chapter 3 concrete

7/21/2019 Chapter 3 Concrete

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Chapter 3: Concrete 30 1stEd, Civil Engineering Materials

Prepared by: Mohammad Soffi Md. Noh, January 2010

Chapter3CONCRETE

3.1 Properties of Fresh Concrete

3.2 Water Cement Ratio and Workability

3.3 Strength and Grade of Concrete

3.4 Concrete Preparation; Mixing, Placing, Delivery, Compaction,

Curing

3.5 Standard Testing for Fresh and Harden Concrete

3.6 Properties of Harden Concrete; Durability and Permeability3.7 Concrete Mixture and Design

3.8 Types of Concrete

3.9 Admixture for Concrete

Concrete a composite man made material, is the most widely used material

in the construction industry. It consist of a rotationally chosen mixture of

binding material such as lime or cement, well graded fine and coarse

aggregate, water and admixture. In a concrete mix, cement and water form apaste or matrix which fills the voids of the fine aggregate and binds them (fine

and coarse) together. The mixture than placed in forms and allowed to cure

and becomes hard like stone. The hardening of concrete is caused by

chemical reaction between water and cement and it continues for a long time,

and consequently the concrete grows stronger with age.

Figure 4.1: Basic concrete materials

Water

Fine Aggregate

Coarse

Cemen


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The strength, durability and other characteristic of concrete depend upon the

properties of its ingredients, the proportion of the mix, the method of

compaction and other controls during placing and curing. Basically, concrete

can be classified into two stages namely;

i) Fresh concrete

ii) Hardened concrete

3.1 Properties of Fresh Concrete

Certain properties are desired of the freshly mixed concrete even though for

a short time only because it affects the quality and cost of hardened concrete.

The properties are defined as follows;

a) The wetness or dryness of the mix that is the consistency or slump.

b) Uniformity of the mix meaning that the concrete is mixed thoroughly

has a standard appearance and all ingredients are evenly distributed

in the mix.

c) Workability of the fresh concrete that is the ease with which concrete

is placed and consolidated.

3.2 Water Cement Ratio and workability

3.2.1 Water Cement Ratio

The quality of concrete is measured by its strength and durability. The

principal factor affecting the strength of concrete is the water/cement ratio of

the mix.

Fresh concrete is a mixture of water, cement,

aggregate and admixture. The constituent materials

should be uniformly distributed after mixing within

the concrete mass during handling and placing.

Harden concreteis a

____________________________________________________________________________________


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The compressive strength of concrete at a given age and under normal

temperature depends primarily on two factors which are the water/cement

ratio and the degree of compaction. When concrete is fully compacted its

strength is taken to be inversely proportional to the water/cement ratio asshown in Figure 3.2, which the lower water/cement ratio, the greater

compressive strength would be achieved.

Figure 3.2: The relationship between compressive strength andwater/cement ratio of concrete

Concrete containing water enough for hydration only, would be very dry and

difficult to place and compact. Therefore, additional water must be added to

make the mix workable enough to be easily place inside the forms and work

around the reinforcement. However, this additional water should be kept to a

minimum. The use of too much of water will weaken the strength of the

concrete.

Water/cement ratiocan be definedas the ratio of weight of water with

the total weight of cement that hasbeen used in the certain of concretemix.


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Water occupies space in concrete, as it evaporates it leaves voids, therefore

the more the uncombined water, the more voids there will be in the set

concrete and the less will be its density, strength and durability.

For proper workability the w/c ratio varies from 0.4-0.6. However, maximumstrength is derived at w/c = 0.4. When it is decreased to less than 0.4 there is

improper consistency and workability of cement and honey combed structure.

However, concrete compacted by vibrator displays higher strength even up to

w/c = 0.3.

3.2.2 Workability

For practical purposes, workability implies the ease with which a concrete

mix can be handled from the mixer to its finally compacted shape without

segregation during placement and compaction.

