Concrete Technology

Materials

• Cement, aggregate and water with the

occasional addition of an admixture
• Cement- Ordinary Portland cement, made
by heating limestone and clay, or other suitable
raw materials, together to form a clinker rich in
calcium silicates
• This clinker is ground to a fine powder with a
small proportion of gypsum (calcium sulphate)
which regulates the rate of setting when the
cement is mixed with water
 Setting of Cement
• The setting of cement is a chemical
reaction between cement and water, not a
drying process
• This reaction is called hydration
• It evolves heat and is irreversible
• Strength continues to gain after hardening
and may take many years to reach its
ultimate value
 Portland cements-
Ordinary and Rapid-hardening
Portland cement
• Rapid-hardening Portland cement (RHPC) is
ground somewhat finer than ordinary Portland
cement (OPC)
• The finer cement gains strength more quickly but
after several months there is little to choose
between the two
• Both kinds of cement are manufactured to
conform with BS 12
• RHPC is not ‘quick setting’ but merely gains
strength more rapidly than OPC after hardening
 Sulphate-resisting Portland
cement (SRPC)
• Sulphates in ground water and sea water can
react with hydrated C3A which can weaken the
cement paste in concrete
• SRPC is a form of Portland cement with a low
C3A content
• A higher content of C4AF than other Portland
cements, which gives it a darker colour
• Concrete made with SRPC is more resistant to
attack by the sulphate compounds which may be
found dissolved in ground water and sea water
• Concrete made with SRPC has been found to be
satisfactory in nearly all troublesome conditions
which arise with below-ground concreting
 White Portland cement

• White cement is distinguished by its low

content of iron compounds which impart the
grey-green colour to ordinary cements.
• It is made by using white china clay and
limestone as the raw materials.
• Gypsum is added to control setting, and
special care is taken at all stages of
processing not to introduce coloured
contaminants.
• It is used for decorative white concrete and
also for some coloured concretes, in which
case a pigment is added to the mix.
 Low heat Portland cement

• Low heat Portland cement (LHPC) produces concrete

which gains strength and evolves heat more slowly
than the normal concrete of similar composition,
• It is intended for use in large masses where the rapid
evolution of heat would cause high temperatures and
stresses which might lead to cracking.
• The heat of hydration of LHPC should not exceed 60
and 70 kcal/kg at 7 and 28 days respectively; typical
figures for ordinary Portland cement are around 84 and
88 kcal/kg at these ages.
 Cements based on Portland
cement clinker
Masonry cement
• Mortars, if made with ordinary Portland cement
and sand, tend to be too harsh or too strong for
rendering and brick-or block laying.
• This consists of ordinary Portland cement with
the addition of a fine powder and a plasticizing
agent.
• Masonry cement is not a suitable substitute for
ordinary Portland cement and lime for mortars of
very high strength, and should never be used in
concrete.
 Hydrophobic cement

• In this cement, the particles are coated during

manufacture with a waterproof skin of oleic acid
or other water-repellent to resist hydration in
case the cement has to be stored for a long time
in a moist atmosphere.
• This coating is rubbed off by friction in the mixer,
and the cement then behaves normally.
• In most cases, less than 0.5% of additive is
required to obtain the designed effect.
• The added material may also serve as a
plasticizer in a concrete mix.
 Water-repellent cement

• If a metallic soap is mixed with cement,

concrete made with the cement becomes
water-repellent and tends to shed the dirt in
rainwater better than normal concrete.
• This is particularly important with decorative
concrete and cast stone which has an open
pore structure.
• This cement is used in pre-cast concrete as
well as in other branches of the construction
industry.
• Water-repellent and hydrophobic properties
can be combined in the same cement.
 Delivery and storage of cements

• Cement may be supplied in bulk, in bags or in drums.

Bulk cement is delivered by tanker, usually in loads of
10 tonnes, and blown into storage silos on site by
compressed air.
• Bagged cement is commonly supplied in bags
containing 50kg (110 Ib), though special cements may
be supplied in bags containing other quantities.
• Problems of long-term storage are usually avoided by
planning cement deliveries so as to anticipate only the
short-term requirements.
• Storage in moist air leads to the phenomenon of ‘air-
setting’, which results in the formation of lumps of
hydrated cement. These lumps should be screened
out and discarded if found in cement
• Bagged cement should be stored on a
raised floor in a damp-proof shed in order
to prevent deterioration.
• Each delivery should be kept separate to
avoid confusion.
• To avoid ‘warehouse set’ which results
from the compaction of the cement, bags
should not be stacked higher than about
1.5 m (5 ft).
 Aggregates

• The term ‘aggregates’ is used to describe

the gravels, crushed stones and other
materials
• Gravels, sands and crushed stone such
as granite, Basalt and the harder types of
limestone and sandstone, are in common
use as aggregates.
Characteristics of aggregates

