3.

0 CONRETE

3.1 Introduction

3.1.1 Concrete: definition and composition

Concrete is a hard strong construction material made by mixing cementing material
(such as Portland cement) and mineral aggregates (such as sand and gravel) with sufficient
water to course the cement to set and bind the entire mass. The concrete can be simply
represented as:

Concrete = Filler + Binder.

According to the type of binder used, there are many different kinds of concrete.
For instance, Portland cement concrete, asphalt concrete, and epoxy concrete. In
concrete construction, the Portland cement concrete is used the most. Thus, in this
chapter, the term concrete usually refers to Portland cement concrete. The composition of
Portland cement concrete can be presented as follows:

Cement
(+ Admixture) → Cement paste
+ Water + → mortar
fine aggregate
+ → Concrete
Coarse aggregate

Admixtures are almost always used in modern practice and thus become an
essential component of modern concrete. Admixtures are defined as materials other than
aggregate (fine and coarse), water, fibre and cement, which are added into concrete
batch immediately before or during mixing. The widespread use of admixture is mainly
due to the many benefits made possible by their application. For instance, chemical
admixtures can modify the setting and hardening characteristic of cement paste by
influencing the rate of cement hydration. Water -reducing admixture can plasticize fresh
concrete mixtures by reducing surface tension of water, air-entraining admixtures can
improve the durability of concrete, and mineral admixtures such as Pozzolans (materials
containing reactive silica) can reduce thermal cracking. A detailed description of
admixtures will be given in latter sections.

3.1.2 Uses, advantages and limitations

Concrete is the most widely used material (construction and others) in the world. It
is used in many different structures such as dam, pavement, building frame, bridge,
retaining walls and diaphragm walls. Its worldwide production exceeds that of steel by
a factor of 10 in tonnage and by more than a factor of 30 in volume. The present
consumption of concrete is about 55 billion tons a year. Second to water, concrete is the
most widely consumed material, with three tons per year used for every person in the
world. It is more than 10 times of the consumption by weight of steel. Concrete is neither
as strong nor as tough as steel, so why is concrete so popular?
Concrete have survived wars and natural disasters, outlasting many civilizations
that built it. Alongside its strength and resilience, it is also a staple of building because it is
relatively cheap easy to make and mold.
Concrete possesses excellent resistance to water
 Aqueducts and waterfront retaining walls

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  Dams, canal linings, and pavements
 Structural elements exposed to moisture, such as piles, foundations, footings, floors,
beams, columns, roofs, exterior walls, and pipes
Cheap and easy to mould
 Freshly made concrete is of a plastic consistency
 Flow into prefabricated formwork
 Formwork can be removed for reuse
 The cheapest and most readily available material on the job
 Aggregate, water, and Portland cement are relatively inexpensive and are commonly
available
 Cost may be as low as U.S. $60 to $70 per cubic meter
Advantages:
a) Economical: Concrete is the most inexpensive and the most readily available
material. The cost of production of concrete is low compared with other
engineering construction materials.

Three major components: water, aggregate and cement. Comparing with steel,
plastic and polymer, they are the most inexpensive materials and available in every
corner of the world. This enables concrete to be locally produced anywhere in the
world, thus avoiding the transportation costs necessary for most other materials.

b). Ambient temperature hardened material: Because cement is a low temperature

bonded inorganic material and its reaction occurs at room temperature, concrete
can gain its strength at ambient temperature.

c) Ability to be cast: It can be formed into different desired shape and sizes right at
the construction site.

d) Energy efficiency: Low energy consumption for production, compare with steel
especially. The energy content of plain concrete is 450-750 kWh / ton and that of
reinforced concrete is 800-3200 kWh/ton, compared with 8000 kWh/ton for
structural steel.

e) Excellent resistance to water. Unlike wood and steel, concrete can harden in
water and can withstand the action of water without serious deterioration. This
makes concrete an ideal material for building structures to control, store, and
transport water. Examples include pipelines (such as the Central Arizona Project,
which provide water from Colorado river to central Arizona. The system contains
1560 pipe sections, each 6.7 m long and 7.5 m in outside diameter 6.4 m inside
diameter), dams, and submarine structures. Contrary to popular belief, pure water
is not deleterious to concrete, even to reinforced concrete: it is the chemicals
dissolved in water, such as chlorides, sulfates, and carbon dioxide, which cause
deterioration of concrete structures.

f). High temperature resistance: Concrete conducts heat slowly and is able to store
considerable quantities of heat from the environment (can stand 6-8 hours in fire)
and thus can be used as protective coating for steel structure.

g). Ability to consume waste: Many industrial wastes can be recycled as a substitute
for cement or aggregate. Examples are fly ash, ground tire and slag.

h). Ability to work with reinforcing steel: Concrete and steel possess similar
coefficient of thermal expansion (steel 1.2 x 10 -5; concrete 1.0-1.5 x 10-5).

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 Concrete also provides good protection to steel due to existing of CH (this is for
normal condition). Therefore, while steel bars provide the necessary tensile
strength, concrete provides a perfect environment for the steel, acting as a physical
barrier to the ingress of aggressive species and preventing steel corrosion by
providing a highly alkaline environment with pH about 13.5 to passivate the steel.

i) Less maintenance required: No coating or painting is needed as for steel

structures.
Limitations:

a) Quasi-brittle failure mode: Concrete is a type of quasi-brittle material. (Solution:

Reinforced concrete)

b) Low tensile strength: About 1/10 of its compressive strength. (Solutions: Fiber
reinforced concrete; polymer concrete)

c) Low toughness: The ability to absorb energy is low. (Solution: Fiber reinforced
concrete)

d) Low strength/BSG ratio (specific strength): Steel (300-600)/7.8. Normal

concrete (35-60)/2.3. Limited to middle-rise buildings. (Solutions: Lightweight
concrete; high strength concrete)

e) Formwork is needed: Formwork fabrication is labour intensive and time

consuming, hence costly (Solution: Precast concrete)

f). Long curing time: Full strength development needs a month. (Improvements:
Steam curing)

g). Working with cracks: Most reinforced concrete structures have cracks under
service load. (Solutions : Prestressed concrete).

3.1.3 Classification of concrete

Based on unit weight

Ultra light concrete <1,200 kg/m3
Lightweight concrete 1200- 1,800 kg/m3
Normal-weight concrete ~ 2,400 kg/m3
Heavyweight concrete > 3,200 kg/m3
Based on strength (of cylindrical sample)
Low-strength concrete < 20 MPa compressive strength
Moderate-strength concrete 20 -50 MPa compressive strength
High-strength concrete 50 - 200 MPa compressive strength
Ultra high-strength concrete > 200 MPa compressive strength

Based on additives:
Normal concrete
Fiber reinforced concrete
Shrinkage-compensating concrete
Polymer concrete

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a) Reinforced cement concrete is a composite material made up of cement concrete and
reinforcement, while the concrete resists compression, reinforcement resist the tension and
shear. It is the most versatile building material available and is extensively used in the
construction industry ranging from small structural elements such as beams and columns to
massive structures like dams and bridges. The idea of reinforcing concrete with steel has
resulted in a composite material, having the potential of resisting significant tensile stresses.
The steel bars are embedded in the tensile zone of concrete to compensate the poor
tensile resistance of concrete (Fig. 3.1 (a) (b)). The bond between steel and the surrounding
concrete ensures strain compatibility. Moreover, the reinforcing steel imparts ductility to
this composite material. The reinforcing steel also supplements concrete in bearing
compressive forces, as in the case of columns. Here the bars are confined with lateral ties,
in order to maintain their positions and to prevent their local buckling. In addition, the lateral
ties also serve to confine the concrete, thereby enhancing its compression load bearing
capacity (Fig. 3.1 (c)). Figure 3.1 depicts the advantages of reinforcing steel bars in beams
and columns.

Fig. 3.1: Contribution of steel bars in reinforced concrete.

b) Prestressed Concrete

One of the serious limitation of reinforced cement concrete is the cracking which is a
natural phenomenon for concrete constructions. Once cracks occur they do not disappear
even after removal of load. If the width of these cracks is to be kept within permissible
limits, the steel stress has to be kept low. Presence of cracks lowers the capacity of
structure to bear reversal of stresses, impact vibration and shocks. Also, the reinforcing
bars may get corroded in due course of time and the concrete deteriorates. Besides these

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disadvantages, the presence of cracks makes theory of reinforced concrete quite irrational.
Efforts were made to eliminate the cracking of concrete by artificially introducing in it either
before or simultaneously with the application of external loads, a compressive force of
permanent nature. This force is so applied that it causes compressive stresses in that zone of
the member where tensile stress will be caused by external loads. The tensile stress in
concrete will thus be neutralized and it will not crack.
A prestressed concrete may thus be defined as a concrete in which stresses of suitable
magnitude and distribution are introduced to counteract, to a desired degree, the stresses
resulting from external loads. The concept of prestressing concrete was first used by Mandl
of France in 1896. In prestressed concrete high strength concrete and steel are desirable. The
former is required because of following:

1. The use of high strength concrete results in smaller cross-section of member and hence
smaller self-weight; longer spans become technically and economically practicable.
2. High bearing stresses are generated in anchorage zones.
3. The shrinkage cracks are reduced, with higher modulus of elasticity and smaller creep
strain resulting in smaller loss of prestress.

The loss of prestress at the initial stages is very high and for this reason high strength steel is
required. High tensile strength wires with ultimate tensile strength up to 2010 N/mm2 are the
choice. For prestressed concrete members, the high tensile steel used generally consists of
coires, bars or strands.
Prestressing is achieved by either pre-tensioning or post-tensioning. In Pre-tensioning, the
wires or cables are anchored, tensioned and concrete is cast in the molds. After the concrete
has gained strength the wires are released. This sets up compression in concrete which
counteracts tension in concrete because of bending in the member. In the post-tensioning
prestressing force is applied to the steel bars or cables, after the concrete has hardened
sufficiently. After applying the full prestress the cable passages are grouted. The minimum
28-day cube compressive strength for concrete is 40 N/mm2 for pre-tensioned members and
30 N/mm2 for post-tensioned members.

Advantages
1. The cracking of concrete is eliminated enabling the entire cross-section of the member to
take part in resisting moment.
2. As dead load moments are neutralized and the shear stresses are reduced, the sections
required are much smaller than those for reinforced concrete. This reduces the dead weight of
structure.
3. In ordinary reinforced concrete (RCC) the economy is not as pronounced as in prestressed
concrete (PSC).
The prestressing force in most cases is computed strictly from dead load of the structure;
consequently, a weight reduction of 25% results in a substantial reduction in the weight of
prestressing tendons.

Uses
It is widely used for construction of precast units such as beams, floors, roofing systems,
bridges, folded plate roofs, marine structures, towers and railway sleepers.

c) Polymer Concrete

The strength of concrete is greatly affected by porosity and attempts to reduce it by vibration,
pressure application, spinning, etc. are of little help in reducing the water voids and the
inherent porosity of gel which is about 28 per cent. The impregnation of monomer and
subsequent polymerisation reduces the inherent porosity of the concrete. Polymers—
polyvinyl acetate, homopolymer emulsions and vinyl acetate copolymer emulsions—are
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added to increase strength, resistance to oil and abrasion. They also improve bond between
new and old concrete and are useful for prefabricated structural element and prestressed
concrete. The disadvantages are that they are very brittle and expensive.
For heavy duty Industrial floor the concrete mix used is 1:2:2. Concrete to PVA emulsion in
the ratio 3:1 is then prepared.
For domestic or office floor cement and sand in the ratio of 1:2 is mixed. The cement mortar:
PVA emulsion is then made in the ratio 2:1.

Types The available polymer concrete materials are polymer impregnated concrete (PIC),
polymer cement concrete (PCC), polymer concrete (PC) and, partially impregnated and
surface coated polymer concrete.

