content of 320 kg/m3, but preferably not
To provide a reference to the surface finishes Types A and B as less than 350 kg/m3
specified in BS 8110, sets of panels have been manufactured and
n A sand content of not more than twice the weight of cement
installed at seven regional centres throughout the UK. They can be
visited freely and should reduce the conflict that can occur due to n Total aggregate not more than six times the weight of cement
differing interpretations of the standard. For details see n Avoidance of an excessive quantity of 10 - 5 mm aggregate
www.construct.org.uk. n Free water/cement ratio of 0.50 or less
n Consistence class normally S2 (50 - 90 mm slump).
Influence of formwork
A concrete surface will reproduce every detail of the form surface The proportion of 1 : 2 for the cement: sand fraction is based on
from which it was cast. The formwork must remain watertight M grade sand. In the case of finer sand, the quantity should be
against the pressure of concrete in order to prevent leakage. Any reduced to compensate for its increased surface area.
loss of water from the fresh concrete will result in a dark area on
Trial mixes and sample panels are essential to determine the
the completed surface, which extends into the concrete and
suitability of the concrete for the particular job in question.
cannot usually be removed by tooling - in fact more often than
not tooling makes the blemish more obvious. Caskets may be
necessary to maintain watertightness at the joints and, in the case
Supervision and workmanship
of exposed-aggregate and tooled finishes, the formwork joints may A high level of supervision is essential, with special attention to the
be sealed with adhesive tape, joints in the formwork, even well following:
made ones, will show in the finished concrete; the position of n The formwork should be checked for alignment, level and
formwork joints should therefore be planned. plumb, for bracing and tightness of all fixings, joints must be
watertight to prevent leakage. The formwork should also be
The standard of finish specified and the number of uses required
checked for accuracy of dimensions
for the formwork usually affect the choice of facing material. The
surface characteristics of the formwork have a profound effect n Reinforcement needs to be checked so that the correct cover
upon the appearance of the concrete. Unsealed plywood - because will be achieved
it is slightly absorbent - will cause relatively few blow-holes, but n The first part discharged from a truck-mixer should either be
variations in the absorbency of the timber grain will produce used in a visually unimportant part of the work, or set aside and
corresponding variations in the colour of the concrete. On the used later after some of the more homogeneous concrete has
other hand, impermeable formwork will give rise to more blow- been placed
holes, although the colour may be more uniform. A further n Once begun, placing should be continuous until the pour has
complication is that concrete cast against smooth impermeable been completed; continuity in the supply of concrete is
surfaces may have a dark, almost black, surface (as described under therefore essential
Surface treatment on page 43). The colour variation may be
n Placing should proceed at a uniform rate of not less than 2 m
reduced if the formwork has a matt surface which will retain the
height per hour. This requires careful consideration and
release agent, rather than a smooth shiny one from which the
planning of pour sizes
release agent is removed during placing and compaction of the
concrete. n The actual rate of placing needs to be restricted by the rate at
which the concrete can be compacted.
A thin coating of release agent should be applied to the formwork
each time before the concrete is placed to prevent adhesion, and
thus make formwork removal easier when the concrete has Plain smooth finishes
hardened. Release agents must be applied sparingly otherwise the Contrary to a widely held belief, plain smooth surfaces are the
surface of the concrete could be adversely affected. There are most difficult to produce to a consistently high standard, because
several different types of release agent and it is important to use of the inherent variation in the constituent materials and the fact
one that is suitable for the form material in question. that even the smallest blemishes are readily visible on plain
Recommendations of types and applications are given earlier (page surfaces.
43) under the heading Surface treatment.
There is still need for a good finish, even when grit blasting or
The quality of surface finish may be significantly improved with the bush hammering is to be used to expose the aggregate. It is not
use of a controlled-permeability formwork (CPF) system; see Types true that bush hammering will make a poor finish acceptable; in
of formwork page 43. Here a patent woven synthetic fabric is practice, any tooling of a poor finish only serves to accentuate the
securely fixed to the form face and the concrete cast in the normal surface defects.
way. Bleed water that would cause unsightly marking of vertical

49
 CONCRETE SURFACE FINISHES
quicker and less sensitive to variations in the finish than other
Textured and profiled methods of exposing the aggregate.
finishes Guidance on exposed aggregate finish is given in the BCA
The simplest textured finishes can be obtained by using formwork publication Appearance Matters - 8.
made from rough-sawn boards, so that the imprint of the wood
grain is reproduced on the surface of the concrete. To be effective,
great care is necessary in the design and fabrication of the Tooled concrete finishes
formwork, particularly the joints and any fixings.
Tooled finishes are not produced until the concrete has achieved a
Formwork linings are made in a variety of patterns from materials compressive strength of at least 20 N/mm2, so this particular
such as expanded polystyrene (which can only be used once) and operation can often be left until near the end of a contract. Tooling
flexible rubber-like plastics (which will give repeated use). removes a layer of concrete from the surface - typically 5 mm with
Manufacturers of these materials usually provide guidance as to bush hammering and 10 mm or even 20 mm with point tooling -
their use. and reveals the colour but not necessarily the shape of the
aggregate. Care is required when working near the edge of an
Ribbed finishes are usually produced by fixing timber battens element, and it is usual to provide a plain margin that is left
securely to the formwork; fixing should be by both gluing and untooled to minimize the risk of breaking the edges.
screwing to prevent grout loss under the battens. The battens must
have a generous taper ('draw') to enable them to be struck from Guidance on tooled finishes is given in the BCA publication
the hardened concrete. The formwork for intricately modelled Appearance Matters - 9.
surfaces is often made from glass fibre reinforced plastics.

Guidance on textured and profiled finish is given in the BCA Remedial work
publication Appearance Matters - 7. Except for the need to cure the concrete, plain and textured
finishes should not need any further attention once the formwork
has been removed. However, it will be necessary to fill any tie-bolt
Exposed-aggregate finishes holes, and these are best dealt with as soon as possible so that the
Coarse aggregate is exposed by removing cement and sand mortar mortar gains strength at the same rate as the concrete itself.
from the face of the concrete. The variability in the distribution of Similarly, if blow-holes are to be filled, these should be 'made-
the coarse aggregate in ordinary concrete tends to give an uneven good' as soon as possible.
appearance when that aggregate is exposed. It is therefore
necessary to use a special prescribed concrete using gap-graded Making-good is a skilled job that should not be entrusted to a
aggregates containing as large a proportion as possible of the general labourer. The mortar that is used has to be blended by
coarse aggregate, and it should be noted that such a concrete will introducing a proportion of white cement, or some white
require particular care in transporting and placing. For example, all limestone sand, to match the grey of the concrete and smoothness
the coarse aggregate should be between 2 0 - 1 0 mm in size of a formed face.
instead of 20 - 5 mm. Trial mixes will have to be produced to
Remedying a fault such as a locally honeycombed area, which may
determine the mix design, firstly to achieve the finish and secondly
have to be cut back to sound, well compacted material before
to satisfy durability and compressive strength requirements.
being reinstated, may be satisfactory from a structural point of
One method of exposing the aggregate is to coat the formwork view, but is unlikely to be acceptable in terms of appearance
with a chemical retarder, which prevents the cement in contact because the patch will often tend to weather differently and
with it from hardening. When the formwork is removed the surface become more obvious with time. It is therefore all the more
mortar is brushed away to uncover the aggregate embedded in the necessary to take particular care in the production of visually
hardened concrete. Timing of the operation is fairly critical since important sections of the work.
some retarders will delay the hardening only while the surface is
Further information is available in the BCA publication Concrete on
tightly covered. When the formwork is removed and air gets to the
site - 8: Making good and finishing.
surface, the mortar soon hardens. It is therefore important to
organize the work so that the surface can be treated within a short
time of the formwork being stripped. Brushing should begin near References/further reading
the bottom of the wall or column, because the concrete will be
TR 52, Plain formed concrete finishes. 1999, The Concrete Society,
harder there than towards the top.
Crowthorne. 48 pp.
The surface mortar can also be removed from hardened concrete
Appearance matters. British Cement Association, Crowthome.
by abrasive blasting. In this technique, compressed air is used to
carry a stream of selected grit along a flexible hose to a nozzle, 1: Visual concrete - Design and production. 1988,
where it emerges as a jet and is directed at the surface of the Ref. 47.101.26 pp.
concrete. The operator must wear a helmet with a piped air supply 7: Textured and profiled concrete finishes. 1986. Ref. 47.107.
to protect him from the dust which is created, and the working 12 pp.
area has to be screened off to avoid endangering other site 8: Exposed aggregate concrete finishes. 1985. Ref. 47.108. 16 pp.
personnel and passers-by. A deep or heavy exposure, in which the
9: Tooled concrete finishes. 1985. Ref. 47.109. 8 pp.
coarse aggregate is exposed by a third of its depth, is best carried
out when the concrete is about two days old. If it is left until the Concrete on site - 8: Making good and finishing. 1993, British
concrete is harder, it will take much longer to produce the finish. A Cement Association, Crowthorne. Ref. 45.208. 12 pp.
medium-depth finish can be carried out at three to four days after
placing the concrete, and a light abrasive-blast finish at seven days
or later. The operator must have considerable experience of
working on concrete but, given this, abrasive blasting is often