• Durability and cleanness

• Durability: Aggregates should be hard and
should not contain materials which are likely to
decompose or change in volume when exposed
to the weather, or to affect the reinforcement
• High-strength mixes may call for additional
special properties
• In particular, the crushing value or impact value,
density, or mineralogical type may be specified
 Cleanness
• Aggregates should be clean and free from
organic impurities as these prevent the proper
bonding of the material.
• Gravels and sands are usually washed by the
suppliers to remove clay, silt and other
impurities.
• However, washing must not be carried to such
an extent that all fine material passing the 300
m (No. 52) sieve is removed; otherwise the
resulting concrete mix will be lacking in cohesion
and in particular may be unsuitable for a mix
which is to be placed by pump.
 Types of aggregate

• The term ‘fine aggregate’ is used to

describe natural sand, crushed rock,
crushed gravel or other material, most of
which passes through a 5 mm (3/16 in.)
BS sieve.
• ‘Coarse aggregate’ is the term used to
describe material such as natural gravel,
crushed gravel or crushed rock, most of
which is retained on this sieve.
 Grading of aggregates

• The proportions of the different sizes of particles

making up the aggregate are found by sieving and are
known as the ‘grading’ of the aggregate.
• The grading is usually given in terms of the percentage
by weight passing the various sieves.
• Continuously graded aggregates for concrete should
contain particles ranging in size from the largest to the
smaller;
• In gap-graded aggregates some of the intermediate
sizes are left out. Gap-grading is not normally
desirable for routine concreting work, though it may be
necessary in order to achieve certain surface finishes.
• Aggregate particles which have sharp edges or a rough surface,
needs more water than smooth and rounded particles to produce
concrete of the same workability.
• Increase the cement content of a mix made with crushed aggregate
to allow water to be added in order to make the concrete sufficiently
workable without reducing the strength below the required level.
• However, due to interlock between aggregate particles, a crushed
aggregate concrete may have a higher strength than a smooth or
rounded aggregate concrete
• This extra strength may be sufficient to offset the effect of the extra
water. This is particularly so at high strength levels
• The fine and coarse aggregates should be
proportioned to obtain the required workability
with the minimum amount of water.
• In BS 882 grading limits in percentages by
weight for coarse aggregate and for four grades
of fine aggregate are given in a typical chart, for
fine aggregate in grading zone 2 and for 20 mm
(3/4 in.) graded coarse aggregate, is shown in
Figure 2.
 Marine aggregates

• Some aggregates are obtained by dredging

marine deposits. Most of these aggregates
contain some shells and salt.
• Broken shells tend to affect the workability of
the concrete, but are not themselves harmful.
• A disadvantage of dredged fine aggregate is
that, in many cases, it has a predominance of
one size of particle, which can make mix
design difficult.
• Beach sands are often single-sized and can also
have much higher concentrations of salt than
dredged material because of the accumulation
of salt crystals above the high tide mark.
• If beach sands have to be used, they should be
washed carefully and their salt content checked
frequently in a suitably equipped laboratory.
• Chloride content 0.15% by weight RCC, 0.1%for
PSC.
 Lightweight and manufactured
aggregates
• Manufactured aggregates .
– Air-cooled blast furnace slag (steel industry)
– Pumice

▪ most lightweight aggregate concrete is made using

manufactured aggregates.
• Lightweight materials are relatively weak because of the porosity
which gives them reduced weight.
• This imposes a limitation on strength, though this is not often a
serious problem because the strength that may be obtained is
comfortably in excess of most structural requirements.
• Lightweight aggregates are used to reduce weight in structural
elements or to give improved thermal insulation.
 Storage of aggregates

• Aggregates should be stored in such away that it is self-draining

and so that it does not become contaminated with other materials.
• Stockpiles should be as large as possible as this helps to ensure
uniformity of moisture content.
• Deliveries should be allowed to stand in the stockpile for 12 hours
before use.
• The use of aggregates from the lower part of the stockpile should
be avoided since dirt and water from the higher layers can
accumulate there.
• The various sizes should be separated from each other by dividng
walls of sleepers or concrete blocks, and the practice of mixing
alternate lorry-loads of, for instance, 10 mm and 20 mm material
in the same day should be avoided.
 Water

• Mixing water for concrete is usually

required to be fit for drinking or to be
taken from an approved source.
 Admixtures