Polymer Impregnated Concrete is a conventional concrete, cured and dried in oven. A low
viscosity manomer is then diffused and polymerised by using radiation, heat or by chemical
initiation. The manomers used are, methylmethacrylate (MMA), styrene, acrylonitrile, t-butyl
styrene, etc.
Polymer Cement Concrete is made by mixing cement, aggregates, water and manomers, such
as polyester-styrene, epoxy styrene, furans, venylidene chloride. The plastic mix is moulded,
cured, dried and polymerised.

Polymer Concrete In this type of concrete cement is not used and the aggregates are bound
with a polymer binder. It is most suitable for structures with a high ratio of live load to dead
load and composite construction.
Partially impregnated and surface coated concrete is made by initially soaking the dried
specimens in liquid manomer like methyl methacrylate and then sealing them by keeping
under hot water at 70°C to prevent loss due to evaporation. The polymerisation is achieved
by adding 3 per cent by weight of benzoyl peroxide to the manomer as catalyst. It finds its
application in improving durability of bridge decks.
Application Polymer concrete finds its application in the production of prefabricated
elements, prestressed concrete, ferrocement products, marine works, nuclear power plants
and industrial applications. Because of its high sulphate and acid resistance properties it is
most suitable for sewage disposal works.

d) FIBER REINFORCED CONCRETE

Conventional concrete is modified by random dispersal of short discrete fine fibres of

asbestos, steel, sisal, glass, carbon, poly-propylene, nylon, etc. Asbestos cement fibres so
far have proved to be commercially successful. The improvement in structural performance
depends on the strength characteristics, volume, spacing, dispersion and orientation, shape
and their aspect ratio (ratio of length to diameter) of fibres. A fibre-reinforced concrete
requires a considerably greater amount of fine aggregate than that for conventional concrete
for convenient handling. For FRC to be fully effective, each fibre needs to be fully embedded
in the matrix, thus the cement paste requirement is more. For FRC the cement paste required
ranges between 35 to 45 per cent as against 25 to 35 per cent in conventional concrete.

The behaviour of fibre reinforced concrete (FRC) is shown in Fig. 3.2. The tensile cracking
strain of cement matrix is about 1/50 of that of yield of steel fibres. Consequently when FRC
is loaded, the matrix cracks long before the fibres are fractured. Once the matrix is cracked
the composites continue to carry increasing tensile stress, provided the pullout resistance of
fibres at the first crack is greater than the load at the first cracking. The bond or the pullout
resistance of the fibres depends on the average bond strength between the fibres and the
matrix, the number of fibres crossing the crack, the length and diameter of fibres, and the
aspect ratio. The first flexural cracking load on a FRC member increases due to crack
arresting mechanism of the closely spaced fibres. After the first crack fibres continue to take
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load provided the bond is good. Thereafter the fibres, reaching the breaking strain fracture.
The neutral axis of the section shifts and the fibres of adjacent layers fracture on reaching the
breaking strain. Failure occurs when the concrete in compression reaches the ultimate strain.

Fig. 3.2: Behaviour of Fiber reinforced Concrete

Advantages

1. Strength of concrete increases.

2. Fibres help to reduce cracking and permit the use of thin concrete sections.
3. Mix becomes cohesive and possibilities of segregation are reduced.
4. Ductility, impact resistance, tensile and bending strength are improved.

Disadvantages

1. Fibres reduce the workability of a mix and may cause the entrainment of air.
2. Steel fibres tend to intermesh and form balls during mixing of concrete.

Application Fibre reinforced concrete is useful in hydraulic structures, airfield pavements,

highways, bridge decks, heavy duty floors, and tunnel linings.

e) FERROCEMENT

Ferrocement is a composite material in which the filler material (called matrix), cement
mortar, is reinforced with fibres, usually steel mesh dispersed throughout the composite,
which results in better structural performances than individual ones. The fibres impart tensile
strength to the mass.
In rationally designed ferrocement structures the reinforcements consist of small diameter
wire meshes wherein uniform distribution of reinforcement is made possible throughout the
thickness of the element. Because of the distribution of such reinforcement over the entire
matrix, high resistance to cracking is achieved. Toughness, fatigue resistance,
impermeability, etc. are also improved. This material which is a special form of reinforced
concrete, exhibits a behavior so different from conventional reinforced concrete in
performance, strength and potential application that it must be classed as a separate material.
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The reinforcement may be hexagonal wire mesh (0.5– 1.00 mm diameter at 5–25 mm
spacing) welded wire mesh (18–19 gauge), woven mesh, expanded metal sheet and Watson
mesh (Fig. 3.3). Generally characteristics of different types of meshes are given in Table 3.3.
The skeletal steel may be placed 300 mm apart to serve as a spacer rod to the mesh
reinforcement. A rich mix of Portland cement and sand usually 1:2 to 1:1.5 is used as mortar.
The thickness of the ferrocement element is kept 10–40 mm, with a clear cover of 1.5– 2 mm
to reinforcement.

Fig. 3.3: Different types of welded wire meshes

Table 3.0: General characteristics of different types of meshes

Steel bars, generally provided to make the formwork of the structure are known as skeletal
steel. The size of the rod varies from 4 to 10 mm and at a spacing of 300 mm apart. In highly
stressed structures like boats, barages and tubular sections this spacing is reduced to 75
mm. For structures steel rods, along with wire mesh, are considered as a component of
reinforcement imparting structural strength and stiffness, whereas in most of the terrestrial
structures wire mesh is treated as the main reinforcement. The steel content in ferrocement
various from 1–8%.
Portland cement, rapid hardening Portland cement, Sulphate resisting Portland cement
or Portland blast-furnace cement may be used.
Ferrocement which is specially advantageous in spatial structure has relatively better
mechanical properties and durability than ordinary reinforced concrete. Within certain
loading limits, it behaves as homogeneous. The high surface area to volume ratio (specific
surface) of the reinforcement results in better crack arrest mechanism, i.e., the propagation of
cracks are arrested resulting in high tensile strength of the material. Recent studies show that
the inclusion of short steel fibres in ferrocement increases further the first crack strength of
the composite. Its ultimate strength depends almost entirely upon the volume fraction of the
wire mesh.

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Ferrocement is used in thin-walled structures where strength and rigidity are developed
through form or shape. It has the distinct advantage of being moldable and of one piece
construction. Other major advantages are its low cost and its non-flamability and high
corrosion resistance characteristics. The advantages of ferrocement are:

1. Easy availability of raw materials.

2. Reduction in weight consequent of thin section.
3. Moulding can be done without any formwork.
4. No machinery or sophistication is required in construction.

Mechanical Properties: Ferrocement, a homogeneous composite material, contains a high

percentage of ductile steel wire mesh with a high surface area to volume ratio in a brittle
cement-mortar matrix, enables the matrix to assume the ductile characteristics of the
reinforcement. The strength of ferrocement, as in ordinary concrete, is commonly considered
as the most valuable property, although in many practical cases other characteristics, such as
durability and permeability may in fact be more important. Nevertheless, strength always
gives an overall picture of the quality of ferrocement, as strength is directly related with the
properties of its hardened cement paste and reinforcement. Some of the properties of steel
were meshes and rainforcement are given in Table 3.2.

Table3.2: Mechanical properties of steek wire meshes and reinforceing bars.

f) LIGHT WEIGHT CONCRETE

Conventional cement concrete is a heavy building material. For structures such as multistorey
buildings it is desirable to reduce the dead loads. Light weight concrete (LWC) is most suitable
for such construction works. Lightweight aggregate concrete is particularly suitable for use
where low density, good thermal insulation or fire protection are required but not all of the
available aggregate are equally suitable for any particular application. It is best produced by
entraining air in the cement concrete and can be obtained by anyone of the following methods:
1. By making concrete with cement and coarse aggregate only. Sometimes such a concrete is
referred to as no-fines concrete. Suitable aggregates are—natural aggregate, blast furnace
slag, clinker, foamed slag, etc. Since fine aggregates are not used, voids will be created and the
concrete produced will be light weight.
2. By replacing coarse aggregate by porous or cellular aggregate. The concrete produced is
known as cellular concrete which is further classified as follows.

Based on Manufacturing Method—classified as foam concrete and gas concrete.

Based on Type of Binding Material—classified as gas and foam concrete (Portland cement),
gas and foam concrete (lime and sand), gas slag and foam slag concretes (lime and finely
divided blast furnace slag or fly ash).

Foam Concrete is obtained by mixing cement paste or mortar with stabilized foam. After
hardening, the foam cells form concrete of a cellular structure. The foam is obtained by stirring a
mixture of resin soap and animal glue. The best foaming agents are alumino sulpho napthene

9
compounds and hydrolysed slaughter blood. This concrete is very suitable for heat insulation
purposes.

Heat insulating foam concrete is cast into blocks measuring 100 × 50 × 50 cm and over, which,
once hardened, can be sawn into slabs of 100 × 50 × 50 to 100 × 50 × 12 cm. Heat insulating
foam concrete has a strength of up to 2.5 N/mm2 and a coefficient of heat conductivity of 0.10–
0.20 k Cal/m.h.°C. This kind of foam concrete is used as heat insulating material for reinforced
concrete floors, partitions, etc.

Structural-and-heat insulating foam concrete has a strength of 2.5 – 7.5 N/mm2 and a
coefficient of heat conductivity of 0.20–0.40 kCal/m.h.°C and is used for exterior walls.

Structural foam concrete is used to make reinforced floor components, the items being
reinforced by two wire meshes from wires of 3–5 mm thick. Structural foam concrete has a
strength up to 15 N/mm2 and a coefficient of heat conductivity of 0.40–0.60 kCal/m.h°C. Heat
insulating foam concrete is widely used in three-layer exterior walls of heated buildings.

Gas Concrete is manufactured by expanding the binding material paste, which may or may not
include aggregates. It is also known as aerated concrete. The mix is expanded by gas forming
substances, but care should be taken to synchronize the end of gas formation with the beginning
of mix setting. The setting time of cement may be regulated with the aid of accelerators (such as
dihydrate gypsum) or retarders (such as industrial sugar, or molasses, introduced in amounts
from 0.1 to 2.5 kg/m3).

The approximate relative proportions of gas concrete ingredients are as follows: 90% Portland
cement, 9.75% powdered lime, 0.25% aluminum power (for a water to cement ratio of 0.55–
0.65). About 2/3 of sand are ground in a wet state. The basic considerations in choosing the
proportion of light weight concrete are economy consistent with place-ability and adequate
strength, and attainment of specified bulk density with the lowest consumption of cement. Lime
for preparing gas concrete should be of the top grade, quick-slaking and low-magnesium variety.
In sand intended for gas concrete, the content of clay impurities should not exceed 1.5% by
weight, since the impurities lower the strength and slow down gas evacuation and concrete
expansion. The gas forming agent used is finely ground aluminum powder. The evolving
hydrogen produced during the chemical reaction between hydrate calcium oxide and aluminum
according to the equation

2Al + 3Ca(OH)2 + 6 H2O = 3 CaO.Al2O3.6H2O + 3H2

expands the cement paste which retains its porous structure as it hardens.

Items from gas concrete are manufactured in the manner described below. A mixture of ground
sand and water is fed to the stirrer and mixed with cement, aluminum powder, water and un-
ground sand, after which the mix is cast into molds. After 4–5 h of hardening, gas concrete is cut
into slabs and loaded into autoclaves where the items finally harden at a temperature of 175°C
and at a pressure of 8 atm. Autoclaving enhances the strength of gas concrete and, in addition,
substantially reduces the consumption of cement which can thus be fully or partially replaced by
lime. Gas concrete is similar to foam concrete in properties and is used for the same purpose.
However, it is simpler to manufacture, and items form it have more stable qualities than from
foam concrete: in particular, this applies to their bulk densities. These are the chief advantages
of gas concrete over foam concrete. Among the main shortcomings of cellular concrete are high
tendency to deformation, shrinkage, etc.