50
 FLOOR FINISHES
A good-quality concrete, well laid and finished makes a satisfactory The concrete is allowed to harden and the surface is then ground
floor for many purposes. The concrete surface can be smooth, to a coarse 'sandpaper' texture using a low-speed grinder to
easily cleaned, have good resistance to abrasion, be non-slip and remove the top 1 - 2 mm of laitance and minor irregularities left
have a low maintenance cost. These properties depend largely on during finishing.
the choice of suitable materials, concrete quality and good
workmanship in construction and finishing. The grinding is not intended to remove gross inaccuracies of level.
The age of the concrete at grinding depends on the gain of
concrete strength, but for maximum economy grinding is usually
Choice of finish carried out between one and seven days after laying, as soon as
the concrete can be ground without tearing sand particles from
The finish of a concrete floor should be chosen after considering
the surface. Some grinders operate dry, but with the bigger
the type of traffic and loading, impact abrasion and chemical
machines the concrete is usually well wetted.
resistance, and such factors as hygiene, dust prevention,
slipperiness and decorative treatment. Guidance on suitable
finishes for various duties is given in a BCA publication Power Vacuum dewatering
trowelling and skip floating. The problems of timing power trowelling - particularly in cold
weather - can be largely overcome by using the vacuum
In many industrial situations a structural slab of adequate quality
dewatering process immediately after initial compaction and
may be direct-finished to give a satisfactory wearing surface. The
levelling of the concrete slab or wearing screed. The concrete
finish may be produced by power trowelling (or hand trowelling if
surface is covered with a vacuum mat, incorporating filter layers,
the operatives are sufficiently skilled) or early-age power grinding.
which is connected to a vacuum pump. By applying a vacuum for
Whichever finish method is used, to achieve good regularity of the
about three minutes per 25 mm depth of concrete, excess water is
final surface it is essential to provide accurately set and levelled
extracted, causing the concrete to stiffen rapidly. The initial power
square-edged side forms, and to give careful attention to concrete
floating can then take place within an hour of the concrete being
placing, compaction and levelling. Compaction and levelling can
placed. The process also improves the wear resistance and general
be done efficiently and speedily by using a double-beam vibrator
durability.
or a tri-screed (known as a razorback) and flat surfaces can be
achieved, even on steep slopes, with a rotating striker tube.
Applied concrete wearing screeds
In some heavy industrial situations, especially where fork-lift trucks Where wearing screeds are considered necessary, it is preferable
with solid wheels are used and where heavy abrasion or impact that they are laid monolithically with the base concrete, i.e. within
and chemical attack will occur, special applied wearing screeds three hours of placing the base. Where screeds are laid separately
may be necessary. and bonded, experience has shown that there is a high failure rate,
mainly due to lack of care in bonding techniques. Where separate
Power trowelling bonding screeds must be used, an extremely high standard of
A power-trowelled finish is obtained by first using a power float to workmanship is necessary to avoid curling and hollowness of the
smooth and close the previously levelled concrete surface after it screed through loss of bond.
has stiffened sufficiently - about three hours after laying. After a
further delay to allow excess surface moisture to evaporate, the
slab surface is further smoothed and made dense with a power
Curing
trowel ( Figure 33). A power float has a rotating circular disc If a hard-wearing surface is to be produced, the concrete surface of
attachment or large flat individual blades. A power trowel has a direct-finished slab or wearing screed must be continuously cured
smaller, tilted individual blades and is used for final finishing. The for at least seven days after being finished.
timing of each application is critical.
An efficient and economical method of curing is to cover the floor
with plastic sheeting held down in close contact with the surface.
Alternatively, a proprietary sprayed resin curing membrane may be
applied, provided it is compatible with any later surface sealing
and hardening process which may have been specified to reduce
the risk of dusting. Curing techniques are dealt with on page 46
under Methods of curing.

References/further reading
BS 8204 : Part 2 :1999. Screeds, bases and in-situ floorings. Concrete
wearing surfaces. Code of practice. British Standards Institution,
London.

CSTR 34, Concrete industrial ground floors - A guide to their design

and construction. 1994, The Concrete Society, Crowthorne. 172 pp.

Figure 33: Power trowelling an industrial floor slab. Power trowelling and skip floating. 1999. BCA Library Reprint, British
Cement Association, Crowthorne. 12 pp.
Power grinding
Floor levelling screeds. 1997, British Cement Association,
Power grinding is a finishing technique intended to provide an
Crowthorne. Ref. 48.061. 16 pp.
acceptable and durable concrete wearing surface without further
treatment. After the concrete has been fully compacted and
accurately levelled with a double-beam vibrator, it is further
smoothed with a large, metal skip float attached to a long handle.

51
 TESTING CONCRETE & CONCRETING MATERIALS
Most tests on concrete and concreting materials fall into one of the have been layer-loaded. When sampling from a stockpile,
following categories: representative samples should be obtained by first digging holes of
1. Tests, carried out mainly in a laboratory, to assist in deciding various depths from the top.
whether particular sources of material are suitable for concrete Table 18: Minimum number of sampling increments for aggregates.
and conform to the specification requirements and, if so, in
what proportions they should be combined to produce
concrete of the required properties. In the case of site-mixed Nominal Minimum number of Minimum
concrete, these will usually consist of tests on the aggregate size of sampling increments sample size for
followed by trial mixes to establish the proportions for the aggregate normal density
various concretes to be used. For ready-mixed concrete, Dmax Large scoop Small scoop aggregate
suppliers normally provide mix design certificates for the
20 mm and 20 - 50 kg
different concrete classes specified along with information
larger
about the aggregates and their properties.
2. Routine on-site tests for both control and conformity. 5 mm to 10 - 25 kg
Aggregates are tested for grading and moisture content in the 20 mm
case of site mixing, and both site- and ready-mixed concrete
are tested for consistence and strength. Site tests are reasonably 5 mm and 10 10 10 kg
simple and, although staff do not need special skills, they must smaller half scoops
know the Standard requirements for those tests which they
have to perform. Meaningful results can be obtained only if The bulk sample usually has to be reduced to a smaller quantity,
tests are carried out strictly in accordance with the relevant depending on the amounts required for the particular tests. A
British Standard method. sample divider (riffle box) is the most convenient method. This is
designed so that material poured in the top is divided
This section concentrates on the site tests, though a few references approximately equally and diverted to two sides; material to one
are made to laboratory tests that, on occasions, may be used in the side of the box is discarded and the remainder tested or divided to
field. a smaller sample (Figure 34). Coarse aggregates may be divided
while damp, but sand should be surface-dry.
Detailed descriptions of the methods of sampling and testing are
given in a number of British Standards that are listed under
References/further reading.

Sampling materials
The object of sampling is to produce a truly representative quantity
of the consignment being sampled and of sufficient quantity for
the tests required. Aggregate and concrete are heterogeneous
materials, so great care is needed in sampling to ensure that the
sample is truly representative if reliable test results are to be
obtained. Materials being delivered to site in relatively small lots
are best sampled during delivery to stockpiles.

Cement
The requirements for sampling cement are given in BS EN 196.
The manufacturer supplies test reports on a regular basis, which
should be studied by the concrete producer. Cement testing is
rarely required on site.

Aggregates
Details of sampling aggregates are given in BS 812 : Part 102 and
BS EN 932-1.

The bulk sample of each type or size of aggregate to be tested

Figure 34: Dividing a sample of aggregate with a riffle box.
should be obtained by collecting increments (scoopsful) to provide
the quantity required for all the tests to be made. The minimum Quartering is the alternative method where a sample divider is not
number of increments should not be less than those shown in available. The bulk sample should be shovelled to form a cone, and
Table 18. turned over to form a new cone, this being done three times. The
third cone should be flattened to an even layer 7 5 - 1 0 0 mm thick
Sampling may have to be done in a wide variety of conditions and
and then divided into four equal quadrants with two diagonally
it is not possible to describe in detail the procedure for obtaining
opposite quarters being discarded (Figure 35). The procedure is
increments in all circumstances. When sampling from heaps, the
then repeated on the remainder until the desired size of sample
increments should be taken from different places in the stockpile,
removing at each position the top 150 mm before digging in the remains. When samples of aggregates have to be despatched to a
scoop. For material being loaded or unloaded from a vehicle, or laboratory, they should be packed securely in containers that will
being discharged from a conveyor belt, the increments should be prevent loss of fine dust and damage in transit, and should be
taken at fairly regular intervals distributed during the movement of clearly labelled. Each sample should be accompanied by a
the quantity being sampled. It is virtually impossible to obtain a certificate, stating that sampling was carried out in accordance
representative sample from lorries, before or after discharge, which with the relevant standard.