• Admixtures are materials which are added to

concrete during mixing.
• Both admixtures and additives can offer
benefits to a concrete.
• Indeed all Portland cements contain at least
one interground additive, gypsum, without
which it would be very difficult to control the
stiffening of a mix within a reasonable period of
time.
• Overdosing, is to be avoided because
excessive doses have adverse effects on the
properties of the concrete.
 Accelerators
• Calcium chloride is the most commonly used
accelerator.
• It is used either on its own or contained in a proprietary
admixture.
• Drawback
– May lead to corrosion of embedded steel, which includes
reinforcement and prestressing wires.
– Calcium chloride should never be used in prestressed
concrete, and should not normally be used in reinforced
concrete.
– Should not be used with sulphate-resting Portland cement
– Calcium chloride is considered for use in the mass concrete
 Water-reducing admixtures

• Inducing a repelling force between cement

particles and therefore act as dispersing
agents in concrete.
• The dispersed particles require less water to
lubricate them and the net result is that, for a
given workability, lower water content is
required in the presence of these chemicals,
which are known as water-reducing
admixtures.
• In trade literature the terms densifiers,
hardeners, water proofers and plasticizers
• An addition of 0.20% of the admixture by weight
of cement will, in a typical case, enable the
water content of a mix to be reduced by 10%
without loss of workability.
• This in turn allows the cement content of a mix
to be reduced without loss of ultimate strength,
which may lead to an over-all saving in cost.
• However, to ensure sufficient durability,
minimum cement content may be specified, and
care should be taken to ensure that the cement
content is not reduced below this level.
 Air-entraining admixtures

• Air-entrained concrete is more durable than non-air

entrained concrete under the action of frost and the
de-icing salts and fluids which are used on roads and
airfield pavements in winter.
• The air bubbles have a plasticizing effect on the mix,
which usually necessitates some minor changes in mix
proportions.
• An air content of 5% by volume is considered optimum
for concrete with 20 mm (3/4 in.) aggregate, and this is
normally achieved with the addition of about 0.1% air-
entraining admixture by weight of cement.

• Each 1% addition of air can reduce the potential

strength by 4 to 7% in a typical concrete, but reduction
in the water content and extra cement in the mix can
compensate for this drop.
 Water-repelling admixtures

• Structural concrete which is designed to resist cracking

and is made from dense concrete cast in thick enough
sections will be virtually impermeable to the flow of
water, even under pressure.
• However, chemicals are sometimes employed as
water-repelling admixtures.
• These are especially useful when the concrete has a
porous texture, as, for example, with cast stone, or
when the section is thin, as in a rendering for lining a
leaky basement. Admixtures, however, can only
marginally improve the impermeability of concrete.
• The usual chemical for this application is calcium
stearate, a metallic soap supplied in powder
form which has a repelling action on water and
is compatible with cement
• A further use of water-repelling admixtures is to
reduce the need for frequent cleaning of
buildings which would otherwise be necessary
when rain has washed dirt into the surface.
 Pumping aids

• When concrete is placed by pump, the applied

pressure tends to make the water in the mix flow
through the solids.
• Mix design for pumping is aimed at minimizing this, but
it is sometimes useful to add a material that has the
property of thickening the water phase.
• When pumping a very rich concrete mix with cement
content over 400 kg/m3 (670 lb/yd3), a reduction in
viscosity of the water may be desirable.
• In these circumstances a simple water-reducing
plasticizer may prove beneficial.
 Pozzolanas

• Pozzolanas are materials which possess cementing properties in

the presence of lime, including the lime derived from Portland
cement.
• Ground granulated blast furnace slag and pulverized-fuel ash,
more commonly referred to as fly-ash, from coal-burning power
stations.
• Not all pfa is suitable for use in concrete, so care is needed when
choosing the source of supply.
• The rate of gain of strength of pozzolanic mixes is fairly slow, with
a corresponding slow rate of heat evolution.
• Pozzolanas have thus been used as a partial substitute for
cement in situations, such as dams and other massive structures,
where it is necessary to reduce the amount of heat evolved or the
rate at which it is liberated.
 Concrete mixes

• Two essential properties of hardened concrete are

durability and strength.
• Both properties are affected by the voids or capillaries
in the concrete .
• The requirement that air voids be kept to a minimum
means that the materials must be so proportioned that
the mix is workable enough to be fully compacted with
the means available.
• The use of mechanical compaction equipment allows a
drier and potentially stronger and more durable mix.
 Workability

• The term ‘workability’ is used to describe

the ease with which the concrete can be
compacted.
• The cement content, the overall grading
of the aggregate and the shape of the
aggregate particles affect the amount of
water required to produce ‘workable’
concrete.
 Strength

• The strength of concrete is usually defined

by the crushing strength of 150 mm (6 in.)
cubes at an age of 28 days.
• The strength will probably be specified as
a characteristic strength.
• This is the strength below which not more
than a stated proportion of test results fall.
Target mean strength
 Characteristic strength
• ‘characteristic strength’‘-below which a
specified proportion of the test results,
often called ‘defectives’, may be expected
to fall.
• The characteristic strength may be defined
to have 5% defective level according to
BS 8110.
 Margin for mix design