Characteristics
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Density The density of LWC varies from 300–1200 kg/m2

Workability Due to low density and the characteristic texture of porous aggregate especially in
the crushed state, the workability of concrete needs special attention. In general, placing
compacting, and finishing lightweight aggregate concrete requires relatively less effort;
therefore, even 50 to 75 mm slum may be sufficient to obtain workability of the type that is
shown by 100 to 125 mm slump of normal-weight concrete.

Unit Weight- Unit weight and strength are the two properties generally sought from lightweight
concrete. With given materials, it is generally desired to have the highest possible strength/ unit
weight ratio with the lowest cost of concrete. The air-dried unit weight of concrete is limited to a
maximum of 18.40 kN/m3. The use of normal sand to control the properties of hardened
concrete tends to increase the unit weight, although this tendency is partially offset from the
balancing effect of entrained air, which is invariably prescribed for improving the workability.
Most structural lightweight concretes weigh between 16.00 to 17.60 kN/m3 ; however, job
specifications in special cases may allow higher than 18.40 kN/m3.

Strength Design strengths of 20 to 35 MPa, 28 day compressive strengths are common, although
by using a high cement content and good quality light weight aggregate of small size (9 to 13
mm maximum) has made it possible, in some precast and prestressing plants, to produce 40 to
48 MPa concrete. Lightweight aggregates with controlled micro porosity have been developed to
produce 70 to 75 MPa lightweight concrete which generally weigh 18.40 to 20.00 kN/m3. The
ratio between the splitting tensile strength and compressive strength decreases significantly with
increasing strength of lightweight concrete.

Thermal Insulation is about 3–4 times more than that of bricks and about 10 times than that of
concrete.
Fire resistance is excellent
Sound insulation is Poor
Durability
Aerated concrete is slightly alkaline. Due to its porosity and low alkalinity the reinforcement
may be subjected to corrosion and as such, require special treatments.

Repair ability Light weight cellular element can be easily sawn, drilled or nailed which makes
for easy construction and repairs.

Economy Due to light weight and high strength to mass ratio, the cellular products are quite
economical.

Advantages
The basic economy of LWC can be demonstrated by the savings achieved in associated
reinforcement requirement. LWC has superior resistance of shear elements to earthquake
loading since seismic forces are largely a direct function of dead weight of a structure, is also
one among the other advantages of LWC. Due to lower handling transportation, the construction
cost, the light weight concrete is ideally suited for the production of precast concrete elements
and prefabricated elements.

Application
1. Low density cellular concrete is used for precast floor and roofing units.
2. As load bearing walls using cellular concrete blocks.
3. As insulation cladding to exterior walls of structures.

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3.3 Fresh concrete

3.3.1 Definition

Fresh concrete is defined as concrete at the state when its components are fully
mixed but its strength has not yet developed. This period corresponds to the cement
hydration stages 1, 2, and 3. The properties of fresh concrete directly influence the
handling, placing and consolidation, as well as the properties of hardened concrete.

3.3.2 Workability

a) Definition

Workability is a general term to describe the properties of fresh concrete.

Workability is often defined as the amount of mechanical work required for full
compaction of the concrete without segregation.

This is a useful definition because the final strength of the concrete is largely
influenced by the degree of compaction. A small increase in void content due to
insufficient compaction could lead to a large decease in strength.

The primary characteristics of workability are consistency (or fluidity) and

cohesiveness. Consistency is used to measure the ease of flow of fresh concrete. And
cohesiveness is used to describe the ability of fresh concrete to hold all ingredients
together without segregation and excessive bleeding.

b). Factors affecting workability

. Water content: Except for the absorption by particle surfaces, water must fill the
spaces among particles. Additional water "lubricates" the particles by separating them with
a water film. Increasing the amount of water will increase the fluidity and make concrete
easy to be compacted. Indeed, the total water content is the most important parameter
governing consistency. But, too much water reduces cohesiveness, leading to segregation
and bleeding. With increasing water content, concrete strength is also reduced.
. Aggregate mix proportion: For a fixed w/c ratio, an increase in the
aggregate/cement ratio will decrease the fluidity. (Note that less cement implies less water,
as w/c is fixed.) Generally speaking, a higher fine aggregate/coarse aggregate ratio leads to
a higher cohesiveness.
. Maximum aggregate size: For a given w/c ratio, as the maximum size of
aggregate increases, the fluidity increases. This is generally due to the overall reduction in
surface area of the aggregates.
. Aggregate properties: The shape and texture of aggregate particles can also
affect the workability. As a general rule, the more nearly spherical and smoother the
particles, the more workable the concrete.
. Cement: Increased fineness will reduce fluidity at a given w/c ratio, but increase
cohesiveness. Under the same w/c ratio, the higher the cement content, the better the
workability (as the total water content increases).

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 . Admixtures: Air entraining agent and superplasticizers can improve the
workability.
. Temperature and time: As temperature increases, the workability decreases. Also,
workability decreases with time. These effects are related to the progression of chemical
reaction.
c). Segregation and bleeding
. Segregation (separation): Segregation means separation of the components of
fresh concrete, resulting in a non-uniform mix. More specifically, this implies some
separation of the coarse aggregate from mortar.
. Bleeding (water concentration): Bleeding means the concentration of water at
certain portions of the concrete. The locations with increased water concentration
are concrete surface, bottom of large aggregate and bottom of reinforcing steel.
Bleed water trapped under aggregates or steel lead to the formation of weak and
porous zones, within which microcracks can easily form and propagate.

3.3.3 Measurement of workability

a). Slump test (BS 1881: 102, ASTM C143):
Three different kinds of possible slumps exist, true slump, shear slump, and
collapse slump. Conventionally, when shear or collapse slump occur, the test is considered
invalid. However, due to recent development of self compact concrete, the term of
collapse slump has to be used with caution.

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 b). Compaction factor test (BS 1881: Part 103):

The compacting test was developed in Great Britain in 1947. As shown in the
figure, the upper hopper is completely filled with concrete, which is then successively
dropped into the lower hopper and then into the cylindrical mould. The excess of concrete
is struck off, and the compacting factor is defined as the weight ratio of the concrete in the
cylinder, mp , to the same concrete fully compacted in the cylinder (filled in four layers
and tamped or vibrated), mf (i.e., compacting factor = mp/mf ). For the normal range of
concrete the compacting factor lies between 0.8 to 0.92 (values less than 0.7 or higher
than 0.98 is regarded as unsuitable). This test is good for very dry mixes.

Three limitations: (i) not suitable for field application; (ii) not consistent; (iii)
mixes can stick to the sides of the hoppers.

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14
 c). Vebe test (BS 1881: Part 104):

The Vebe consistometer was developed in 1940 and is probably the most suitable
test for determining differences in consistency of very dry mixes. This test method is
widely used in Europe and is described in BS 1881: Part 104. It is, however, only
applicable to concrete with a maximum size of aggregate of less than 40 mm. For the
test, a standard cone is cast. The mould is removed, and a transparent disk is placed on the
top of the cone. Then it is vibrated at a controlled frequency and amplitude until the lower
surface of the disk is completely covered with grout. The time in seconds for this to occur
is the Vebe time. The test is probably most suitable for concrete with Vebe times of 5 to
30s. The only difficulty is that mortar may not wet the disc in a uniform manner, and it
may be difficult to pick out the end point of the test.

15
 d). Ball-penetration test

A measure of consistency may also be determined by ball penetration (ASTM

C360). Essentially, this test consists of placing a 30-lb metal cylindrical weight, 6" in
diameter and 4-5/8" in height, having a hemispherically shaped bottom, on the smooth
surface of fresh concrete and determining the depth to which it will sink when released
slowly. During penetration the handle attached to the weight slides freely through a hole in
the center of the stirrup which rests on large bearing areas set far enough away from the
ball to avoid disturbance when penetration occurs. The depth of penetration is obtained
from the scale reading penetration of the handle, using the top edge of the independent
stirrup as the line of reference. Penetration is measured to the nearest 1/4", and each
reported value should be the average of at least three penetration tests. The depth of
concrete to be tested should not be less than 8". This test is quickly made and is less prone
to personal errors.

The ratio of slump to penetration is usually between 1.3 and 2.0.

16
3.3.4 Setting of concrete

a). Definition: Setting is defined as the onset of rigidity in fresh concrete. It is

different from hardening, which describes the development of useful and
measurable strength. Setting precedes hardening although both are controlled by
the continuing hydration of the cement.

b). Abnormal setting

False setting: If concrete stiffens rapidly in a short time right after mixing but
restores its fluidity by remixing, and then set normally, the phenomenon is called
false setting. The main reason causing the false setting is crystallization of gypsum.
In the process of cement production, gypsum is added into clinker through inter-
grinding. During grinding, the temperature can rise to about 120oC, thus causing
the following reaction:

CS H2 → CS H1/2

The C S H1/2 is called plaster. During mixing, when water is added, the plaster will
re-hydrate to gypsum and form a crystalline matrix that provides ‘stiffness’ to the
mix. However, due to the small amount of plaster in the mix, very little strength
will actually develop. Fluidity can be easily restored by further mixing to break up
the matrix structure.

Flash setting: Flash setting is caused by the formation of large quantities of

monosulfoaluminate or other calcium aluminate hydrates due to quick reactivity

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17
of C3A. This is a rapid set with the development of strength and thus is more
severe than false setting. However, as we mentioned before, flash setting can be
eliminated by the addition of 3-5% gypsum into cement.

Thixotropic set is due to the presence of abnormally high surface charges on the
cement particles. It can be taken care of by additional mixing.

As the hydration reaction progresses with time, the concrete becomes less flowable,
and the slump value will naturally decrease. However, if the slump value decreases
at an abnormally fast rate, the phenomenon is called “slump loss”. It is often due to
the use of abnormal setting cement, the unusually long time taken in the mixing and
placing operations, or the high temperature of the mix (e.g., when concrete is
placed under hot weather, or when ingredients have been stored under high
temperature). In the last case, ice chips can be used to replace part of water to lower
the temperature.

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18
3.3.5 Placing, Compacting and Curing

Concrete should be place close to its final position as possible. To minimize

segregation, it should not be moved over too long a distance. After concrete is placed in
the formwork, it has to be compacted to remove entrapped air. Compaction can be carried
out by hand rodding or tamping, or by the use of mechanical vibrators.

For concrete to develop strength, the chemical reactions need to proceed

continuously. Curing refers to procedure of maintaining of a proper environment for the
hydration reactions to proceed. It is therefore very important for the production of strong,
durable and watertight concrete. In concrete curing, the critical thing is to provide
sufficient water to the concrete, so the chemical reaction will not stop. Moist curing is
provided by water spraying, ponding or covering the concrete surface with wet sand,
plastic sheets, burlaps or mats. Curing compounds, which can be sprayed onto the
concrete surface to form a thin continuous sheet, are also commonly used. Loss of water to
the surrounding should be minimized. If concrete is cast on soil subgrade, the subgrade
should be wetted to prevent water absorption. In exposed areas (such as a slope),
windbreaks and sunshades are often built to reduce water evaporation. For portland
cement concrete, a minimum period of 7 days of moist curing is generally recommended.

Under normal curing (at room temperature), it takes one week for concrete to reach
about 70% of its long-term strength. Strength development can be accelerated with a
higher curing temperature. In the fabrication of pre-cast concrete components, steam
curing is often employed, and the 7-day strength under normal curing can be achieved in
one day. The mold can then be re -used, leading to more rapid turnover. If curing is carried
out at a higher temperature, the hydration products form faster, but they do not form as
uniformly. As a result, the long-term strength is reduced. This is something we need to
worry about when we are casting under hot weather. The concrete may need to be cooled
down by the use of chilled water or crushed ice. In large concrete structures, cooling of
the interior (e.g., by circulation of water in embedded pipes) is important, not only to
prevent the reduction of concrete strength, but also to avoid thermal cracking as a result of
non-uniform heating/cooling of the structure.