Factors that affect the workability of concrete are;

a) Water/cement ratio

b) Aggregate

c) Admixture

d) Fineness of cement

e) Time and temperature

There are four main characteristic of workability, which are;

1) ConsistencyA state of f luidity of concrete mix, includingthe wettest and densest type and thisdepends on the water content in the mix.

2) MobilityThe ease with which concrete can flow intothe moulds and around steel(reinforcement) thus completely filling the

moulds (formworks).

3) CompactibilityThe ease with which the concrete mixescan be completely compacted and the airvoids removed.

4) StabilityThe ability of the concrete to maintain itsuniformity i.e to remain a stable coherenthomogenous mass during handling andvibration without constituents segregating.


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The strength of concrete is defined

as the maximum stress it can resist or

the maximum load it can carry.

Optimal workability would give maximum density, minimum voids and no

segregation. The workability tests that are commonly conducted are slump

test, Vebe test and compacting factor test.

3.3 Strength and Grade of Concrete

3.3.1 Strength of Concrete

Strength of concrete is commonly considered as its most valuable property,

other than durability and impermeability. Nevertheless strength usually

gives an overall picture of the quality of concrete because strength is

directly related to the structure of the hardened cement paste.

Cubes, cylinders and prisms are the three types of compression test

specimens used to determine the compressive strength. The cubes

are usually of 100 mm or 150 mm side, the cylinders are 150 mmdiameter by 300 mm height, and the prisms are 100 mm x l00 mm x

500 mm in size.

To estimate the load at which the concrete members may crack,

normally flexural tensile strength test will be conducted. From this test,

the flexural tensile strength or the modulus of rupture is thus

determined. The modulus of rupture is determine by testing standard

test specimens of 150 mm x 150 mm x 700 mm over a span 600mm or

100mm x 100mm x 500mm over a span 400 mm under symmetricaltwo point loading.


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3.3.1.1 Factors Influencing the Strength of Concrete

There are several factors that influence the strength development of

concrete. Normally as time passes by, with proper curing the concrete

strength should increase. Nevertheless the strength gain can be put to ahalt and consequently creating durability problems if proper curing is not

done and the durability aspects are not considered. Factors influencing the

strength of concrete can be grouped into two categories:

1) Factors Depending On Testing Method

a. Size of test specimen;

b. Size of specimen in relation to the size of aggregate;

c. Support conditions of specimen;d. Moisture conditions of the specimen;

e. Type of loading adopted;

f. Rate of loading of the specimen;

g. Type of testing machine; and

h. The assumption in the analysis relating stress to failure load.

2) Factors Independent of the Type of Test

a. The type of cement, age, type of aggregate and admixture;b. Degree of compaction;

a. Concrete mix proportions, i.e. cement content, aggregate-

cement ratio, amount of voids and water-cement ratio;

b. Type of curing and temperature of curing:

c. Nature of loading to which the specimen is subjected, i.e. static,

sustained, dynamic etc.; and

d. Type of stress situation that may exist.

3.3.2 Grade of ConcreteThe concrete is generally graded according to its compressive strength at

28 days. The various grades of concrete as stipulated in codes of practice

BS 8110 grouped the grade in nine categories which is based on their

characteristic strength in N/mm2. Table 3.1 shows the tabulation of concrete

grade based on BS 8110.

Concrete of grade 7 and 10 is suitable for lean concrete bases and for mass

concrete and these need not be designed. The concrete of grade lower than

grade 15 is not suitable for reinforced concrete works and grades of lowerthan grade 30 are not to be used in the prestressed concrete works.


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Table 3.1: Grades of concrete proposed by BS 8110

GradeCharacteristic Strength

(N/mm2)

Lowest grade suitable for specific

purposes

710

7.010.0

Mass concrete

15 15.0Reinforced concrete using Lightweight

aggregate

20

25

20.0

25.0

Reinforced concrete using Heavyweight

aggregate

30 30.0 Prestressed Post-tensioned concrete

40

50

60

40.0

50.0

60.0

Prestressed Pre-tensioned concrete

3.4 Concrete Preparation

3.4.1 Selection of Materials

To achieve the desired qualities, the materials for making concrete - cement,

fine and coarse aggregates, and water are selected cautiously on the basis

of the quality acceptance tests.