• Fm = fc + ks
Where fm = the target mean strength
f c = the specified characteristic strength
ks = the margin, which is the product of the constant and
standard deviation
s = the standard deviation
k = a constant
• The constant k is derived from the mathematics of the normal
distribution and increases as the proportion of defectives is
decreased, thus:
•
• k for l0% defectives =1.28
• k for 5% defectives =1.64
• k for 2.5% defectives =1.96
• k for 1%.defectives =2.33
• For the 5% defective level specified in BS 5328,
k 1.64
• Fm + 1.64s. relates to a concrete having a
• specified characteristic strength of 30N/mm2
and a standard deviation of 6.1 N/mm2.
• Target mean strength
= 30 + (1.64 x 6.1)
=30+10
= 40 N/mm2
 Recommended grades of
concrete
Grade Characteristic Lbf/in Application
strength 2 **

N/mm2

7 7.0 1000 Lowest grade for compliance strength

10 10.0 1450 Plain concrete
15 15.0 2200 reinforced concrete with lightweight agg.
20 20.0 2900 reinforced concrete with dense aggregate
25 25.0 3600
30 30.0 4350 concrete with post-tensioned tendons
40 40.0 5800 concrete with post-tensioned tendons
50 50.0 7250
60 60.0 8700
 Design of mixes

• Once an average strength has been decided

upon, the mix must be designed to meet this and
any other requirements, including workability.
• All accurate mix design requires a trial mix, the
proportions of which may need modification in
the light of experience.
• This approach, of using a theoretical method to
arrive at a first approximation followed by
modification by trial mixes to achieve a practical
end result.
Designed mix
• The mix is specified by its required performance in terms of a
strength grade, subject to any special requirements for materials,
minimum or maximum cement content, maximum free water/cement
ratio and any other properties required.
Prescribed mix
• The mix is specified by its constituent materials and the properties or
quantities of those constituents to produce a concrete with the
required performance.
Standard mix
• The standard mixes are given in the standard documents based on
the characteristic compressive strength.
Designated mix
• The mix shall be specified by considering the site conditions and
then identifying the application for which the concrete is to be used.
Standard mix
DESIGNATED MIXES
 Batching

• Weigh-batching is preferable in most cases,

though in some circumstances lightweight
aggregates may be measured by volume.
• Cement should always be measured by weight
because its bulk volume is affected by the
compaction it receives in handling.
• It is now usual to specify all materials by weight
 Trial mixes

• For designed mixes, trial mixes should be

prepared by the supplier
• Three cubes made from each batch for test at 28
days.
• A further three cubes from each batch should be
made for test at an earlier age if required.
 Concrete properties

Strength of concrete
Characteristic compressive strength
• As defined in BS 8110, characteristic strength
means the value of the strength of concrete
below which not more than 5% of the test results
fall
• The characteristic strength of the concrete
should be specified at one age only
• Unless specified otherwise, the strength tests
should be carried out at an age of 28 days
Increase in strength with age

• Owing to continuing hydration of the

cement, the strength of concrete increases
with age
• Higher gains in strength may be found
where the concrete is kept wet for a long
time or where the volume of concrete is
large enough to prevent drying out within
the mass.
 Table 5: Increase in strength with age
Grade Characteristic Cube strength at an age of 3 months 6 months 1 year
strength 7 days 2 months
N/mm2 Ibf/in2

N/mm2 Ibf/in2 N/mm2 Ibf/in2 N/mm2 Ibf/in2 N/mm2 Ibf/in2 N/mm2 Ibf/in2

20 20.0 2900 13.5 1960 22 3190 23 3340 24 3480 25 3600

25 25.0 3600 16.5 2390 27.5 3980 29 4200 30 4350 31 4500
30 30.0 4350 20 2900 33 4780 35 5070 36 5220 37 5360
40 40.0 5800 28 4060 44 6370 45.5 6600 47.5 6880 50 7250
50 50.0 7250 36 5220 54 7830 55.5 8050 57.5 8330 60 8700
 Durability of concrete

• The resistance of concrete to weathering, chemical

attack, abrasion, frost and fire depends largely upon its
quality and constituent materials.
• In reinforced and prestressed concrete, protection of the
steel against corrosion is influenced by the depth of
cover provided and the permeability of the concrete.

• The crushing strength alone is not a reliable guide to the

quality and durability of concrete; it must also have
adequate cement content and a low water/cement ratio.
• Furthermore, the mix must be workable so that it can be
fully compacted to produce a dense, impermeable
concrete and provide the required finish.