After concrete is cast, if surface water evaporation is not prevented, plastic

shrinkage may occur. It is the reduction of concrete volume due to the loss of water. It
occurs if the rate of water loss (due to evaporation) exceeds the rate of bleeding. As
concrete is still at the plastic state (not completely stiffened), a small amount of volume
reduction is still possible, and this is accompanied by the downward movement of material.
If this downward movement is restraint, by steel reinforcements or large aggregates, cracks
will form as long as the low concrete strength is exceeded. Plastic shrinkage cracks often
run perpendicular to the concrete surface, above the steel reinforcements. Their presence
can affect the durability of the structure, as they allow corrosive agents to reach the steel
easily. If care is taken to cover the concrete surface and reduce other water loss (such as
absorption by formwork or subgrade), plastic shrinkage cracking can be avoided. If
noticed at an early stage, they can be removed by re-vibration.

19
3.4 Admixtures used in concretes
Historically, admixtures are almost as old as concrete itself. It is known that the
Romans used animal fat, milk, and blood to improve their concrete properties. Although
these were added to improve workability, blood is a very effective air-entraining agent and
might well have improved the durability of Roman concrete. In more recent times, calcium
chloride was often used to accelerate hydration of cement. The systematic study of
admixtures began with the introduction of air-entraining agents in the 1930s when people
accidentally found that cement ground with beef tallow (grinding aid) had more resistance
to freezing and thawing than a cement ground without it. Nowadays, as we mentioned
earlier, admixtures are important and necessary components for modern concrete
technology. The concrete properties, both in fresh and hardened states, can be modified or
improved by admixtures. In some countries, 70-80% of concrete (88% in Canada, 85% in
Australia, and 71% in US) contains one or more admixtures. It is thus important for civil
engineers to be familiar with commonly used admixtures.
3.4.1 Definition and classifications
Admixture is defined as a material other than water, aggregates, cement and
reinforcing fibers that is used in concrete as an ingredient and added to the batch
immediately before or during mixing.
Admixtures can be roughly divided into the following groups.
i). Air-entraining agents (ASTM C260): This kind of admixture is used to improve
the frost resistance of concrete (i.e., resistance to stresses arising from the freezing of
water in concrete).
ii). Chemical admixtures (ASTM C494 and BS 5075): This kind of admixture is
mainly used to control the setting and hardening properties for concrete, or to reduce its
water requirements.
iii). Mineral admixtures: They are finely divided solids added to concrete to
improve its workability, durability and strength. Slags and pozzolans are important
categories of mineral admixtures.
iv). Miscellaneous admixtures include all those materials that do not come under
the above mentioned categories such as latexes, corrosion inhibitors, and expansive
admixtures.

20
3.4.2 Chemical admixtures
This includes soluble chemicals that are added to concrete for the purpose of
modifying setting times and reducing the water requirements of concrete mixes.
3.4.2.1 Water reducing admixtures
Water reducing admixtures lower the water required to attain a given slump value
for a batch of concrete. The use of water reducing admixture can achieve different
purposes as listed in the following table.

Water-reducing admixture are surface-active chemicals. It can separate the cement

particles by increasing the static charge on the particle surfaces and thus releasing the
water entrapped by cement particle clusters (see Figure below). More water is then
available to ‘lubricate’ the mix.

A water-reducing admixture lowers the water required to attain a given workability.

This means an effective lowering of the w/c ratio that leads to high strength, low
permeability, and improved durability. According to their efficiency, water-reducing
admixtures can be divided into two categories, normal range and high range water-
reducing admixture. The normal range water reducing admixtures or conventional water-
reducing admixtures can reduce 5-10% of water at normal dosages. The high range water-
reducing admixture, also called superplasticizer, can reduce water requirement by 15-30%.
Superplasticizers were initially developed in Japan and Germany. They are long-chained
molecules with a large number of polar groups attached to the main hydrocarbon chain.

69

21
Once they are adsorbed on a cement particle, strong negative charge is introduced on the
particle surface. The surface tension of the surrounding water is hence greatly reduced and
the fluidity of the system is significantly improved. In normal applications, the dosage of
superplasticizer ranges from 0.6 to 3.0% the weight of cement.

Superplasticizers are used for two main purposes:

a). To produce high strength concrete at w/c ratio in a range of 0.23-0.3.

b). To create "flowing" concrete with high slumps in the range of 175 to 225 mm.
(useful in applications involving: rapid pumping of concrete, areas with congested
reinforcements or poor assess – placing can be done with reduced vibration effort )

Associated with the reduced w/c ratio, additional benefits on hardened concrete
include better durability and lower creep and shrinkage.

The major drawbacks of superplasticizer are:

(i). retarding of setting (especially at high dosage)
- introduction of surface charges makes it more difficult for hydration products to
‘bond’ together
(ii). cause more bleeding
- dispersion of cement grains releases ‘trapped’ water
(iii). entrained too much air
- reduced surface tension of water makes it easier for bubbles to form.

3.4.2.2 Setting control admixtures

a). Mechanism

The setting and hardening phenomena of Portland cement paste are derived from
the progressive crystallization of the hydration products. As discussed before,
setting, or the start of significant crystallization of hydration products, occurs at the
end of the induction period, when the concentration of ions (calcium, aluminate
and silicates etc) has reached a critical state. Since the solubility of ions is sensitive
to the presence of other ions in solution, it is possible to change the dissolution rate
of ions from the cement, by introducing other ions. This is the principle behind
retarding and accelerating admixtures. Retarding admixtures are chemicals that can
slow down the dissolution of ions from the cement, thus extending the induction
period and delaying initial set. However, the overall strength development (during
stages 3 and 4) may not be much slower than that without the retarder.
Accelerating admixtures have opposite effect to retarding admixtures. They often
reduce the induction period, and also increase the hardening rate at stages 3 and 4.

b) Applications

i) Retarding admixtures: Mainly used to: 1. Offset fast setting caused by ambient
temperature particularly in hot weather; 2. Control setting of large structural units
to keep concrete workable throughout the entire placing period. Examples include:

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22
lignosulfonic acids and their salts, hydroxycarboxylic acids and their salts as well
as sugars and their derivatives.

ii)Accelerators: Used for plugging leaks, emergency repair, shotcreting and winter
construction in cold region. They are mostly soluble inorganic salts. Calcium
chloride is by far the best known and most widely used accelerator, and its effect is
illustrated in the figure below. However, the introduction of chloride ions can
accelerate the corrosion of steel. Other common accelerators include calcium
acetate and calcium formate.

23
5.4.3 Air-entraining admixtures (0.05% of air entraining agent by weight of cement)

Air-entraining admixtures entrain air in concrete. Air-entraining admixture contain

surface- active agents concentrated at the air-water interface. By lowering the surface
tension, bubbles can form more readily, and remains more stable after they are formed.
With air-entraining admixtures, the mixing water tends to foam and the foam is locked
into the paste during hardening.

The entrained air voids is different from entrapped air voids. The entrained air void
is formed on purpose while the entrapped air void forms by chance when air gets into the
fresh concrete during mixing. Entrapped air voids may be as large as 3 mm; entrained air
voids usually range from 50 to 200 microns. The size distribution of the solids and pores
in a hydrated cement paste is give in the following figure.

Air is entrained into concrete to provide resistance against frost action, or the
freeze/thaw of water in the capillary pores. When freezing occurs, there is a net increase in
volume. If the saturation of the pores is below 91%, the volume increase can be
accommodated. With a higher saturation, however, the volume of ice will be larger than
the pore size. Internal stresses are hence introduced, and cracking may occur. With the
presence of closely spaced air bubbles in the hardened cement paste, when ice starts to
form and grow, the remaining water in the capillary pore can move (through smaller
‘channels’ in the paste) into the air bubbles. The air bubbles thus act as a water reservoir
and help to relieve internal stress arising from water freezing. The effectiveness of air-
entrainment depends on the spacing among the air bubbles. As shown in the following
figure, the smaller the spacing factor (which is defined as the average maximum distance
from any point in the paste to the edge of a void), the more durable the concrete. For
spacing factor beyond 0.3 mm, the entrained air has little effect on durability.

The presence of entrained air also reduces the effective modulus of the hardened
cement paste. With a more flexible paste, the resistance to internal expansion is improved.
The concrete is hence more durable against expansive reactions in general. Also, small air
bubbles act like ‘bearings’ between aggregates. Their size compensates for the lack of fine
particles in sand. Air-entrainment can hence improve both consistency and cohesion.

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24
(Note: durability factor is defined as the percentage of original Young’s modulus retained
after 300 freeze/thaw cycles)

With entrained air, the improved workability allows the reduction of w/c ratio. This
can partly compensate for the reduced strength due to the presence of air bubbles. In
normal air-entrained concrete, the strength loss is in the order of 10-20%.

With increasing entrained air content, the internal stress due to freezing is reduced.
However, the strength of the concrete itself is also decreasing, making it easier for damage
to occur. Hence, there exists an optimal air content that provides the highest durability.

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25
 The volume of air required to give optimum durability is about 4-8% by volume of
concrete as observed from the above figure. The actual fraction depends on the maximum
size of aggregate. With larger aggregate size, the required air content is reduced (see the
Table below). This is because less paste is required to provide workable concrete with
larger size (and hence smaller surface area per unit weight) of the coarse aggregates.

Maximum Recommended Concrete Mortar Paste Spacing

aggregate size Air Content factor (mm)

63.5 mm 4.0% 4.5% 9.1% 16.7% 0.18

38 5.0 4.5 8.5 16.4 0.20
19 6.0 5.0 8.3 16.9 0.23
9.5 7.5 6.5 8.7 19.7 0.28

If entrained air is added into cement paste, the formula for the gel space ratio has to
be modified as follows.

volume of gel (including gel pores)

X=
volume of gel + volume of capillary pores + entrained air
= 0.68α
0.32α + w / c + entrained air

3.4.4 Mineral admixtures

Mineral admixtures are finely divided siliceous materials that are added into
concrete in relatively large amounts (above 10% the weight of the cement). Industrial by-
products are the primary source of mineral admixtures. Common mineral admixtures
include fly ash, condensed silica fume and blast furnace slag. Typical oxide compositions
are given in the table below:

Oxide Fly Ash Blast Furnace Silica Fume Portland

Low Ca High Ca Slag Cement
_____________________________________________________________________
(% by weight)

SiO2 48 40 36 97 20
Al2O3 27 18 9 2 5
Fe2O3 9 8 1 0.1 4
MgO 2 4 11 0.1 1
CaO 3 20 40 --- 64
Na2O 1 --- --- --- 0.2
K2O 4 --- --- --- 0.5
_____________________________________________________________________

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26
 When low- calcium fly ash (<10% CaO) and silica fume are added to cement, the
following reaction occurs.

S+CH+H→ C-S-H

The pozzolanic reaction given by the above equation is of great significance in

concrete technology. The chemical reaction between silicon dioxide (S), and calcium
hydroxide (CH) results in the formation of additional C-S-H. In other words, a weak phase
is converted into a stronger phase. As a result, the ultimate concrete strength is improved.
Materials with no cementing property on its own, but can react with CH at ordinary
temperature to form cementitious compounds, are called pozzolans. It should be noted that
the conversion of CH to C-S- H is also beneficial to concrete durability, as the
permeability of concrete is reduced (due to a denser microstructure) and the resistance to
acidic chemicals and alkali-aggregate reactions is improved (as there is less alkalis).