3.4.2 Batching

The design of concrete mixture involves the determination of the most

economical and practical combination of ingredients to make the concrete

workable in its plastic state and to make it develop the required qualities

when hardened.

A proper and accurate measurement of all materials used in the production of

concrete is essential to ensure uniformity of proportions and aggregate

grading in successive batches. The quality of the concrete produced will

depend on the accuracy of the batching operation. There are two types of

batching which are;

1) Weight batching

For most large and important jobs the batching of materials is usually

done by weighing. Batching by weight eliminates error due to


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variations in the properties of voids contained in a specified volume,

especially with the batching of sand.

Measurement by weight is therefore logical and provided the weighing

machine retains its accuracy at the site, error in proportioning shouldbe negligible. An important advantage with weight batching is the

greater uniformity between successive batches of concrete.

Figure 3.3: Weigh batching machine

2) Volume Batching

For most small job, volume batching is adopted by the amount of each

solid ingredient is measured by loose volume using measuring boxes,gauge box, hopper or wheel barrows. In batching by volume,

allowance has to be made for the moisture present in sand which

results in its bulking. It also advisable to set the volumes in term of

whole bags of cement.

Figure 4.4: Wooden box for gauging aggregates

3.4.3 Mixing

The objective of mixing is to coat the surface of all aggregate particles with

cement paste and blend the ingredients into a uniform mass. It must beensure that each constituent is thoroughly dispersed throughout the mix to


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give a homogeneous product. Concrete mixing is normally done by

mechanical means called mixer, but sometime the mixing of concrete is done

by hand.

a) Machine mixingMachine mixing can either be in rotation or stirring operation. The

rotation operation is used in tilting drum mixer, non-tilting drum mixer,

and dual drum mixer and continues mixer, while the stirring operation

is used in pan-type mixer see Figure 3.5.

(a) (b)

Figure 3.5: (a) Drum mixer, (b) Pan-type mixer

Under normal machine mixing conditions, about 10 percent of the

mixing water should be placed in the mixing drum before the dry

materials are added. Water then is added uniformly with the dry

materials until about 10 percent of the water remains. This water is

added after all of the other materials are in the drum or pan.

b) Hand mixing

There may be occasions when the concrete has to be mixed by hand,and because of this case uniformity is more difficult to achieve,

therefore particular care and effort are necessary.

The aggregate should be spread in a uniform layer on a hard, clean

and non-porous base. Cement is then spread over the aggregate and

the dry material are mixed by turning over from one end of the heap to

another and cutting with a shovel until the mix appear uniform. The

water is gradually added to the trough formed by the uniform dry mix

and the mix is turned over until a homogeneous mixture of uniformcolor and consistency is obtained.


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Figure 3.6: Mixing the concrete by hand

3.4.4 Transporting

The various methods used to move the concrete from the mixer or truck to

the forms depend largely upon the job conditions.

On small jobs, wheel barrows are the usual means of transportation.

However, concrete can be handled and transported by many methods

including the of chutes, buggies operated over runways, buckets handled by

cranes or cable ways, trucks, pump to force the concrete through pipelinesand equipment to force the concrete thorough hoses pneumatically.

(a) (b)

Figure 3.7: (a) Wheel barrow; (b) Bucket


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Figure 3.8: Pump concrete

The main consideration in selecting the type of equipment to be used is an

economic one, however, certain jobs require specialized equipment and thus

the cost is a secondary consideration.

Others factors that need to be looked into when selecting the transporting

and handling equipment are the segregation of aggregates, loss of entrained

air, loss of cement paste, and change in slump.

3.4.5 Placing

The methods used in placing concrete in its final position have an important

effect on its homogeneity, density and behavior in service. To secure good

concrete it is necessary to make certain preparations before placing.