52
 TESTING CONCRETE & CONCRETING MATERIALS

Figure 35: Reducing a sample of coarse aggregate by quartering

Scoop dimensions
Dimension Small scoop Large scoop
Concrete
Use for sampling Use for sampling
Correct sampling of concrete is essential to obtain representative
aggregates only aggregates and
test results of a batch of concrete. Sampling is fully described in
concrete
BS 1881: Part 101 and BS EN 12350-1, which also give the
quantities required for the different tests. Whenever possible, the
Length 200 mm 250 mm
sampling should be done when the concrete is moving in a
stream, such as when it flows down the discharge chute of a mixer Diameter 100 mm 125 mm
or is being conveyed on a belt. Concrete may be sampled from a
stationary lorry or heap, but this method is less satisfactory.
Concrete cannot be sampled satisfactorily from a discharging Figure 36: Sampling scoop.
tipper lorry or dumper.
Where the point of mixing and the point of placing are some
A sample consists of a number of standard scoopsful taken from a distance apart, there is the choice of taking samples at either place.
batch; Table 19 gives the required number of scoopsful for some of Sampling and testing at the mixer has the advantage of enabling
the more usual tests. Figure 36 shows a suitable scoop that will adjustments to the concrete to be made more quickly, but
provide about 5 kg of normal-weight concrete. As a general rule, consistence tests can be more easily related to the placing
the sample size should be about 1½ times the quantity required conditions if done at the point of placing. On some jobs it may be
for testing. useful to carry out a few tests at both places so as to ascertain, for
example, the change in consistence during transport in hot
Table 19: Quantities of concrete required. weather or when long delays, of half an hour or so, occur between
mixing and placing.
Test or specimen Number of standard
In the case of ready-mixed concrete an alternative method of
scoopsful
sampling is permitted for the slump test only. For this, six standard
scoopsful should be collected in a bucket or other suitable
Slump 4
container after about 0.3 m3 has been allowed to discharge, so
Compacting factor 6 that the load can be tested before the main discharge takes place.
Degree of compactability 12
For all tests the sample, consisting of a number of scoopfuls in one
Air content 4 or more buckets, will require thorough mixing. This should be
100 mm cube (per pair of cubes) 4 done by emptying it from the container onto a non-absorbent
surface, preferably a 900 mm square metal tray, and shovelling it
150 mm cube (per pair of cubes) 4
to form a cone, which should be turned over to form a new cone
NOTE three times. When forming the cones each shovelful should be
Take sufficient scoopsful for tests to be made on different placed on the apex of the cone so that the portions that slide
samples down the sides are distributed as evenly as possible. BS 1881:
Part 101 requires each sample to be accompanied by a certificate
The batch should be nominally divided into a number of parts from the person responsible for taking the sample stating that the
equal to the required number of scoopsful. For samples taken from sampling was done in accordance with British Standard.
a moving stream, such as a batch-mixer or ready-mixed concrete Appendix 2 shows a suitable certificate.
truck-mixer, the scoopsful should be taken at equally spaced
intervals; avoiding the very first and very last parts of the
discharge. Thus, when four scoopsful are needed to make up the
Testing materials
required test sample, they should be taken about the time when Cement
one-fifth, two-fifths, three-fifths and four-fifths have been The testing of cement is done in accordance with the standard
discharged, the scoop being passed through the whole width and procedures in BS EN 196, which is published in different parts for
thickness of the stream in a single movement. the various physical and chemical tests. The British Standard -
which now implements the European Standard - for cement
When sampling from lorries or heaps, the scoopsful should be
testing covers a wide range of properties including chemical
distributed through the depth of concrete as well as over the
analysis, fineness, strength, setting time, soundness and, in special
exposed surface.
cases, heat of hydration.

53
 TESTING CONCRETE & CONCRETING MATERIALS
Aggregates upon the grading analysis (see below) to show whether there is an
excess of fine dust in the material. Problems arise, however,
For good quality control, it is important to ensure that the
because of the tendency for fines to adhere to the rough surface of
aggregate is clean and does not contain any organic impurities
crushed coarse aggregate.
which might retard or prevent the setting of the cement, and that
the proportions of the different sizes or particles within a graded Similarly, there is no suitable site test for the cleanness of a gravel
material remain uniform. coarse aggregate. It is important to ensure that aggregate particles
are not coated with clay and that lumps of clay are not mixed in
Cleanness
the aggregate. The presence of clay indicates that the aggregate
Accurate tests for determining the proportion of fines (clay, silt and
has not been washed adequately before delivery or that the
dust) in sand and coarse aggregates are given in Standards, but
aggregate has subsequently become contaminated.
these tests are only suitable for the laboratory. On site, cleanness
can be assessed visually, although for natural sands the 'field settling' Accurate determinations of the fines content in aggregates are
test will give an approximate guide to the amount of fines; the test made in laboratories using either sedimentation or (more usually)
is not appropriate for coarse aggregates or for crushed-rock sand. decantation or wet sieving methods in accordance with Standard
procedures.
The test entails placing about 50 ml of a 1% solution of common
salt in water (roughly two teaspoonsful per litre) in a 250 ml Organic and other impurities
measuring cylinder. Sand is then added gradually until the level of Coarse aggregates from any source, and crushed-rock sands, are
the top of the sand is at the 100 ml mark, and more solution is unlikely to contain organic impurities, though natural sands may
added to bring the liquid level to the 150 ml mark. The cylinder is do so. Current aggregate standards include tests for organic
shaken vigorously, and the contents allowed to settle for three impurities. If there is reason to suspect the presence of organic
hours. The thickness of the layer of fines that settles above the sand impurities that could retard the hydration of the cement, the
(Figure 37) is then measured and expressed as a percentage of the effects should be determined by performance tests on concrete
height of the sand below the layer. The amount of fines in the sand made with the aggregate in question.
is compared against previous test results to give a warning of
changes in cleanness. Where appearance is an essential feature of the concrete,
aggregates should be selected from sources known to be free from
materials such as iron pyrites or particles of coal that could mar the
surface. The only guide is a knowledge of the source and of similar
work that has been carried out with the aggregate in question.

Sieve analysis
The grading of an aggregate is found by passing a representative
sample of dry aggregate through a series of sieves, starting with
the largest mesh. If the sieving is carried out by hand, each sieve is
shaken separately over a clean tray for not less than two minutes.
For many routine purposes mechanical sieving is advantageous,
but if this method is used care should be taken to ensure that
sieving is complete.

The material retained on each sieve, together with any material

cleaned from the mesh, is weighed and recorded. The amount
passing each sieve is then calculated as a percentage by weight of
the total. Table 20 gives an example of a method for recording a
sieve analysis and calculating the percentage passing each sieve.
Comparison with Table 6 on page 12 will show that this sample is
a sand falling within grading limit M.

Table 20: Example of the method of recording sieve analysis of

sand.

BS sieve Mass Total mass Percentage

size retained on passing passing
each sieve each sieve each sieve
Figure 37: The field settling test for fines in sand. (g) (g)
10 mm 0 275 100
If a measuring cylinder is not available, a jam jar or bottle filled to a
depth of 50 mm with sand and to a total depth of 75 mm with the 5 mm 8 267 97
salt solution will give comparable results if the contents are allowed 2.36 mm 36 231 84
to settle for three hours.
1.18 mm 33 198 72
The field settling test gives only an approximate guide. Sands 600 µm 38 160 58
apparently containing large amounts of fines cannot be regarded as 300 µm 91 69 25
having failed to comply with the specification, and further
laboratory tests to assess their suitability must be carried out. It is, 150 µm 47 22 8
however, a useful and quick way of detecting changes in the Sieve pan 22 0 0
cleanness of sand. Total 275
There is no suitable site test for the cleanness of coarse or all-in
aggregates or of crushed-rock sand, and reliance is usually placed