With high calcium content, blast furnace slag is mainly cementitious on its own.
High calcium fly ash has both cementing and pozzolanic properties. In the following, our
discussion will focus on silica fume and low-calcium fly ash. Behaviour of concrete with
high-calcium fly ash and blast furnace slag is intermediate between that of portland cement
concrete and concrete with low-calcium fly ash.

A) Condensed silica fume

Silica fume is a by-product of the induction arc furnaces in the silicon metal and
ferrosilicon alloy industries. Reduction of quartz to silicon at temperature up to 2000oC
produces SiO vapours, which oxidize and condense in the low temperature zone to tiny
spherical particles consisting of noncrystalline silica. The material removed by filtering
the outgoing gases in bag filters. A size distribution of silica fume, relative to portland
cement and fly ash, is shown in the following figure.

27
 More accurately, the size distribution of a typical silica fume product is provided as:
20% below 0.05 micron
70% below 0.10 micron
95% below 0.20 micron
99% below 0.50 micron.

The surface area is around 20 m2/g and its average bulk density is 586 kg/m3.
Compared with normal portland cement and typical fly ashes, silica fume is two orders of
magnitude finer. With such a small size, the pozzolanic reaction can occur very fast. The
incorporation of silica fume in concrete can hence increase concrete strength at both early
age (due to rapid pozzolanic reaction) and later stage. Besides the pozzolanic reaction, the
development of a denser C-S-H structure through better packing (achievable with the small
silica fume particles) also contributes to the strength improvement. Indeed, the use of silica
fume is an effective way to produce high strength concrete. For strength over 100 MPa, the
addition of silica fume is mandatory. The small size of silica fume creates problems of
handling. It is often mixed in water to avoid inhalation, which is detrimental to human
health. Also, with its large surface area, the water requirement to make workable concrete
is significantly increased. A superplasticizer must be used together with silica fume.

B) Fly ash

Fly ash (pulverized fuel ash) is a by-product of electricity generating plant using
coal as fuel. During combustion of powdered coal in modern power plants, as coal passes
through the high temperature zone in the furnace, the volatile matter and carbon are burned
off, whereas most of the mineral impurities, such as clays, quartz, and feldspar, will melt at
the high temperature. The fused matter is quickly transported to lower temperature zones,
where it solidifies as spherical particles of glass. Some of the mineral matter agglomerates
to form bottom ash, but most of it flies out with the flue gas stream and thus is called fly
ash. This ash is subsequently removed from the gas by electrostatic precipitators.

Fly ash can be divided into two categories according to the calcium content. The
ash containing less than 10% CaO (from bituminous coal) is called low-calcium fly ash
(Class F) and the ash typically containing 15% to 30% of CaO (from lignite coal) is called
high-calcium fly ash (Class C).

By replacing cement with low calcium fly ash, the cohesiveness is improved (small
particles are always helpful to prevent segregation). The water requirement to achieve the
same consistency is reduced, as the near-spherical fly ash particles makes it easier for the
concrete mix to flow. As the pozzolanic reaction does not occur until later, the early
strength of concrete is reduced, with a corresponding reduction in heat of hydration. Fly
ash can hence be used in mass concrete construction. The ultimate strength is higher than
that for concrete without fly ash replacing part of the cement. This is due to the conversion
of CH to C-S-H in the long term.

28
3.5 Hardened concrete

3.5.1 Strength of hardened concrete

3.5.1.1 Introduction

A). Definition
Strength is defined as the ability of a material to resist stress without failure. The
failure of concrete is due to cracking. Under direct tension, concrete failure is due to the
propagation of a single major crack. In compression, failure involves the propagation of
a large number of cracks, leading to a mode of disintegration commonly referred to as
‘crushing’. The strength is the property generally specified in construction design and
quality control, for the following reasons:
(1) It is relatively easy to measure, and
(2) Other properties are related to the strength and can be deduced from strength
data.

The 28 days compressive strength of concrete determined by a standard uniaxial

compression test is accepted universally as a general index of concrete strength.
3.5.1.2 Compressive strength and corresponding tests

a). Failure mechanism

a. b. c. d.

a. At about 25-30% of the ultimate strength, random cracking (usually in

transition zone around large aggregates) are observed
b. At about 50% of ultimate strength, cracks grow stably from transition zone
into paste. Also, micro cracks start to develop in the paste.
c. At about 75% of the ultimate strength, paste cracks and bond cracks start to
join together, forming major cracks. The major cracks keep growing while
smaller cracks tend to close.
d. At the ultimate load, failure occurs when the major cracks link up along the
vertical direction and split the specimen

The development of the vertical cracks result in expansion of concrete in the lateral
directions. If concrete is confined (i.e., it is not allow to expand freely in the lateral
directions), growth of the vertical cracks will be resisted. The strength is hence
increased, together with an increase in failure strain. In the design of concrete
columns, steel stirrups are placed around the vertical reinforcing steel. They serve
to prevent the lateral displacement of the interior concrete and hence increase the
concrete strength. In composite construction (steel + reinforced concrete),

29
 steel tubes are often used to encase reinforced concrete columns. The tube is very
effective in providing the confinement.

The above figure illustrates the case when the concrete member is under ideal
uniaxial loading. In reality, when we are running a compressive test, friction exists
at the top and bottom surfaces of a concrete specimen, to prevent the lateral
movement of the specimen. As a result, confining stresses are generated around the
two ends of the specimen. If the specimen has a low aspect ratio (in terms of height
vs width), such as a cube (aspect ratio = 1.0), the confining stresses will increase
the apparent strength of the material. For a cylinder with aspect ratio beyond 2.0,
the confining effect is not too significant at the middle of the specimen (where
failure occurs). The strength obtained from a cylinder is hence closer to the actual
uniaxial strength of concrete. Note that in a cylinder test, the cracks propagate
vertically in the middle of the specimen. When they get close to the ends, due to
the confining stresses, they propagate in an inclined direction, leading to the
formation of two cones at the ends.

(b) Specimen for compressive strength determination

Note that the cube specimen is popular in U.K. and Europe while the cylinder
specimen is commonly used in the U.S.

i) Cube specimen
BS 1881: Part 108: 1983. Filling in 3 layers with 50 mm for each layer (2
layers for 100 mm cube). Strokes 35 times for 150 mm cube and 25 times
for 100 mm cube. Curing at 20±5 0C and 90% relative humility.
ii) Cylinder specimen

ASTM C470-81. Standard cylinder size is 150 x 300 mm. Curing condition
is temperature of 23±1.7 0C and moist condition. Grinding or capping are
needed to provide level and smooth compression surface.

30
(c). Factors influencing experiment results

(i) End condition: Due to influence of platen restraint, cube's apparent strength is
about 1.15 time of cylinders. In assessing report on concrete strength, it is
IMPORTANT to know which type of specimen has been employed.
(ii) Loading rate: The faster the load rate, the higher the ultimate load obtained. The
standard load rate is 0.15 -0.34 MPa/s for ASTM and 0.2-0.4 MPa/s for BS.

(iii) Size effect: The probability of having larger defects (such as voids and cracks)
increases with size. Thus smaller size specimen will give higher apparent strength.
Standard specimen size is mentioned above. Test results for small size specimen
needs to be modified.

3.5.1.3 Tensile strength and corresponding tests

a). Failure mechanism

a. b. c.

a. Random crack development (micro cracks usually form at transition zone)

b. Localization of micro cracks
c. Major crack propagation

It is important to notice that cracks form and propagate a lot easier in tension than
in compression. The tensile strength is hence much lower than the compressive strength.
An empirical relation between ft and f c is given by:

31
 ft = 0.615 (fc)0.5 (for 21 MPa < fc < 83 MPa)

Substituting numerical values for fc, ft is found to be around 7 to 13% of the

compressive strength, with a lower ft/fc ratio for higher concrete strength. In the
above formula, fc is obtained from the direct compression of cylinders while ft is
measured with the splitting tensile test, to be described below.

b). Direct tension test methods

Direct tension tests of concrete are seldom carried out because it is very
difficult to control. Also, perfect alignment is difficult to ensure and the specimen
holding devices introduce secondary stress that cannot be ignored. In practice, it is
common to carry out the splitting tensile test or flexural test.

c). Indirect tension test (split cylinder test or Brazilian test)

BS 1881: Part 117:1983.

Specimen 150 x 300 mm cylinder. Loading rate 0.02 to 0.04 MPa/s
ASTM C496-71:
Specimen 150 x 300 mm cylinder. Loading rate 0.011 to 0.023 MPa/s

The splitting test is carried out by applying compression loads along two
axial lines that are diametrically opposite. This test is based on the
following observation from elastic analysis. Under vertical loading acting
on the two ends of the vertical diametrical line, uniform tension is
introduced along the central part of the specimen.

32
 The splitting tensile strength can be obtained using the following formula:

f = 2P
st π LD
According to the comparison of test results on the same concrete, fst is
about 10-15% higher than direct tensile strength, ft.

d) Flexural strength and corresponding tests

BS 1881: Part 118: 1983. Flexural test. 150 x 150 x 750 mm or 100 x 100 x 500
(Max. size of aggregate is less than 25 mm). The arrangement for modulus of rupture is
shown in the above figure.

From Mechanics of Materials, we know that the maximum tension stress should
occur at the bottom of the constant moment region. The modulus of rapture can be
calculated as:

f = Pl
bt bd 2

This formula is for the case of fracture taking place within the middle one third of
the beam. If fracture occurs outside of the middle one- third (constant moment zone), the
modulus of rupture can be computed from the moment at the crack location according to
ASTM standard, with the following formula.
3Pa
f =
bt bd 2
However, according to British Standards, once fracture occurs outside of the
constant moment zone, the test result should be discarded.

33
 Although the modulus of rupture is a kind of tensile strength, it is much higher
than the results obtained from a direct tension test. This is because concrete can still carry
stress after a crack is formed. The maximum load in a bending test does not correspond to
the start of cracking, but correspond to a situation when the crack has propagated. The
stress distribution along the vertical section through the crack is no longer varying in a
linear manner. The above equations are therefore not exact.

3.5.1.4 Factors affecting concrete strength

Water/Cement Ratio

In the discussion of cement hydration, it has been pointed out that the density of
hardened cement (in terms of a gel/space ratio) is governed by the water/cement ratio.
With higher w/c ratio, the paste is more porous and hence the strength is lower. An
empirical formula relating fc and w/c was proposed by Abrams in 1919:
A
f =
c
B1.5(w/c)

For 28 days strength, A is approximately 100 MPa. B is usually taken to be 4. It

should be noted that the equation gives very conservative estimates for concrete with low
w/c ratio (below about 0.38). Also, the strength continues to increase with decreasing w/c
ratio only if the concrete can be fully compacted. For concrete with very low w/c ratio, if
no water-reducing agent is employed, the workability can be so poor that a lot of air voids
are entrapped in the hardened material. The strength can then be lower than that for
concrete with higher w/c ratio.

While w/c ratio is the most important parameter governing the strength of concrete,
it is not the only parameter. Strictly speaking, the above equation is not correct. However,
with no test results available, an estimation of fc from w/c is a good first approximation.
Indeed, under the American practice of mix proportioning (ACI 211.1), the compression
strength is estimated (in a conservative way) from the water/cement ratio. Under British
practice, design tables and charts that take into consideration the types of cement and
aggregate are employed. More details will be provided in a later section on concrete mix
design.

Age and Curing Condition

The effect of curing temperature on concrete strength has already been discussed
before. Provided the concrete is properly cured, the strength increases with time due to the
increased degree of hydration. As a rule of thumb, for type I cement, the 7 day strength
can range from 60 – 80% of the 28 day strength, with a higher percentage for a lower
w/c ratio. After 28 days, the strength can continue to go up. Experimental data indicates
that the strength after one year can be over 20% higher than the 28 day strength. The
reliance on such strength increase in structural design needs to be done with caution, as the
progress of cement hydration under real world conditions may vary greatly from site to
site.