The formworks must be examined for correct alignment and adequate rigidity

to withstand the weight of concrete, impact loads during construction without

undue deformation.

Formworks should be moistened before the concrete is placed, otherwise

they will absorb water from the concrete and swell. In addition, the forms

should be oiled to make form removal easier.

The concrete should be placed in its final position rapidly so that it is not too

stiff to work. Water should not be added after the concrete has left the mixer.

When placing the concrete, care should be taken to drop the concrete

vertically and not too great a height.


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3.4.6 Compaction

The objective of compaction is to eliminate air holes and to achieve

maximum density of concrete.

During mixing of concrete a considerable quantity of air is entrapped andduring its transportation there is a possibility of partial segregation taking

place. If the entrapped air is not removed and the segregation of coarse

aggregate not corrected, the concrete may be porous, non-homogeneous

and reduced the concrete strength.

Therefore the process of removal of entrapped air and of uniform placement

of concrete to form homogeneous dense mass is termed compaction.

To compact the concrete, it should be mechanically vibrated or hand spadingas it goes into the form. The reason for compaction are to ensure the

requirement of strength, impermeability and durability of harden concrete.

The process of compaction consists of elimination of entrapped air and

forcing the particles into a close configuration.

Method of compaction can either be hand compaction or machine

compaction;

a) Hand CompactionHand compaction methods consist of rodding, tamping and spading

with suitable tools. Concrete mixes that normally use for hand

compaction are of fairly workable mix if the sections are at narrow and

the reinforcement closely packed.

b) Machine compaction

Compaction by using vibrators makes possible the placement of stiff,

harsh concrete mixes that cannot be placed and consolidated readily

by hand. Vibration makes it possible to use less workable mixes,resulting in increased strength and lower drying shrinkage for given

mix proportions. Vibrating machines are usually operated by petrol

engines, compressed air or electricity. The vibrating machines that are

suitable for site use are of 3 main types, namely:

o Internal vibrator-pocker

o External vibrator-clamp to formwork

o Vibrating tables


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The internal vibrator head should be kept moving in and up-and-down

direction to consolidate the concrete. Thus, preventing honeycombing

and air voids at the surface of the form. Systematic vibration will

consolidate the concrete adequately.

Figure 3.9: Vibrator pocker

3.4.7 Curing

In order to obtain good concrete the placing of an appropriate mix must be

followed by curing in a suitable environment during the early stages ofhardening.

The purpose of curing is to promote the hydration of cement, thus the

development of strength and durability of concrete. It also controls the

temperature and moisture movement from and into the concrete.

More specifically, the objective of curing is to keep concrete saturated, or as

nearly saturated as possible, until the originally water-filled space in the fresh

cement paste has been filled to the desired extent by the products ofhydration of cement.

In the case of site concrete, active curing stops nearly always long before the

maximum possible hydration has taken place. Normal curing keeps concrete

saturated or as nearly saturated as possible until water-filled space has been

occupied by the product of hydration. Inadequate curing through loss of water

by evaporation will fail to gain strength.

The necessity for curing arises from the fact that hydration of cement can

take place only in water-filled capillaries. This is why a loss of water byevaporation from the capillaries must be prevented. Furthermore, water lost


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internally by self desiccation has to be replaced by water from outside, i.e.

ingress of water into the concrete must be made possible.

3.4.7.1 Method of Curing

In general, concrete should be cured for at least three days and preferablefor a week after it is placed. The curing time depends on the temperature of

the concrete. The length of time that the concrete is to be protected against

the loss of moisture depends on:-

The cement content

Mix proportions

Required strength

Size and shape of the concrete mass - Weather

Future exposure conditions

Curing can be divided into two classifications:

Those which supply additional moisture to the concrete, and

Those which prevent loss of moisture from the concrete by sealing

the surface.