54
 TESTING CONCRETE & CONCRETING MATERIALS
Sieving will not be accurate if there is too much material left on at 105±5°C in an oven overnight. An important point to note is
any mesh after shaking. The maximum weights of aggregate to be that the SSD moisture content must be used in conjunction with a
retained on a sieve to avoid overloading are given in BS 812. specified saturated surface-dry water/cement ratio referred to as
the 'free' water/cement ratio; similarly, the total moisture content
The size of the sample tested depends upon the maximum size of must be used with a 'total' water/cement ratio.
the aggregate. For nominal 40 mm aggregate the sample should
weigh at least 5 kg, for 20 mm at least 2 kg, for 10 mm at least Other methods. Most of the other methods of determining the
0.5 kg and for sand at least 0.2 kg. moisture content of an aggregate are proprietary methods and
instructions for carrying out the test are supplied with the
The test results can be plotted on a chart similar to that shown in apparatus.
Figure 2 on page 11 so that the specified gradings and the sample
gradings can be more easily compared. It should be noted that the In the calcium carbide method, using the 'Speedy' apparatus, a
points representing the percentage of material passing the various sample of aggregate is mixed with an excess of calcium carbide in
sieve sizes are joined by straight lines and not by curves. Although a sealed metal flask.
the BS 812 sieve test should be made on dried samples, an
approximate grading, accurate enough for routine site testing, The pressure produced by the acetylene liberated in the reaction
may be obtained by sieving coarse aggregates in a damp between water and carbide is related to the moisture content,
condition. which can be read off a dial. This test is very quick, but the size of
sample is small and two or three samples may be advisable to give
Moisture content a reliable measure of the moisture content in a stockpile of
The purpose of measuring the moisture content of aggregate is to aggregate.
enable an estimate to be made of the quantity of water contained
Care must be taken when releasing the pressure to avoid any
within it so that the water added at the mixer can be adjusted to
possible source of flame as acetylene is flammable and can be
control the required free water in the mix; as mentioned on
explosive. There must be no smoking during this test.
page 1 3 (under Storage of aggregates), the moisture content of
aggregates, especially the sand, can vary considerably from load to Mechanical properties of aggregates
load or in the stockpile. Ideally, the weight of aggregate in each The facilities of a laboratory are needed for determining the
batch of concrete should be adjusted to allow for changes in the mechanical properties of aggregates. The tests include crushing or
moisture content of the aggregate, but this is seldom practicable; abrading samples of an aggregate to give a measure of its strength
adjustments to the batch weight of dry aggregate are therefore or resistance to wear. The methods of test are described in BS 812.
usually based on an average moisture content. There are various
methods of determining the moisture content, which are fully
described in BS 812 : Part 109 and outlined below.
Water
BS 3148 / BS EN 1008* describes the testing of water for concrete
Drying methods. Methods involving the drying of representative by comparing the properties of concrete made with any particular
samples of aggregate are often used. On site the quickest and sample of water with those of an otherwise similar concrete made
most direct way of measuring the moisture content of both coarse with distilled water.
aggregate and sand is the 'frying pan' technique in which the
aggregate is dried by heating it in an open pan. The sample of The Standards specify acceptance limits for these tests, and
aggregate to be tested should weigh between 1.8 and 2.2 kg for guidance is given on the interpretation of the results. The tests will
coarse aggregate. The sample size for sand may be reduced to not usually be performed in a laboratory.
less than 0.5 kg if an accurate balance is used for weighing.
*At the time of writing, BS EN 1008 is at draft stage.
The aggregate is first weighed (W1), then dried and re-weighed
(W2). The moisture content is then calculated as:
Testing fresh concrete
W1 - W2 The measurement of the consistence of fresh concrete is of
x 100% importance in assessing the practicability of placing and
w2 compacting it and also in maintaining uniformity throughout the
When the 'saturated surface-dry' (SSD) moisture content is job. In addition, consistence tests can be used as an indirect check
measured, coarse aggregate should be dried until surface moisture on the water content and, therefore on the free water/cement ratio
has evaporated (this is often accompanied by a slight change in of the concrete. In this instance, the relationship between water
colour), but any further heating should be avoided. Sand should content and consistence is established in the laboratory or early in
be dried until it just fails to adhere to a glass rod when stirred. the site work. Then, by maintaining the correct proportions of
cement, aggregate and any admixture, the free water/cement ratio
If an open source of heat is used it is important not to overheat the is controlled by adding water to maintain the consistence. It is
aggregate or heat it too rapidly, which could cause the particles to essential that all test results are based on representative samples of
break up and spit out of the pan. A microwave oven may be more concrete. Tests on fresh concrete are described in BS 1881 and
convenient. BS EN 12350.

This method gives the water content as a percentage of the SSD

weight of the aggregate, which is the measure of moisture content Slump test
generally used on site. It takes into account water on the surface of The slump test is suitable for normal cohesive concretes of low to
all particles of aggregate, but does not include water absorbed high consistence and is the test most commonly used. Changes in
into the pores of the aggregate. The 'total' moisture content, the value of the slump may indicate changes in materials, in the
which includes water absorbed into the pores of the aggregate water content or in the proportions of the mix, so it is useful in
particles, is sometimes used instead of the SSD moisture content. controlling the quality of the concrete as produced.
When the total moisture content is to be measured, the aggregate
should be dried thoroughly; it is necessary to heat the aggregate The apparatus consists of a truncated conical mould 100 mm
diameter at the top, 200 mm diameter at the bottom and 300 mm

55
 TESTING CONCRETE & CONCRETING MATERIALS
high, with a steel tamping rod 16 mm diameter and 600 mm long
with both ends hemispherical. The inside of the mould should be (Dimensions in millimetres) Straightedge,
clean and damp before each test and it should be placed on a not less than 200
smooth, horizontal and rigid impervious surface. The mould is held Container
down using the foot rests and filled with three layers of
approximately equal depth. Each layer is tamped with 25 strokes of 110
the tamping rod, the strokes being uniformly distributed over the
cross-section of the layer. When tamping the first layer, the rod 400
should be inclined slightly, and about half of the 25 strokes should
spiral towards the centre. Each layer should be tamped to its full 160
Trowel
depth, allowing the rod to penetrate through into the layer below.
The concrete should be heaped above the mould before the top
90
layer is tamped. 200
200
After the top layer has been tamped the concrete should be struck
off level with the top of the mould by a sawing and rolling motion Figure 39: Compactability test apparatus.
of the tamping rod. Any spillage is cleaned away from around the
base of the mould and, taking 5 to 10 seconds, the mould is then
slowly lifted vertically from the concrete.
Flow table test
The growing use of concrete that flows into place and is self-
The slump is the difference between the height of the mould and compacting calls for a test method that is more sensitive than the
of the highest point of the concrete being tested. If it collapses or slump test. The slump test should not be used for any concrete
shears off laterally, the test should be repeated with another whose slump could exceed 210 mm as the test results would
sample of the same concrete and the type of slump noted. The simply be recorded as 'collapse' every time. The flow table test
slump should be recorded to the nearest 10 mm (Figure 38). enables quite fine distinctions to be made between batches of
concrete with differing degrees of very high consistence.

The main item of apparatus, shown in Figure 40, is a 700 mm

square table that is attached by a hinge along one edge to a firm
base. A handle is fixed to the edge opposite the hinge and the
table is fitted with stops, which prevent it rising off the base by
more than 40 mm. A truncated cone is used but it is quite
different from the standard slump cone by being only 200 mm
high. A wooden tamping bar 40 mm square completes the
apparatus.

Figure 38: Measuring the slump of concrete.

After the slump measurement has been completed, if the side of

the concrete is tapped gently with the tamping rod, a well-
proportioned cohesive concrete will gradually slump further, but if
it is a harsh or uncohesive it is likely to shear or collapse.

A formal certificate should be completed for every slump test. See

Appendix 3.
Figure 40: Flow table apparatus.
Compactability
It is very important to set the base plate on a level surface to avoid
The compacting factor test of BS 1881: Part 103 has been replaced the sample running off one side during the test.
by a new test, described in BS EN 12350-4, which represents the
way in which concrete may respond to vibration on site. A simple Before starting the test, any dry surfaces of the table top, cone or
rectangular metal mould (Figure 39), is filled to the top using a tamping bar are moistened with a clean damp cloth. The person
special trowel, struck off with a straightedge and compacted using doing the test presses down on the footpieces of the cone so that
a small poker vibrator. The extent to which the concrete no movement or leakage of concrete occurs.
consolidates after complete vibration is measured and the
A representative sample of fresh concrete is remixed and placed
compactability is calculated from the expression:
into the cone in two layers of equal depth, each layer being
Degree of compactability = 400 tamped exactly ten times. The tamping is rather different from that
400-S of other tests such as the slump test, because the bar should not
penetrate far into the concrete and more of a levelling action is
where S is the distance (in millimetres) from the top of the mould used for this fluid concrete. After the second layer has been
to the surface of the concrete after vibration. tamped, the surface is ruled off with a suitable straightedge, and
the surcharge removed from the table. Thirty seconds are allowed
to pass between striking off the surface and starting to lift the
cone, which is done in 3 to 6 seconds.