34
 Aggregates

For the same w/c ratio, mixes with larger aggregates give lower strength. This is
due to the presence of a weak zone at the aggregate/paste interface, where cracking will
first occur. With larger aggregates, larger cracks can form at the interface, and they can
interact easier with paste cracks as well as other interfacial cracks.

With the same mix proportion, rougher and more angular aggregates give
higher strength than smooth and round aggregates. However, with smooth aggregates,
a lower w/c ratio can be employed to achieve the same workability. Therefore, it is
possible to achieve similar strength with smooth and rough aggregates, by adopting
slightly different w/c ratios.

For a fixed w/c ratio, the strength increases slightly with the aggregate/cement ratio.
This is because aggregates are often denser than the cement paste. With less paste in the
concrete, the overall density is increased.

For normal strength concrete, the aggregate strength is seldom a concern.

However, in the development of high strength concrete, it is important to select aggregates
with strength higher than that of the hardened paste.

Admixtures

Air-entraining agents decrease concrete strength by incorporation of bubbles. Set-

retarding and accelerating agents affect the early strength development but have little
effect on ultimate strength. Incorporation of mineral admixtures increases ultimate
strength through the pozzolanic reaction.

3.5.1.5 Rate effect on concrete strength and creep rupture

35
 The strength of concrete is found to decrease with decreasing loading rate. At slower
loading rates, more time is allowed for the crack to propagate, and hence a lower stress is
required for the joining of cracks to cause failure. Indeed, concrete failure can occur if
loading is increased to 70-80% of the ultimate short-term strength and then kept constant for
a period of time. This phenomenon is referred to as creep rupture or static fatigue.

3.5.1.6 Fatigue strength of concrete

The fatigue of concrete is often analyzed using empirical approaches, following the

same concept of the Goodman’s law, i.e., the allowable stress oscillation decreases

linearly with the mean stress. An example design diagram is shown in the figure

below.

100 100

Minimum Maximum
80 80
stress as a stress as a
percentage percentage
of static of static
60 60
strength
strength

40 40

106 cycles
20 20

0 0

For a life of 106 cycles, if the minimum stress is zero, the allowable maximum
stress is 50% the static strength. If the minimum stress is non-zero, the allowable
maximum stress can be found in a way illustrated by the arrows in the figure. Note that the
above diagram gives conservative estimates of life time.

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36
3.5.2 Stress-strain curve and modulus of elasticity

The elastic modulus of concrete is usually determined from test data up to about
40% of the ultimate strength (i.e., within or slightly beyond the linear range). Since
concrete is a composite material making up of the hardened paste (with pores) and the
aggregates, its modulus can be predicted from composite models. The modulus of the
paste increases with decreasing porosity so paste with lower w/c ratio is stiffer.
Empirically, the paste modulus is found to vary with (1- p)3, where p is the porosity. To
obtain the concrete modulus Ec from the paste modulus Ep and aggregate modulus Ea,
three models have been proposed.

(i) Parallel model (aggregate and paste under the same strain)

Ec = VaEa + VpEp

Va: volume fraction of aggregate

Vp = 1 – Va = volume fraction of paste

(ii) Series model (aggregate and paste under the same stress)

Va Vp -1
E= +
Ea Ep

(iii) Square in square model

Aggregates are assumed to be completed surrounded by cement, and the composite

is simplified into a system of square in square. To find the concrete modulus, the system is
assumed to be one made of two layers of pure paste in series with a layer consisting of
paste and aggregate in parallel. This is illustrated in the figure below.

Paste
Unit length alone

Va0.5

Paste and
Aggregate
in Parallel

Paste
Aggregate
Paste
alone

37
 For the square in square system,

Concrete modulus predicted from this model is found to agree much better with
experimental results than predictions from the parallel and series models.

Concrete modulus generally increases with strength. To achieve higher strength,

the water/cement ratio is reduced, and hence the hardened cement paste is denser and
stiffer. The composite modulus will therefore also increase. For high strength concrete, the
higher modulus is also due to the use of high quality (and hence strong and stiff)
aggregates in the mix.

In practice, the concrete modulus is usually not measured, but estimated from the
concrete compressive strength using empirical formula. According to the British Standard
for the structural used of concrete (BS 8110: part 2), the Young's Modulus of concrete (in
GPa) can be related to the cube compressive strength (in MPa) by the expression

Ec = 9.1 fc0.33

for concrete with density of 2320 kg/m3, i.e. for typical normal weight concrete.

If the density of concrete is between 1400 and 2320 kg/m3, the expression for
Young's modulus is

Ec = 1.7 ρ 2 fc0.33 × 10−6

where ρ is the density of concrete in kg/m3.

According to ACI Building Code 318-83, the Young's Modulus of normal weight
concrete is,

Ec = 4.70 fc0.5

where fc is the cylinder compressive strength.

For concrete with density from 1,500 to 2,500 kg/m3, the relationship changes to
−6
Ec = 43ρ 1.5 fc0.5 × 10

38
3.5.3 Dimensional stability--Shrinkage and creep

Dimensional stability of a construction material refers to its dimensional change

over a long period of time. If the change is so small that it will not cause any structural
problems, the material is dimensionally stable. For concrete, drying shrinkage and creep
are two phenomena that compromise its dimensional stability.

Shrinkage and creep are often discussed together because they are both governed
by the deformation of hydrated cement paste within concrete. The aggregate in concrete
does not shrink or creep, and they serve to restrain the deformation.

3.5.3.1 Drying shrinkage

After concrete has been cured and begins to dry, the excessive water that has not
reacted with the cement will begin to migrate from the interior of the concrete mass to the
surface. As the moisture evaporates, the concrete volume shrinks. The loss of moisture
from the concrete varies with distance from the surface. The shortening of concrete per
unit length associated with the reduction in volume due to moisture loss is termed the
shrinkage. Shrinkage is sensitive to the relative humidity. For higher relative humidity,
there is less evaporation and hence reduced shrinkage. When concrete is exposed to
100% relative humidity or submerged in water, it will actually swell slightly.

Shrinkage can create stress inside concrete. Because concrete adjacent to the
surface of a member dries more rapidly than the interior, shrinkage strains are initially
larger near the surface than in the interior. As a result of the differential shrinkage, a set of
internal self-balancing forces, i.e. compression in the interior and tension on the outside, is
set up.

In additional to the self-balancing stresses set up by differential shrinkage, the

overall shrinkage creates stresses if members are restrained in the direction along which
shrinkage occurs. If the tensile stress induced by restrained shrinkage exceeds the tensile
strength of concrete, cracking will take place in the restrained structural element. If
shrinkage cracks are not properly controlled, they permit the passage of water, expose
steel reinforcements to the atmosphere, reduce shear strength of the member and are bad
for appearance of the structure. Shrinkage cracking is often controlled with the
incorporation of sufficient reinforcing steel, or the provision of joints to allow movement.

After drying shrinkage occurs, if the concrete member is allowed to absorb water,
only part of the shrinkage is reversible. This is because water is lost from the capillary
pores, the gel pores (i.e., the pore within the C-S-H), as well as the space between the C-S-
H layers. The water lost from the capillary and gel pores can be easily replenished.
However, once water is lost from the interlayer space, the bond between the layers
becomes stronger as they get closer to one another. On wetting, it is more difficult for
water to re-enter. As a result, part of the shrinkage is irreversible.

Gel pore
Interlayer space
Internal Structure
of C-S-H

39
 The magnitude of the ultimate shrinkage is primarily a function of initial water
content of the concrete and the relative humidity of the surrounding environment. For
the same w/c ratio, with increasing aggregate content, shrinkage is reduced. For concrete
with fixed aggregate/cement ratio, as the w/c ratio increases, the cement becomes more
porous and can hold more water. Its ultimate shrinkage is hence also higher. If a stiffer
aggregate is used, shrinkage is reduced.

The shrinkage strain, εsh, is time dependent. Approximately 90% of the ultimate
shrinkage occurs during the first year.

Both the rate at which shrinkage occurs and the magnitude of the total shrinkage
increase as the ratio of surface to volume increases. This is because the larger the surface
area, the more rapidly moisture can evaporate.

Based on a number of local investigations in Hong Kong, the value of shrinkage

strain (after one year) for plain concrete members appears to lie between 0.0004 and
0.0007 (although value as high as 0.0009 has been reported). For reinforced concrete
members, the shrinkage strain values is reduced, as reinforcement is helpful in reducing
shrinkage.

3.5.3.2 Creep

Creep is defined as the time-dependent deformation under a constant load.

Water movement under stress is a major mechanism leading to creeping of concrete. As a
result, factors affecting shrinkage also affect creep in a similar way. Besides moisture
movement, there is evidence that creep may also be due to time-dependent formation and
propagation of microcracks, as well as microstructural adjustment under high stresses
(where stress concentration exists). These mechanisms, together with water loss from the
gel interlayer, lead to irreversible creep. Creeping develops rapidly at the beginning and
gradually decreases with time. Approximately 75% of ultimate creep occurs during the first
year. The ultimate creep strain (after 20 years) can be 3-6 times the elastic strain.

Creep can influence reinforced concrete in the following aspects.

i). Due to the delayed effects of creep, the long-term deflection of a beam can be 2-
3 times larger than the initial deflection.

ii). Creeping results in the reduction of stress in pre-stressed concrete which can
lead to increased cracking and deflection under service load.

iii). In a R.C column supporting a constant load, creep can cause the initial stress in
the steel to double or triple with time because steel is non-creeping and thus take over the
force reduced in concrete due to creep.

Creep is significantly influenced by the stress level. For concrete stress less than
50% of its strength, creep is linear with stress. In this case, the burger’s body, which is a
combination of Maxwell and Kelvin models, is a reasonable representation of creep
behaviour. For stress more than 50% of the strength, the creep is a nonlinear function of
stress, and linear viscoelastic models are no longer applicable. For stress level higher than

40
75-80% of strength, creep rupture can occur. It is therefore very important to keep in mind
that in the design of concrete column, 0.8 fc is taken to be the strength limit.

Factors affecting Creep of concrete

a). w/c ratio: The higher the w/c ratio, the higher is the creep.

b). Aggregate stiffness (elastic modulus): The stiffer the aggregate, the smaller the

creep.

c). Aggregate fraction: higher aggregate fraction leads to reduced creep.

d). Theoretical thickness: The theoretical thickness is defined as the ratio of section
area to the semi-perimeter in contact with the atmosphere. The higher the
theoretical thickness the smaller the creep and shrinkage.

e). Temperature: with increasing temperature, both the rate of creep and the
ultimate creep increase. This is due to the increase in diffusion rate with
temperature, as well as the removal of more water at a higher temperature.

f). Humidity: with higher humidity in the air, there is less exchange of moisture
between the concrete and the surrounding environment. The amount of creep is
hence reduced.

g). Age of concrete at loading: The creep strain at a given time after the application
of loading is lower if loading is applied to concrete at a higher age. For example, if
the same loading is applied to 14 day and 56 day concrete (of the same mix), and
creep strain is measured at 28 and 70 days respectively (i.e., 14 days after load
application), the 56 day concrete is found to creep less. This is because the
hydration reaction has progressed to a greater extent in the 56 day concrete. With
less capillary pores to hold water, creep is reduced.

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41
3.5.4 Durability

Almost universally, concrete has been specified principally on the basis of its
compressive strength at 28 days after casting. Since reinforced concrete structures are
usually designed with a sufficiently high safety factor, it is rare for concrete structures to
fail due to lack of intrinsic strength. However, gradual deterioration caused by the lack of
durability reduces the safety margin of concrete structures to an extent that serious
concerns have been raised. The extent of the problem is such that concrete durability has
been recently described as a "multimillion dollar opportunity". In the U. S. A., the cost of
repairing the interstate highway bridges alone is 20 Billion U.S. dollars. In developed
countries, it is estimated that over 40% of the total resources of the building industry are
applied to repair and maintenance of existing structures. In Hong Kong, steel corrosion
and chemical attack on concrete have created a lot of problems. Some new pipelines can
not even last for five years. Steel corrosion has led to the collapse of canopies, resulting in
death and injury of people.