1) Water Curing

Curing by flooding, ponding, or mist spraying is widely used. It is the

most effective of all known curing methods for the prevention of mixwater evaporation. This method is not always practical, however,

because of job conditions. Continuous sprinkling with water is also

an excellent method of curing. If the sprinkling is done at intervals,

the concrete must not be allowed to dry between applications of

water. A constant supply of water prevents the .possibility of crazing

or cracking due to alternate wetting and drying.

2) Water Retaining Methods

These methods involve the use of coverings that are keptcontinuously wet, as such as sand, earth, canvas, sawdust or straw.

When concrete is cured by one of these methods, the entire

concrete surface, including exposed edges or sides, must be

covered. The material is kept moist by periodical sprinkling of water.


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Figure 3.10: Curing a concrete slab by flooding

Figure 3.11: Concrete have been cured using blanket

3) Waterproof Mechanical Barriers

Barriers of waterproof or plastic film seal in the water and prevent

evaporation. Advantages of mechanical barriers is that periodic

additions of water are not required, provide protection againstdamage from subsequent construction activity as well as protection

from the sun. This waterproof paper if of a good quality can be

reused. These materials are applied as soon as the concrete is hard

enough to resist surface damage.

4) Chemical Membranes

Chemicals can be sprayed on the surface to cure concrete. Liquid

membrane-forming curing compounds retard or prevent the

evaporation of moisture from the concrete. The chemical applicationshould be made as soon as the concrete is finished. If there is any


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delay in the application, the concrete must be kept moist until the

membrane is applied. The membrane curing compound must not be

applied when there is free water on the surface or after the concrete

is dry.

5) Steam Curing

In steam curing, the heating of the concrete products is caused by

steam either at low pressure or high pressure. The method ensures

even heating of products all over, even if the space between the

stacked precast concrete products is very small.

Steam curing is more favourable to mixes of concrete with low water-

cement ratio than mixes with higher water-cement ratio. The choice of

steam curing cycle will be governed by:

The pre-curing period

The rate of increase and decrease of temperature.

The level and time of constant temperature.

An early rise in temperature at the time of setting of concrete may be

detrimental to concrete because the green concrete may be too weak

to resist the air pressure set up in the pores by the increased

temperature. Too high a rate of increase or decrease in temperatureintroduces thermal shocks and the rates should generally not exceed

10 C to 20 C per hour.

Figure 3.12: Autoclave for steam curing large diameter hoses

3.5 Standard Testing for Fresh and Harden Concrete


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3.5.1 Testing of Fresh Concrete

In fresh concrete, concrete is in the plastic state which can be moulded into

desired shape. Therefore, in order to handling concrete without segregation,

to placing without loss of homogeneity and to compacting with specified effort,

the certain testing should be performed to ensure that the concrete areworkable enough.

a) Slump Test

This method of test specifies the procedure to be adopted, either in the

laboratory or during the progress of work in the field, for determining the

consistency of concrete where the nominal maximum size of the aggregate

does not exceed 38 mm.

The internal dimensions of the mould for the test specimen shown in Figure4.13 are bottom diameter = 200 mm, top diameter = 100 mm, and height =

300 mm.

The mould is filled in with fresh concrete in four layers, each approximately

one-quarter of the height and tamped with twenty-five strokes of the rounded

end of the tamping rod. The strokes are distributed in a uniform manner over

the cross -section and for the second and subsequent layers should

penetrate into the underlying layer. The bottom layer is tamped throughout its

depth. After the top layer has been rodded, the concrete is struck off levelwith a trowel or the tamping rod, so that the mould is exactly filled.

Figure 3.13: Mould for Slump test

The mould is removed immediately by raising it slowly and carefully in a

vertical direction. This allows the concrete to subside and the slump is

measured immediately by determining the difference between the height ofthe mould and that of the highest point of the specimen being tested (Fig.

Plan view

Side view


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4.14). The slump measured is recorded in terms of millimeters of subsidence

of the specimen.