56
 TESTING CONCRETE & CONCRETING MATERIALS
With one foot on the base plate and one hand on the handle fixed exceed 75 mm. The object of tamping or vibrating the concrete is
to the table top, the tester then raises the table top as far as the to attain full compaction without removing an appreciable
stops and then lets it drop onto the base. This is repeated every proportion of any deliberately entrained air.
four seconds until the table has been given 15 drops. The concrete
The concrete should be compacted until it just makes good
spreads across the table as a result and the maximum diameter is
contact with the sides of the container; prolonged working should
measured in two directions parallel to the table edges. The average
be avoided. After compacting each layer, the side of the apparatus
of the two dimensions is calculated and reported as the flow of the
is tapped using a 250 g soft mallet to remove any large air voids
concrete, recorded to the nearest 5 mm.
entrapped in the concrete. Provided that the air has been correctly
If any aggregate particles shake loose during the test or the final entrained, the amount of air removed by excessive compaction is
shape of the concrete is markedly asymmetrical, the fact should be likely to be small.
reported because these are indications of possible problems with
The correct operating pressure (which is predetermined by
cohesion that could lead to segregation in the concrete. Also it is a
calibration) is applied to the concrete and the volume of entrained
good idea to leave the concrete on the table for a few minutes
air is read off the water column (method A) or the pressure gauge
after the test is completed: any tendency to bleeding will become
(method B).
apparent. The test may, therefore, be regarded as a useful means
of giving early warning of potential problems with placing and An aggregate correction factor is necessary and will vary with
compacting concrete on site but it should be remembered that the different aggregates. This can be determined only by test, since it
flow table test is suitable only for mixes with very high and flowing is not directly related to the water absorption of the particles.
consistence classes. Ordinarily the factor will remain reasonably constant for a
particular aggregate, but an occasional check is recommended.
The normal tolerance for flow, according to BS 5328 : Part 4, is
±50 mm. Full details of the air meter test are given in the Standard and a
proper certificate should be completed. See Appendix 4.
It is permissible, in the case of extreme consistence - where the
concrete might be in danger of flowing off the table - to reduce
the number of drops from 15 to a number determined by trials
and agreed by all the interested parties.
Testing hardened concrete
The strength of hardened concrete is usually measured on
Full details of the flow table test procedure are given in BS 1881 : specimens that are tested in compression. Other tests are non-
Part 105 and BS EN 12350-5 destructive, such as the rebound hammer, ultrasonic pulse, or
various pull-off/break-off techniques, that can recognize variations
Air content test in concrete strength and quality.
If an air-entrained concrete is used, the air content of the fresh
concrete should be determined in accordance with either of the Manufacture of test cubes
two methods given in BS 1881: Part 106 and BS EN 12350-7. Compressive strength tests for UK concretes with maximum
Typical air meters as used for method A and method B are shown aggregate sizes (Dmax) of 10, 14 or 20 mm are usually made on
in Figure 4 1 . 100 mm cubes. For aggregate with a D max greater than 20 mm,
150 mm cubes are used. Details of the making and curing of test
cubes are given in Parts 108 and 111 respectively of BS 1881, and
A B BSEN 12390-2.

A summary of the procedure on site is given below. It should be

emphasized, however, that cubes should always be made by
personnel trained in the work and that it is preferable for the same
personnel to make all the cubes throughout the job.

The moulds for test cubes are traditionally made of steel or cast
iron, with very close tolerances for dimensions, flatness,
squareness, parallelism, and surface texture and hardness. Each
mould should have a removable steel base plate with a true surface
to support the mould and prevent leakage. It is essential to keep
the mould and base plate clean, and both should be thinly coated
with release agent to prevent the concrete sticking to them. No
undue force should be used during assembly.

Hard plastic cube moulds may be permitted where equal levels of

Figure 4 1 : Air-entrainment meters - A, water column type,
precision can be assured and, if plastic moulds are deemed to be
B, pressure gauge type.
acceptable, hand tamping may be done with a slump rod to avoid
The test involves measuring the reduction in volume of air the damage likely to be caused by the traditional square bar.
resulting from an increase in the applied air pressure. The air meter
should be of a type in which the air content is read off while the It is essential that the concrete in the cubes should be fully
concrete is under an operating pressure of approximately compacted. A 100 mm cube mould should be filled in two layers
1 bar. The container of the meter should have a nominal capacity and a 150 mm mould in three layers. When compaction is by
of at least 5 litres, and should be filled with concrete in three hand each layer should be tamped with at least 25 strokes for
approximately equal layers. Each layer is compacted with at least 100 mm cubes and at least 35 strokes for 150 mm cubes, with a
25 strokes of a steel tamping bar weighing 1.8 kg, and having a steel bar, weighing 1.8 kg and having a tamping face 25 mm
tamping face 25 mm square (as used for cube making). square. More strokes should be used if required to ensure full
Alternatively, vibration may be used, provided the slump does not compaction. The tamping of the concrete should be carried out

57
 TESTING CONCRETE & CONCRETING MATERIALS
methodically, the strokes being evenly distributed over the surface the type of failure should be noted. The cube should be
of the concrete in a regular pattern and not concentrated in one retained for a minimum period of one month
particular spot. n The compressive strength is recorded to the nearest
0.5 N/mm2
Alternatively, the concrete can be compacted by vibration, again in
layers, using either an electric or pneumatic hammer or a suitable n It is important to maintain testing machines in good working
table vibrator. In all cases the amount of compaction is recorded condition ensuring, for example, that the spherical seating can
either as the number of strokes per layer or as the duration of move correctly. The seating must move freely as the slack in the
vibration. After compaction, the surface of the concrete should be machine is taken up but then must lock and remain rigid until
trowelled as smooth as practicable, level with the top of the the cube fails; otherwise low failure loads will be recorded, and
mould. the shape of the failure will be one-sided. The type of failure
should always be noted, because an unusual shape of failure
Normal curing of test cubes surface may indicate a defective machine. Compression testing
machine requirements are specified in BS 1881: Part 115 and
Immediately after making, cubes should be stored under damp
BS EN 12390-4. It is important that machines are regularly
matting or similar material in a place free from vibration, and
calibrated.
wrapped completely with plastic sheeting to prevent loss of
moisture. Cubes to be tested at an age of seven days or more
should be kept at a temperature of 20 ±5°C; cubes to be tested at Test cores
earlier age should be kept at a temperature of 20 ±2°C, during this Core tests are useful for assessing the quality of hardened concrete
initial moist-air curing period. in situ. Not only can the compressive strength of the concrete be
assessed, but a description of the aggregate, including maximum
Cubes to be tested at 24 h should be demoulded just before size, shape, surface texture and type, can be obtained and the
testing; cubes for testing at greater ages should be demoulded degree of compaction noted. The results of the tests can be used
within the period 16 - 28 h after the time of making. Each cube to give the 'estimated in-situ cube strength' or to give an estimate
should be clearly and indelibly marked for later identification and of the potential strength of the standard test cube. While the
immediately submerged in a tank of water maintained at a relationship of core strength with in-situ cube strength is fairly
temperature of 20 ±2°C until the time of testing. Cubes that have straightforward, the relationship between core strength and
to be transported to another location for testing should be standard cube strength is complex and will vary with particular
removed from their moulds or from the curing tank and packed in conditions.
such a way that they do not become damaged or dry. This can be
done by enclosing them in plastic bags and transporting them in Before deciding to drill cores for compressive testing, full
purpose-made boxes. Cubes can be transferred any time after consideration must be given to the aims and value of the results
demoulding, but they must arrive at the place of testing at least that will be obtained. Reference should be made to specialist
24 hours before the time of testing, where they must be stored in literature for the number of cores and the assessment of results -
water at 20 ±2°C. BS 6089, BS EN 13791, Concrete Society Digest No. 9 and
Concrete Society Technical Report No. 11.
A record should be kept of maximum and minimum temperatures,
both for the initial moist-air curing period and for the subsequent The usual diameter of a core (Figure 42) is 150 mm or 100 mm
water curing. A formal certificate should be completed for every and the length/diameter ratio must be between 1 and 2,
set of test cubes made, see Appendices 5 and 6. preferably between 1 and 1.2. In thin members, or where
reinforcement is congested, smaller diameter cores may be
Testing cubes necessary but these give less reliable strength test results. Core
cutting is a skilled operation, usually performed by specialist
Details of the testing of cubes are given in BS 1881 : Part 116 and
subcontractors.
BS EN 12390-3. Specimens will usually be sent to a laboratory for
testing and it is recommended that the laboratory is one which is Testing cores
accredited for cube testing by the United Kingdom Accreditation
Core tests may be used when the result of a cube test has proved
Service ( UKAS). For site testing, reference should be made to the
unsatisfactory. A cube may have failed to give a desired result
Standard for full details of requirements for the testing procedure,
because of a defect in the testing procedure, in which case it is
but some of the more important points relating to testing
usual to examine the concrete in the structure in an attempt to
procedure for cubes are as follows:
assess its properties. Core tests need careful interpretation because
n The cube should be stored in water, as described above, and the strength of a core is dependent on:
tested immediately on removal from the water. Surface water,
n Quality of concrete
grit and projecting fins should be removed and the dimensions
and weight recorded, noting any unusual features, such as n Degree of compaction
honeycombing. Cubes that are clearly misshapen should not be n Location in the structure
tested n Curing
n The bearing surfaces of the testing machine should be wiped n Presence of reinforcement
clean and the cube should be placed in the machine in such a
n Method and direction of cutting
way that the load is applied to faces other than the top and
bottom of the cube as cast. The cube must be carefully centred n Preparation of specimen
on the lower platen n Testing procedure
n The load must be applied without shock and increased n Age at test
.
continuously at a rate within the range of 0.2 - 0.4 N/mm2 per
second until no greater load can be sustained. The maximum
load applied to the cube is recorded. Any unusual features in