Durability of concrete can be defined as its ability to resist weathering action,

chemical attack, abrasion, or any other process of deterioration, and hence to retain
its original shape, dimension, quality and serviceability.

3.5.4.1 Causes of deterioration and main durability problems

The causes of concrete deterioration is grouped into two categories

Physical causes:

Surface wear (abrasion, erosion and cavitation)

Cracking (Volume changes, loading damage, and extreme

temperature damage)

Chemical causes:

Alkali-aggregate reaction

Sulphate attack

Steel corrosion

The most severe durability problems always involve the penetration of water (with
corrosive agents) into the concrete. Physical action (e.g., water freezing) or chemical
reaction (e.g., alkali-aggregate reaction) will then lead to internal expansion, resulting in
significant cracking/spalling of the concrete. Durability of concrete is hence related to the
ease of ingress of water and chemicals. Concrete permeability and diffusivity are hence
important parameters to be considered.

42
3.5.4.2. Basic factors influencing the durability

The durability of concrete depends, to a large extent, on permeability, K, and

diffusivity, D. Permeability is defined as the property that governs the rate of flow of a
fluid into a porous material under a hydraulic gradient. For steady-state flow, the
Darcy’s law states:

∂ h
u x = −K ∂ x
where ux is the velocity of flow in the x-direction, h the pressure head and K the
coefficient of permeability.

Obviously the permeability is a function of pores inside materials. It is affected by

both the percentage of porosity and size distribution of pores.

Porosity(%) Average pore size Permeability coefficient

Hardened 40 110 x 10-12 cm/sec.

Cement 30 20 x 10-12 cm/sec.
Paste 20 100 nm 6 x 10-12 cm/sec.

Agg. 3-10 10 μm 1-10 x 10-12 cm/sec.

Concrete 20-40 nm-mm 100 - 300 x 10-12 cm/sec.

Diffusivity is defined by Fick's law, and it is the rate of moisture migration under a
concentration gradient at the equilibrium diffusion condition.

∂C
Q = −Dp
∂x

wher Q is the mass transport rate (kg/m2.s); Dp the diffusion coefficient (m2/s); C the
concentration of a particular ion or gas (kg/m3).
The two parameters apply to different situations. When water is flowing through a
piece of concrete, or from one part to another part under a hydraulic gradient, permeability
is the governing parameter. When gases (e.g. oxygen) move through concrete (either dry
or wet) or ions (e.g. chloride) move through the pore solution, the process is governed by
the diffusion coefficient (or diffusivity). Note that the diffusion coefficient varies for
different diffusing substances. Generally speaking, since both the permeability and
diffusivity are related to the pore structure of concrete, concrete with low permeability will
also possess low diffusivity. Means to reduce permeability and diffusivity (e.g. use lower
w/c ratio to reduce capillary porosity, specification of cement content high enough to
ensure sufficient consistency and hence proper compaction, proper curing to reduce
surface cracks) are generally helpful to concrete durability.

43
3.5.4.3 Alkali-Aggregate Reaction

Alkali-Aggregate Reaction (AAR) is the reaction between alkalis from cement and
constituents present in some aggregates. It can be further classified into alkali-silica
reaction (ASR) and alkali-carbonate reaction. The small amounts of Na2 O and K2O
present in the cement clinker are responsible for the reaction. In the cement paste, these
form hydroxides and raise the pH level from 12.5 to 13.5. In such highly alkaline
solutions, reaction can occur for some aggregates. ASR occurs if an aggregate contains
glassy silicates. Examples include chert, flint, opal, cristoballite, chalcedony and volcanic
glasses. The reaction products absorb water and expand. Stresses induced by the
expansion lead to cracking and spalling. Alkali -carbonate reaction occurs for dolomite. In
this case, the reaction products do not expand, but the reaction exposes clay minerals that
tend to absorb water and expand. Cracking will then occur. In Hong Kong AAR has only
been observed for one source of aggregates from mainland China used in some projects in
Northern New Territories.

Although it is possible to tell (through mineral identification) if an aggregate will

react with cement, it is impossible to predict whether its use will result in excessive
expansion or not. Various tests for AAR have therefore been developed. In the chemical
test (ASTM C289), powdered aggregate is put into sodium hydroxide, and the solubility is
measured to assess the reactivity of aggregate with an alkali. Since the presence of various
minerals may affect the solubility, the result is not reliable. In the mortar bar test (ASTM
C227), crushed aggregates are used to cast mortar bars of standard size. The bars are
stored moist at 38 oC to accelerate the reaction. Expansion should not exceed 0.05% after
3 months or 0.1% after 6 months. Since significant expansion may start after 6 months, the
test should ideally be continued further. However, the time involved may make it
impractical. Recently, a few rapid testing methods have been developed, such as
dynamic modulus test, and gel fluorescence test. Dynamic modulus, measured from
pulse velocity (Note: v = (E/ρ)1/2) is a good indication of deterioration due to AAR. The
measurement can even detect deterioration before any expansion and visible cracking
occurs. In the gel fluorescence test, 5% solution of uranyl acetate is applied on the
specimen surface, and the specimen is then viewed under ultraviolet (UV) light. A
yellowish green fluorescent glow means that reaction products from AAR are present. The
new rapid testing methods are promising but they are yet to be standardized.

To minimize the risk of AAR, one can:

A) Use non-deleterious aggregate when the alkali content of the cement is high;

B) Use low-alkali cement (<0.6% Na2O equivalent) when the silica content of
aggregate is high

C) Keep concrete dry (relative humidity of the concrete < 80%).

The choice on types of cements and aggregates at a construction site is usually

very limited, and the environment surrounding the concrete is obviously unchangeable.
Therefore, the above guidelines may not be practical in a real world. In such cases, the
only effective way to reduce the risk of AAR is to control moisture migration in the
concrete. No AAR will occur in dry concrete even if reactive aggregates are present.
Control of moisture flow can be achieved with external coating on the structural member,

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44
or local diffusion coating around the aggregates (formed by placing the aggregates into a
slurry of the protective material before casting).

3.5.4.4 Sulphate Attack

Sulphates may be present in ground water, particularly from clay soils.

Underground concrete members such as foundations and pipes are therefore subject to
sulphate attack. The severity of sulphate attack depends on the type of sulphate present.
Calcium sulphate reacts with monosulphate (formed from C3A in the cement) to form
ettringite. Since the reaction products occupy a higher volume, internal stresses are
generated to disrupt the concrete. (Note: when ettringite is initially formed during the first
few hours of cement hydration, expansion also occurs. However, since the concrete is still
plastic, the expansion can be accommodated.) When sodium sulphate is present, it also
reacts with calcium hydroxide to form gypsum, leading to a loss in strength and stiffness.
Magnesium sulphate can react with calcium hydroxide and attack the C-S-H as well. The
resulting degradation is hence even more serious. Sulphate is also present in seawater.
However, in the presence of chloride ions (which are also abundant in seawater), the
resulting ettringite stays in solution. Hence, there will be no expansion and internal
stresses. Therefore, sulphate attack in seawater is not as severe as that in ground water.

To minimize sulphate attack, several approaches can be employed. Firstly, the C3A
content of the cement can be reduced. This is the rationale behind using type V (sulphate
resistant) cement. Secondly, a lower w/c ratio can be used to reduce the permeability of
concrete. While reducing the w/c ratio, it is important to keep a minimum cement content.
Otherwise, the water content may become too low for the concrete to be properly
compacted. Thirdly, fly ash and silica fume can be added to reduce the permeability.
Replacement of cement with pozzolans can also ‘dilute’ the C3A.

3.5.4.5 Corrosion of Steel Reinforcement

The high pH in concrete offers a protective environment to the corrosion of steel.

In such an environment, steel oxidizes to form Fe(OH)2 first. Part of the oxide will further
react to form FeO.OH. With pH > 11.5, and in the absence of chloride ions, both oxides
are stable. They form a thin protective film on top of the steel surface to prevent further
corrosion. Steel is said to be ‘passivated’ under such a condition. The initiation of steel
corrosion is usually due to either carbonation of the concrete, or the penetration of chloride
ions. Carbonation is the reaction between carbon dioxide in the air, and calcium
hydroxide in the hardened cement to form calcium carbonate. With calcium hydroxide
removed by this reaction, the pH drops to below 11.5. When carbonation proceeds to the
level of the steel reinforcement, the protective layer is no longer stable. Steel is then
‘depassivated’ and significant rusting will start. In this case, relatively uniform rusting
occurs on the steel.

In cities near the ocean, such as Hong Kong, or in cold regions where salt is used
for deicing of road pavements, the penetration of chloride ions is a major cause for steel
corrosion. When the chloride concentration at the steel level reaches a critical value (0.6 to
0.9 kg/m3 of concrete for pH value of 12-13), it will react with the Fe(OH)2 (the
remaining ferrous oxide that has not converted into FeO.OH) to form a water soluble
compound. The protective surface is hence destroyed. Since the part of the surface covered
with FeO.OH is not affected, corrosion only occurs at isolated spots where the

45
Fe(OH)2 formerly exists. This type of corrosion is referred to as ‘pitting’ corrosion, and it
is more dangerous than uniform corrosion, as large reduction in steel cross section can
occur locally with little loss in total mass.

Once steel is depassivated, corrosion occurs through an electrochemical process,

consisting of both oxidation and reduction reactions. Four components must be present for
corrosion to occur. The four components include anode, cathode, electrolyte and metallic
path. The anode is the electrode at which oxidation occurs. Oxidation involves the loss of
electrons and formation of metal ions. Hence, material is lost at the anode. The cathode is
the electrode where reduction occurs. Reduction is the gain of electrons in a chemical
reaction. The electrolyte is a chemical mixture, usually liquid, containing ions that migrate
in an electric field. A metallic path between anode and cathode is essential for electron
movement between the anode and cathode. For steel corrosion in concrete, the anode and
cathode are both on the steel and the steel itself is the metallic path. The electrolyte is the
moisture in concrete surrounding the steel. The specific reactions are given below.

At the anode:
Fe → Fe++ +2e-
At the cathode:
4e- +O2 +2H2O → 4(OH)-
In the electroyte:
Fe++ + 2(OH)- → Fe(OH)2

4 Fe(OH)2 + 2 H2O + O2 → 4 Fe(OH)3

(further reaction with sufficient water supply)

As steel oxidizes, the corrosion products occupy a higher volume. The unit volume
of Fe can be doubled if FeO is formed. The unit volume of the final corrosion product,
Fe(OH)3 . 3H2O, is as large as six and a half times of the original Fe. Expansion leads to
cracking and surface spalling of concrete. Once the concrete cover spalls and steel is
exposed to the atmosphere, the corrosion rate will increase significantly. Eventually, the
excessive loss of steel area, if left unnoticed, can lead to collapse of the structure.

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46
 Based on the above discussions on the corrosion process, various approaches for
corrosion control can be proposed. The most cost-effective corrosion control method is to
use a low w/c concrete and a relatively thick cover on the steel. (Note: the cover cannot be
excessive thick as this will significantly increase the member size. Also, a thick
unreinforced cover can easily crack due to shrinkage or thermal effects.) With thick cover
and low concrete diffusivity, it takes a long time for carbon dioxide or chloride ion to
reach the reinforcing steel. Corrosion initiation is therefore greatly reduced. Also, with low
water permeability associated with low w/c ratio, once water is used up in the corrosion
reactions, it takes a longer time for it to be replenished. In other words, the electrolyte, a
critical component in the electrochemical reaction, is removed. Similarly, with low oxygen
diffusivity, the replenishment of reacted oxygen is slow. The corrosion rate after initiation
is hence reduced.

Another approach to stop corrosion is to isolate the anode and cathode from the
electrolyte. This is the principle behind epoxy-coated rebars (reinforcing bars). In general,
epoxy-coated rebars have performed well in bridge decks and parking garages. When
epoxy coated rebars are used, it is important to minimize damage to the coating during the
casting procedure. Training is thus necessary for proper handling of epoxy-coated bars.
Recently, notable problems with corrosion of epoxy-coated bars were reported for bridge
columns in Florida. Epoxy coating, though intact, was found to separate from the steel.
Further research is hence required to understand the degradation mechanism and to
improve the coating performance.

Instead of isolating steel from the electrolyte, one can also connect steel to either a
voltage supply, or a metal higher in the electrochemical series (and hence have a higher
tendency to oxidize, e.g. zinc), so the whole piece of steel becomes the cathode. This
technique is called cathodic protection. Further discussions on cathodic protection will be
given in the section on steel.

Corrosion can also be controlled through chemical means, through the

incorporation of corrosion inhibitors. The most common corrosion inhibitor is calcium
nitrite. Its presence facilitates the conversion of Fe(OH)2 to FeO.OH. In other words, it is
competing with chloride ions for Fe (II) ions. If the nitrite/chloride concentration is high,
the chloride cannot react with Fe(OH)2 to turn it into a water soluble compound.
Therefore, pitting will not occur.

47
3.6 Concrete Mix Design Procedures

In the previous chapters, we have discussed the various concrete properties and the
factors influencing the properties. Now, we are ready to apply this knowledge to design a
concrete mix. Mix design, or mix proportioning, is a process by which one arrives at the
right combination of cement, aggregate, water, and admixtures to produce concrete to
satisfy given specifications. It should be indicated that this process is considered an art
rather than a science.

3.6.1. Principal requirements for concrete

The main purpose of mix design is to obtain a product that will perform according
to predetermined requirements. These requirements include the following concrete
properties:

i). Quality (strength and durability)

Strength and permeability of hydrated cement paste are mutually related through
the capillary porosity that is controlled by w/c ratio and degree of hydration. Since
durability of concrete is controlled mainly by its permeability, there is a relationship
between strength and durability. Consequently, routine mix design usually focuses on
strength and workability only. When the concrete is exposed to special environmental
conditions, provisions on durability (e.g. limit on w/c ratio, minimum cement content,
minimum cover to steel reinforcement, etc) will also be considered.

ii).Workability

As we mentioned earlier, workability is a complicated concept for fresh concrete

and embodies various properties including consistency and cohesiveness. There is still not
a single test method that can fully reflect workability. Since the slump represents the ease
with which the concrete mixture will flow during placement, and the slump test is simple
and quantitative, most mix design procedures rely on slump as a crude index of
workability. Sometimes, the Vebe time may be employed.

iii).Economy

Among all the constituents of the concrete, the admixture has the highest unit cost,
followed by cement. The cost of aggregates is about one-tenth that of cement. Admixtures
are often used in small amounts, or they are required to achieve certain properties. To
minimize cost of concrete, the key consideration is the cement cost. Therefore, all possible
steps should be taken to reduce the cement content of a concrete mixture without
sacrificing the desirable properties, such as strength and durability. The scope for cost
reduction can be enlarged further by replacing a part of the portland cement with cheaper
materials such as fly ash or ground blast-furnace slag.

As mentioned earlier, under normal conditions, it is sufficient to consider

workability and strength for concrete design. For special conditions, additional
considerations on dimensional stability and durability have to be taken.

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 3.6.2 Fundamentals of mix design
i) w/c ratio
Water/cement ratio is the most important factor influencing various kinds of
concrete properties. For the strength of concern, the Abrams's law states:
f = A
c B1.5 (w / c )

where fc is the compressive strength, A is an empirical constant (usually about 100 MPa),
and B is a constant depends mostly on the cement properties (usually 4).
In practice, tables or charts are available for the determination of fc from w/c, as
well as cement and aggregate type.
ii) Cement content
At a given w/c ratio, increasing the cement content will increase workability and
durability. However, the cost and hydration heat will also be increased. To solve such a
problem, part of the cement can be replaced with fly ash or slag.
iii) Major aggregate properties
a) maximum aggregate size:
The maximum aggregate size influences the paste requirement and optimum
grading. The larger the maximum size, the lower the paste requirement to achieve a given
workability. However, the larger the maximum aggregate size, the lower the strength.
The following considerations should be taken into account when choosing
maximum aggregate size: (a). For reinforced concrete, the maximum size should not
exceed one-fifth of the minimum dimension, or three-fourths of the minimum clear
spacing between bars. (b) For slabs on grade, the maximum size may not exceed one-
third the slab depth.
b) aggregate grading
The grading of aggregate is important to concrete because a good grading will
decrease the cement content and void in concrete and thus produce economical and better
concrete. For practical purpose it is adequate to follow grading limits specified by various
organizations (e.g. British Standards, ASTM), which are not only broad and therefore
economically feasible, but are also based on practical experience.

3.6.3 Weight method and volume method

Usually the unit weight of fresh concrete can be known from previous experience
for the commonly used raw materials. Thus we have,
W(wet concrete) = W(cement)+W(water)+W(aggregate)+W(sand)
The unit weight of wet concrete is usually ranged from 2300 to 2400 kg/m3.
In the case of the absolute volume method the total volume (1 m3) is equal to the
sum of volume of each ingredient (i.e., water, air, cement, and coarse aggregate). Thus we
have
W
W cement + W water + aggregate + W sand + volume%(air) = 1
ρ ρ ρ ρ
cemen water aggregate sand

Since the weight of each ingredient is easy to measure than volume, the design proportion
of concrete is usually expressed as a weight ratio. Hence, the proportion obtained in
volume method have to be converted to weight units by multiplying it by the density of
material.

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3.6.4 Design procedures

Basic Steps for Weight and Absolute Volume Methods

The basic steps required for determining mix design proportions for both weight and
absolute volume methods are as follows (Kosmatka et al., 2008):

1. Evaluate strength requirements.

2. Determine the water-cement (water–cementitious materials) ratio required.
3. Evaluate coarse aggregate requirements.
■ maximum aggregate size of the coarse aggregate
■ quantity of the coarse aggregate
4. Determine air entrainment requirements.
5. Evaluate workability requirements of the plastic concrete. 6. Estimate the water content
requirements of the mix.
7. Determine cementing materials content and type needed.
8. Evaluate the need and application rate of admixtures.
9. Evaluate fine aggregate requirements.
10. Determine moisture corrections.
11. Make and test trial mixes

There are many different methods for designing concrete mix. For example, ACI
method and UK method. However, there is no fundamental difference among these
methods. Thus, it is sufficient to introduce one method. Here, the method adopted in
U. K. (Department of the Environment, DoE, 1988) is introduced.

Step 1. Set a target mean compressive strength, f m

Two terms about strength are important in concrete design. One is target mean
strength, fm, and another is specified design (characteristic) strength, fc. The
characteristic strength, fc, is the strength to be used in structural design, and is hence the
objective strength to be achieved. The target strength is the strength to be obtained with
the concrete mix design. Due to the variations of concrete quality, characteristic strength,
fc, is defined for a permissible percentage of failure. For example, a characteristic strength,
fc, of 30 MPa with 5% failures, implies that 95% of test results have to be equal to or
higher than 30 MPa. Thus, the target mean strength (fm) should be higher than the
characteristic strength, fc, and can be obtained from the following equation

fm = fc + ks

where k is a factor dependent on the failure percentage and s is the standard deviation. k
and s can be obtained from the following table and figure.

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Step 2. Obtain required material information, including availability of materials (what
kind of aggregate and cement available), sieve analysis of both coarse and fine
aggregate, BD, UW and MC of aggregate, and maximum size of aggregate.

Step 3. Determine the w/c ratio according to the following empirical table and
figures. To do this, first get a predicted compressive strength according to the
type of cement and type of coarse aggregate for a w/c ratio of 0.5 from the
following table. Then, plot this value on the following figure on the w/c =0.5
vertical line. Draw a curve through this point, parallel to the printed curves,
until it intercepts a horizontal line passing through the ordinate of
predetermined mean strength. The value of the w/c ratio corresponding to mean
strength can then be obtained.

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Step 4. Specify the slump value. Usually, slump values of the concrete to be designed
will be specified according to the job nature of the concrete construction. For
an inexperienced person, the following table shall provide sufficient
information.

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Step 5. The free water content is then obtained from the following table according to the
type of aggregate and the specified workability.

Step 6 The cement content can then be calculated by dividing the water content by w/c
ratio.
Water content / m3
Cement content / m3 =
w/c

Compare this value with the specified minimum required cement content
determined by durability consideration. If it is below such a value, the value
specified must be used and the modified w/c ratio calculated.

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Step 7. The total aggregate content is then obtained by estimating the wet density of the
fully compacted mix from following figure. The total SSD aggregate content is
then equal to (wet density - cement content - free water content) for 1 m3
concrete.

Step 8. Determine the proportion of sand. The proportion of fine aggregate to total
aggregate depends on the grading of the sand and maximum aggregate size, the
workability and w/c ratio. The value can be determined from the following
figures, which plot the percent of fine aggregate versus w/c ratio for different
maximum aggregate size, workability, and the aggregate zone (represented by
the percentage of fine aggregates going through the 600 μm sieve).

For a given slump, as the w/c ratio increases (with fixed water content), a
higher percentage of fine aggregates is employed as there is less cement in the
mix. For the same w/c, when one goes from one figure to another figure of
higher slump (for the same max. aggregate size), the water content increases,
and the percentage of fines can also increase. When the maximum aggregate
size increases (e.g. going from Fig. (a) to (b) below), the amount of water
decreases, so the percentage of fines (for the same slump and w/c ratio)
decreases.

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103

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Step 9. The proportion of coarse aggregate is then the total aggregate content minus
the amount of fine aggregate.

Step 10. Obtain the mix proportion for the standard material conditions. Usually, take
cement as 1 and other materials and the ratio of the weight of cement.

Step 11. For field applications, make adjustment for moisture in the aggregates.

For stock aggregate with surface moisture, the actual weight of stock aggregate
for mixing should be more than the weight of aggregate specified in the
proportion while the amount of water (available for achieving desired
workability) should be decreased. For stock aggregate with absorption
capability, the actual weight of aggregate for mixing should be less than the
weight of above proportioned aggregate while the amount of water should be
increased.

Step 12. Calculate the quantities in kg per cubic meter for the raw materials.

Step 13. Make the trial batch to check the validity of the concrete design.
The calculated mix proportions should be checked by making trial mixes. Only a
sufficient amount of water to produce the required workability should be used, regardless
of the amount calculated. Trial mix should be tested for flow ability, cohesiveness,
finishing properties and air content, as well as for yield and density (unit weight). If any
one of these properties, expect the last two, is unsatisfactory, adjustments to the mix
proportions are necessary. For example, lack of cohesiveness can be corrected by
increasing the fine aggregate content at the expense of the coarse aggregate content. The
‘rules of thumb’ are as follows:
(a) If the correct slump is not achieved, the estimated water content is increased (or
decreased) by 6 kg/m3 for every 25 mm increase (or decrease) in slump.
(b) If the desired air content is not achieved, the dosage of the air-entraining admixture
should be adjusted to produce the specified air content. The water content is then
increased (or decreased) by 3 kg/m3 for each 1 per cent decrease (or increase) in air
content.
(c) If the estimated density (unit weight) of fresh concrete by mass method is not
achieved and is of importance, mix proportions should be adjusted, allowance being
made for a change in air content.
Step 14. Finalize the mix proportion based on the adjustment of the trial batch.

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