58
 TESTING CONCRETE & CONCRETING MATERIALS
To standardize readings as far as possible it is recommended that
they should be taken on cubes held in the testing machine under a
stress of about 7 N/mm2. Since readings taken on a trowelled face
are often more variable than those on a moulded face, it is
recommended that the hammer should be used on a vertical face of
the cube as cast.

A rebound hammer may also be useful for indicating the variability

of concrete in a structure as a guide to the significance of a core
test. The use of rebound hammers is described in BS 1881: Part 202
and BS EN 12504-2.

Figure 42: As well as providing a specimen for strength testing,

the core gives a good indication of the distribution of the
aggregate and the degree of compaction.

In contrast, the cube test, if properly performed, uses standardized

preparation and testing procedures in an attempt to eliminate all
variables except that of concrete quality. Concrete in a structure
cannot be expected to have had the same treatment as a standard
cube. Accordingly, cores give variable results and the equivalent
cube strengths are usually lower than the standard cube strength Figure 43: Measuring the surface hardness of concrete with a
of the concrete. Schmidt hammer.
Because of the variability of the core test a number of cores are
needed to form a reasonable estimate as to the acceptability of the Ultrasonic test
concrete. If possible a set of cores should also be tested from Ultrasonic tests give a comparative measure of concrete quality as
comparable concrete that is known to be acceptable. indicated by the time taken by an ultrasonic pulse to travel through
a section of concrete. Ultrasonic equipment, such as the Pundit
Cores must be prepared to give end surfaces that are plane,
(Figure 44) is portable and consistent in its behaviour. Its use is
parallel and at right angles to the axis. The length and diameter of
described in BS 1881: Part 203 and BS EN 12504-1.
the core are accurately measured because they are necessary for
the calculation of strength. Core tests should be carried out only in
a UKAS accredited laboratory; details of the preparation and test
procedure are given in BS 1881 : Part 120 and BS EN 12504-1.

The compressive strength of each core is calculated by dividing the

maximum load by the cross-sectional area calculated from the
average diameter. Correction factors dependent upon the
length/diameter ratio of the specimen after preparation of the
ends, the direction of drilling and the presence of reinforcement
are applied to give the estimated in-situ cube strength. Other
factors may then have to be applied if an estimate of the standard
cube strength is required.

Non-destructive testing
Rebound hammer test
The rebound hammer or Schmidt hammer (Figure 43) gives a
comparative measure of concrete quality as indicated by surface
hardness, and can enable an approximate estimate to be made of Figure 44: Measuring the transit time of an ultrasonic pulse
concrete strength provided that users have prepared their own though a concrete column.
calibration charts by recording, as part of their quality control
routine, the results of regular tests with the hammer on cubes and
units made from the same concrete.

59
 TESTING CONCRETE & CONCRETING MATERIALS
The test consists of measuring the velocity of an ultrasonic pulse
through the concrete. A transducer placed in close contact with the
surface of the concrete transmits vibrations into the concrete that
are picked up by another transducer on the opposite face of the
specimen or member under test. The time taken by the pulse to
travel through the concrete is accurately measured by the apparatus,
and the velocity calculated knowing the distance between the
transducers (thickness of member).

This method of test is essentially comparative and, because so many

factors affect the pulse-velocity/strength relationship, it will not
generally be possible to relate pulse-velocity values and strength
without knowledge of the site conditions and constituents of the
concrete under test. Interpretation of the results requires care and
experience.

Pull-out test
This test can determine the strength of concrete that is less than
three days old. It uses pull-out inserts that are cast into the slab or
column, and the load is applied through a manually operated jack
(Figure 45). The peak tensile force is recorded and correlated against
the equivalent concrete cube strength. For more information see
Figure 46: Using the covermeter to estimate the depth and
BCA publication Early age strength assessment of concrete on site, and
direction of the reinforcement in a structure.
CIRIA Report 1 36.
The accuracy likely to be obtained on the average site can be
within about ±10% or ±2 mm, whichever is the greater, when
used to detect normal reinforcement. Calibration by breaking out
some concrete to reveal the reinforcement is recommended in all
cases and is essential when very small bars or stainless steel are
being detected. Calibration methods are given in the standard.

Analysis of fresh concrete

The Rapid Analysis Machine (RAM) (Figure 47) is a floor-mounted
unit which enables the cement content of a sample of fresh
concrete to be easily and rapidly determined in accordance with
BS 1881: Part 128. It provides a practical and accurate method of
analysing a concrete for cement content in the short time available
after mixing and before placing the concrete. The total time for
carrying out a test from loading the RAM to reading off the
cement content from a calibration graph is less than ten minutes.

The operating cycle of the RAM is fully automatic, and is controlled

by an electronic timer that is started after the machine has been
Figure 45: Measuring the force required to pull out an insert cast loaded with an 8 kg sample of fresh concrete. Water is pumped
into a floor. through the sample at a carefully controlled rate to separate the
cement size particles and wash them up and over the top of the
elutriation column. Three sampling channels at the top of the
Electromagnetic covermeter
column remove 10% of the cement slurry, with the remainder
The covermeter (Figure 46) described in BS 1881: Part 204, is a non- going to waste.
destructive method of locating the depth, size and direction of
reinforcement in hardened concrete. It is a portable electromagnetic The 10% sample passes through a 150 µm vibrating sieve, which
instrument. Recent models have a working range of up to 100 mm removes all the small particles greater than cement size from the
cover and operate from rechargeable batteries. A typical instrument slurry. After these particles have been removed, the slurry passes
consists of a search head connected to the main unit containing the into a conditioning vessel where chemical agents are stirred into
battery circuits and display panel. When the instrument is switched the slurry causing the cement particles to cling together and drop
on, or switched from one scale to another, it is necessary to place out of suspension to the bottom of the vessel. Excess water is
the search head well away from any steel and to calibrate the removed by siphons until a constant volume is obtained in the
instrument by adjusting the 'zero set'. The search head is then removable pot at the bottom of the conditioning vessel. After all
positioned on the surface of the concrete and, if any steel exists the cement particles have settled, this vessel is removed from the
below the surface within a depth of 100 mm, the depth will be RAM and weighed on a balance (to an accuracy of 1 g), and the
shown, usually by digital display. cement content of the concrete sample is determined from a
calibration graph.
The search head is rotated and moved methodically across the
surface until the minimum reading is obtained: this reading indicates Modules are available for determining the water/cement ratio of
the depth of cover and the direction of the steel. Some covermeters the concrete and for determining the quantities of any mineral
also give a strong audible signal that assists in locating bars by rising additions.
when approaching steel.

60
 TESTING CONCRETE & CONCRETING MATERIALS
Part 116: 1983, Method for determination of compressive strength
of concrete cubes. (BS EN 12390-3, Compressive strength of test
specimens).
Part 118: 1983, Method for determination of flexural strength.
Part 120: 1983, Method for determination of compressive strength
of concrete cores. (BS EN 12504-1, Cored specimens - Taking,
examining and testing in compression).
Part 128: 1997, Methods for analysis of fresh concrete.
Part 130:1996, Method for temperature-matched curing of
concrete specimens.
Part 201: 1986, Guide to the use of non-destructive methods of
test for hardened concrete (BS EN 1 3791, Assessment of concrete
compressive strength in structures or in structural elements).
Part 202: 1986, Recommendations for surface hardness testing by
rebound hammer (BS EN 12504-2, Non-destructive testing -
Determination of rebound number).
Part 203: 1986, Recommendations for measurement of velocity of
ultrasonic pulses in concrete (BS EN 12504-4, Determination of
ultrasonic pulse velocity).
Part 204: 1988, Recommendations on the use of electromagnetic
covermeters.
BS 3148 : 1980, Methods of test for water for making concrete
(including notes on the suitability of the water) (to be superseded by
BS EN 1008, currently prEN 1008 : 1997 Mixing water for concrete -
specifications for sampling, testing and assesing the suitability of water,
including waste water from recycling installations in the concrete
industry as mixing water for concrete). British Standards Institution,
London.

Figure 47: Measuring the cement content of fresh concrete with BS 6089 : 1981, Guide to the assessment of concrete strength in
the RAM. existing structures. British Standards Institution, London.
BS EN 196 : Parts 1 - 7, Methods of testing cement. British Standards
Institution, London.
References/further reading BS EN 932 : 1997, Tests for general properties of aggregates. Part 1:
BS 812, Testing aggregates. British Standards Institution, London. Methods for sampling. British Standards Institution, London.
Part 102: 1989, Methods for sampling. BS EN 12350, Testing fresh concrete. Part 4 : 2000, Degree of
Part 103: 1985, Methods for determination of particle size compactability. British Standards Institution, London.
distribution.
BS EN 12390, Testing hardened concrete. British Standards
Part 109: 1990, Methods for determination of moisture content.
Institution, London.
BS 1881 Testing concrete. British Standards Institution, London. To Part 1 : 2000, Shape, dimensions and other requirements for test
be replaced by BS EN 12350 : 2000 Testing fresh concrete; BS EN specimens and moulds.
12390 : 2000 Testing hardened concrete; BS EN 12504: 2000, Part 3 : 2002, Compressive strength of test specimens.
Testing concrete in structures; and BS EN 13791, Assessment of Part 5 : 2000, Flexural strength of test specimens, (currently
concrete compressive strength in structures or in precast concrete BS1881 : Part 1 1 8 : 1983).
products, as indicated.
BS EN 12504, Testing concrete in structures. British Standards
Part 101:1983, Method of sampling fresh concrete on site.
Institution, London.
(BSEN 12350-1, Sampling)
Part 1 : 2000, Cored specimens. Taking, examining and testing in
Part 102: 1983, Method for determination of slump.
compression.
(BSEN 12350-2, Slump test)
Part 2 : 2000, Non-destructive testing. Determination of rebound
Part 103: 1983, Method for determination of compacting factor.
number.
Part 104: 1983, Method for determination of Vebe time.
Part 3 : Determination of pull-out force (not yet published).
(BS EN 12350-3, Vebe test)
Part 4 : Determination of ultrasonic pulse velocity (not yet
Part 105: 1984, Method for determination of flow.
published).
(BS EN 12350-5, Flow table test).
Part 106: 1983, Method for determination of air content of fresh CIRIA Report 136. Formwork striking times - criteria, prediction and
concrete. (BS EN 12350-7, Air content of fresh concrete - Pressure methods of assessment. 1995, Construction Industry Research and
methods). Information Association, London.
Part 108: 1983, Method for making test cubes from fresh CSTR 11, Core testing for strength. 1987, The Concrete Society,
concrete. (BS EN 12390-2, Making and curing specimens for Crowthorne. 44 pp.
strength tests). CS020, Digest No. 9, Concrete core testing for strength. 1988, The
Part 111:1983, Method of normal curing of test specimens (20°C Concrete Society, Crowthorne. 8 pp.
method.) (BS EN 12390-2, Making and curing specimens for
Early age strength assessment of concrete on site. 2000, RCC/The
strength tests).
British Cement Association. Ref. 97.503. 4 pp. For free download
Part 115:1986, Specification for compression testing machines for visit www.rcc-info.org.uk
concrete (BS EN 12390-4, Compressive strength - Specification of
compression testing machines).

61
 APPENDICES
1- SCHEDULES FOR SPECIFYING CONCRETE (TAKEN FROM BS5328)
Appendix 1a
Form A. Schedule for the specification requirements of designed mixes required for use on contract
............................................................................................................................
The mixes below shall be supplied as designed mixes in accordance with the relevant clauses of BS 5328 Parts 2, 3 and 4

1. Mix reference

2. Strength grade

3. Nominal maximum size of aggregate (mm)

4. Types of aggregate Coarse BS 882 BS 882 BS 882 BS 882

BS 1047 BS 1047 BS1047 BS 1047
Other
Ring those permitted
Fine BS 882 BS 882 BS 882 BS 882
Other

5. Sulfate classes (see Note 1) (ring if applicable) Class 2 Class 2 Class 2 Class 2
Class 3 Class 3 Class 3 Class 3
Class 4A Class 4A Class 4A Class 4A
Class 4B Class 4B Class 4B Class 4B
Includes adjustment for acidity (yes/no) (see Note 1) (yes/no) (yes/no) (yes/no) (yes/no)

6. Cement type(s) or combinations conforming to

BS 12 PC PC PC PC
BS 146 PBFC PBFC PBFC PBFC
BS 6588 Ring those permitted PPFAC PPFAC PPFAC PPFAC
BS 4027 SRPC SRPC SRPC SRPC
BS 7583 (see Note 2) PLC PLC PLC PLC
Others

7. Minimum cement content (see Note 3) (kg/m 3 )

8. Maximum free water/cement ratio (see Note 3)

9. Quality assurance requirements

10. Rate of sampling intended by the purchaser for strength testing

(for information only)

11. Other requirements (e.g. maximum chloride, alkali, etc.)

The following section to be completed by purchaser of fresh concrete only

12. Workability Slump (mm)

Compacting factor Ring method
Vebe(s) and give target
Flow (mm)

13 Method of placing (for information only)

14. Other requirements by purchaser of fresh concrete (if appropriate)

NOTES
1. The purchaser should ensure that the sulfate class specified accounts for the modifications required in tables 7c and 7d of BS 5328 :
Part 1: 1997 where appropriate. If the classification includes an adjustment for acidity (see table 7d of BS 5328 : Part 1 : 1997), this
should be specified in item 5
2. Reference should be made to 4.2.4 of BS 5328 : Part 1 : 1997 before specifying this cement
3. Where a sulfate class is ringed and no preference for a cement type is indicated, the minimum cement content and maximum free
water/cement ratio for the cement type to be used should be in accordance with table 7a of BS 5328 : Part 1: 1997

Readers should be aware that this is a representation of the form from BS 5328, where the following terminology is still used:
'Mix' is used instead of 'concrete' 'PC instead of CEM I
'Workability' instead of 'consistence' 'PBFC instead of CEM III A or CIIIA
'Fine aggregate' instead of 'sand' 'PPFAC instead of CEM II/B-V or CIIB-V
'Grade' instead of 'strength class' 'PLC instead of CEM II/A-L and CEM II/A-LL

62
 APPENDICES
1 - SCHEDULES FOR SPECIFYING CONCRETE (TAKEN FROM BS 5328)
Appendix 1b
Form B. Schedule for the specification requirements of prescribed mixes required for use on contract
.......................................................................................................................................................................................................................................................

The mixes below shall be supplied as prescribed mixes in accordance with the relevant clauses of BS 5328 : Parts 2, 3 and 4

1. Mix reference

2. Type(s) and standard strength class(es) of the cement(s) or

combination(s)

3. Nominal maximum size of aggregate (mm)

4. Types of aggregate Coarse

Fine

5. Mix proportions
Cement (kg)
Fine aggregate (kg)
Coarse aggregate (kg)
Admixtures
Other

6. Workability
Slump (mm)
Compacting factor Ring method
Vebe(s) and give target
Flow (mm)

7. Quality assurance requirements

8. Method of compliance and rate of sampling

(for information only)

9. Other requirements
(e.g. maximum chloride, etc., if appropriate)

Readers should be aware that this is a representation of the form from BS 5328, where the following terminology is still used:
'Mix' is used instead of 'concrete'
'Workability' instead of 'consistence'
'Fine aggregate' instead of 'sand'

63
 APPENDICES
1-SCHEDULES FOR SPECIFYING CONCRETE (TAKEN FROM BS 5328)
Appendix 1c
Form C. Schedule for the specification requirements of standard mixes required for use on contract

(This form also applies to standardized prescribed concretes)

The mixes below shall be supplied as standard mixes in accordance with the relevant clauses of BS 5328 : Parts 2, 3 and 4 Designated
mixes agreed as equivalent will be acceptable/unacceptable (delete one) as alternative mixes to those below.

1. Standard mix Ring those required ST1 ST2 ST3 ST4 ST5

2. Class of concrete (for information only) Unreinforced Unreinforced Unreinforced Unreinforced Unreinforced
Ring the appropriate Reinforced Reinforced Reinforced Reinforced

3. Cement type(s) or combinations conforming to

BS 12 PC PC PC PC PC
BS 146 PBFC PBFC PBFC PBFC PBFC
BS 6588 Ring as PPFAC PPFAC PPFAC PPFAC PPFAC
BS 4027 (see Note 1) permitted SRPC SRPC
BS 4027 (see Note 1) LASRPC LASRPC
BS 7583 (see Note 2) PLC PLC PLC PLC PLC

4. Nominal maximum size of aggregate (mm) 40 40 40 40 40

Ring as appropriate 20 20 20 20 20

5.Types of aggregate BS 882 BS 882 BS 882 BS 882 BS 882

Coarse BS 1047 BS 1047 BS 1047 BS 1047 BS 1047
Ring those
Fine BS 882 BS 882 BS 882 BS 882 BS 882
permitted
All in ST1, ST2, S BS 882 BS 882 BS 882

6. Workability Very low

Ring as
Slump appropriate 75 mm 75 mm 75 mm 75 mm 75 mm
125 mm 125 mm 125 m m 125 m m 125 m m

The following section to be completed by purchaser of fresh concrete only

7. Quality assurance requirements

8. Other requirements (if appropriate)

NOTE
1. Reference should be made to 8.2.4 of BS 5328 : Part 1: 1997 before specifying these cements
2. Reference should be made to 4.2.4 of BS 5328 : Part 1 : 1997 before specifying this cement

Readers should be aware that this is a representation of the form from BS 5328, where the following terminology is still used:
'Mix' is used instead of 'concrete' 'PC instead of CEM I
'Workability' instead of 'consistence' 'PBFC instead of CEM III A or CII1A
'Fine aggregate' instead of 'sand' 'PPFAC instead of CEM II/B-V or CIIB-V
'PLC instead of CEM II/A-L and CEM II/A-LL

64
 APPENDICES
1 - SCHEDULES FOR SPECIFYING CONCRETE (TAKEN FROM BS 5328)
Appendix 1d
Form D. Schedule for the specification requirements for designated mixes required for use on contract

....................................................................................................................................................
The mixes below shall be supplied as designated mixes in accordance w i t h the relevant clauses of BS 5328 : Parts 2, 3 and 4 \. Standard
mixes agreed as equivalent will be acceptable/unacceptable (delete as appropriate) as alternative mixes to those below.

1. Mix designation
If the FND mix includes an adjustment for acidity (y/n) (yes/no) (yes/no) (yes/no) (yes/no)

2. Use of concrete
For unreinforced concrete Ring one only U U U U
For reinforced concrete R R R R
For reinforced concrete that will be heat cured HR HR HR HR
For prestressed concrete PS PS PS PS

3. Environment (see table 5 of BS 5328 : Part 1 : 1997)

Chloride bearing
Non-chloride bearing Ring as appropriate CB CB CB CB
Severe freeze/thaw NCB NCB NCB NCB
F/T F/T F/T F/T

4. Nominal m a x i m u m size of aggregate ( m m )

(enter 10 or 40, or leave blank for 20 mm

5. Other requirements (if appropriate)

6. Workability 50 50 50 50
Ring one or give value
Slump ( m m ) 75 75 75 75
Other (to be completed by the purchserlaser of the fresh concrete) 125 125 125 125

7. M e t h o d of placing (for information only)

8. M e t h o d of finishing (for information only)

NOTE
The purchaser should ensure that when specifying FND mixes, the mix indicated includes the modifications recommended in tables 7c and 7d of
BS 5328 : Part 1 : 1997 if appropriate. If the FND mix has included an adjustment for acidity (see table 7d of BS 5328 : Part 1 : 1997), this should be specified.

Readers should be aware that this is a representation of the form from BS 5328, where the following terminology is still used:
'Mix' is used instead of 'concrete'
'Workability' instead of 'consistence'

65
 APPENDICES
APPENDIX 2-CERTIFICATE OF SAMPLING FRESH CONCRETE

Sample taken in accordance with: Please indicate

BS 1881: Part 101 (General method)
BS 1881: Part 102, Clause 4.2 (Alternative method from truck-mixer for slump test only) (BS EN 12350-1, Spot sample)
BS EN 12350-1, (Incremental sample)

Date of sampling

Time

Name of works

Location in works of the concrete which the sample represents

Location of sampling (e.g. discharge from truck or from a heap of concrete)

Delivery note number or other means of identifying batch

Sample identity number

Ambient temperature

Weather conditions

Name of sampler

Affirmation that the sampling was done in accordance with: BS 1881: Part 101 or Part 102, Clause 4.2 or BS EN 12350-1 as
indicated above

Signature of person responsible for sampling

66
 APPENDICES
APPENDIX 3-CERTIFICATION OF SLUMP TEST

For test in accordance with: Please indicate

BS 1881: Part 102
BS EN 12350-2

Essential information

Date of test

Sampling: Time Place

Sampling method: General - BS 1881: Part 101

Or alternative - BS 1881: Part 102, Clause 4.2

Sample identity number

Sampling certificate available (copy attached)

Slump: Time of test Place

Time from sampling to beginning of test

Form of slump: true / shear / collapse

Measured true slump

Name of tester

Optional information to be supplied if requested

Name of project

Part of works where concrete used

Name of supplier

Source of concrete

Time of production or delivery to site

Specification of concrete mix (e.g. strength class)

Affirmation that the test was made in accordance with BS 1881: Part 102 or BS EN 12350-2 as indicated above

Signature of person responsible for test

67
 APPENDICES
APPENDIX 4 - CERTIFICATE OF AIR CONTENT TEST
For test made in accordance with: Please indicate
BS 1881: Part 106
BS EN 12350-7

Essential information

Date of test

Sampling: Time Place

Sample identity number

Sampling certificate available (copy attached)

Air content: Time of test Place

Type of apparatus (A or B)

Aggregate correction factor

Method of compaction (hand or vibration)

Type of vibration equipment

Number of strokes of bar or duration of vibration

Measured air content

Name of tester

Optional information to be supplied if requested

Name of project

Part of works where concrete used

Name of supplier

Source of concrete

Time of production or delivery to site

Specification of concrete mix (e.g. strength class)

Temperature of concrete at time of sampling

Density of concrete

Consistence class of concrete

Calculated air content of mortar fraction

Affirmation that the test was made in accordance with BS 1881: Part 106 or BS EN 12350-7 as indicated above

Signature of person responsible for test

68
 APPENDICES
APPENDIX 5-CERTIFICATE OF CUBE MAKING
For concrete test cubes made in accordance with: Please indicate
BS 1881: Part 108
BS EN 12390-2

Essential information

Sample identity number

Date and time of sampling

Place of sampling

Time of making the cubes

Place where cubes were made

Number of cubes made in the set

Size of cubes

Method of compaction (hand/vibration)

For hand tamping: number of strokes used

For vibration: type of equipment and duration of vibration

Identification number or codes of the cubes

Age(s) at which cubes are to be tested

Name of person making cubes

Optional information to be supplied if requested

Name of project

Location of concrete represented by the cubes

Name of supplier and source of the concrete

Time of production

Time of delivery to site

Specification of the concrete

Measured consistence

Air content of concrete (if air-entrained)

Affirmation that the cubes were made in accordance with BS 1881 : Part 108 or BS EN 12390-2 as indicated above

Signature of person making cubes.

69
 APPENDICES
APPENDIX 6-CERTIFICATE OF STANDARD CURING OF TEST CUBES

For concrete test cubes cured in accordance with: Please indicate

BS 1881: Part 111
BS EN 12390-2

Essential information

Sample identity number

Identification number or codes of the cubes

Location of moist air curing

Method of moist air curing

Period of moist air curing

Maximum and minimum moist air curing temperatures

Maximum and minimum water curing temperatures

Name of person responsible for curing cubes

Optional information to be supplied if requested

Time of adding water to the concrete

Time of making the specimens

Time of immersion of specimens in curing tank

Time of removal of specimens from curing tank

Temperature record during moist air curing

Temperature record during water curing

Age(s) at which cubes are to be tested

Affirmation that the cubes were cured in accordance with BS 1881 : Part 111 or BS EN 12390-2 as indicated above

Signature of person responsible for curing cubes

70
 ACKNOWLEDGEMENTS

Our appreciation goes to the following for

permission to use their photographs

Alstom Transport - Figure 24

Building Research Establishment - Figure 16

Controls Testing Equipment - Figures 36, 40, 41

Dow Construction Products - Figure 23

Germann Instruments - Figure 45

Protovale Oxford - Figure 46

Wexham Developments - Figure 47

71
Concrete practice
G F Blackledge and R A Binns

BRITISH CEMENT ASSOCIATION PUBLICATION 48.037