Figure 3.14: Measuring slump

b) Compacting Factor TestThis test is more precise and sensitive than the slump test and is particularly

useful for concrete mixes of very low workability as are normally used when

concrete is to be compacted by vibration; such concrete may consistently fail

to slump. A diagram of the apparatus used in compacting factor test is shown

in Figure 4.15.

The sample of concrete to be tested is placed gently in the upper hopper.

Hopper clamper is fiIIed level with its brim and the trap-door is opened to

allow the concrete to fall into the lower hopper.

Figure 3.15: Compaction factor apparatus

Certain mixes have a tendency to stick in one or both of the hoppers. If thisoccurs, the concrete may be helped through by pushing the rod gently into


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the concrete from the top. During this process, the cylinder should be

covered by the trowels. Immediately after the concrete has come to rest, the

cylinder is uncovered, the trap-door of the lower hopper is opened, and the

concrete is aIIowed to fall into the cylinder. The excess of concrete remaining

above the level of the top of the cylinder is then cut off.

The weight of the concrete in the cylinder is then determined to the nearest l0

g as the weight of partiaIIy compacted concrete. The cylinder is refilled with

concrete from the same sample in layers of approximately 50 mm, the layers

being heavily rammed or preferably vibrated so as to obtain full compaction.

The top surface of the fully compacted concrete is carefully struck off level

with the top of the cylinder. The compacting factor is defined as the ratio of

the weight of partially compacted concrete to the weight of fully compacted

concrete. It is normally stated to the nearest second decimal place.

c) Vebe Consistometer Test

The test determines the time required for transforming, by vibration, a

concrete specimen in the shape of a conical frustum into a cylinder. The

apparatus (Figure 3.16) consists of a vibrator table resting upon elastic

supports, a metal pot, a sheet metal cone, open at both ends, and a standard

iron rod.

A slump test as described earlier is performed in the cylindrical pot of theconsistometer. The glass disc attached to the swivel arm is moved and

placed just on the top of the slump cone in the pot and before the cone is

lifted up, the position of the concrete cone is noted by adjusting the glass disc

attached to the swivel arm . The cone is then lifted up and the slump noted

on the graduated rod by lowering the glass disc on top of the concrete cone.

The electrical vibrator is switched on and the concrete is allowed to spread

out in the pot. The vibration is continued until the whole concrete surface

uniformly adheres to the glass disc and the time taken for this to be attainedis noted with a stop watch. The consistency of the concrete is expressed in

VB-degree which is equal to the recorded time in seconds. The required

slump is obtained on the basis of the consistency scale given in Table 3.1.

The curve in Figure 3.17 indicates the relationship between slump in mm and

the degrees covered by the consistency scale given in Table 3.1.


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Figure 3.16: Vebe Consistometer

Figure 3.17: Relationship between slump and Vebe degrees


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Table 3.2: Values of Workability for Different Placing Conditions


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3.5.2 Testing of Hardened Concrete

There are several reasons why testing of hardened concrete is important,

there are:

(1) Test can investigate the fundamental physical behavior of concretesuch as elastic properties and strength characteristics;

(2) When physical laws are not fully understood testing can simulate

expected conditions to evaluate performance;

(3) Tests to determined physical material constants like the modulus of

elasticity; and

(4) Quality control.

Common characteristics of concrete like strength and durability should not be

considered fundamental material properties. Variables like specimengeometry and preparation, moisture content, temperature, loading rate, and

the type of testing device will affect the mechanical behavior. Therefore,

when defining some mechanical property it is necessary to specify the test

used to determine the value.

The testing of hardened concrete can be classified into two types which are

destructive test and non-destructive test.

a) Destructive test

1) Cube test (BS 1881: Part 116)

This is currently the most common type of destructive test for concrete, owing

to the cheapness of the cube moulds and the comparative simplicity of

manufacture and testing of cubes.

Carefully obtained samples of the concrete mix are placed and compacted in

accurately formed steel moulds, with machine inner surface. Bonding with the

steel mould is prevented by coating with release agent. The surface of eachcube is covered with impermeable sheet or the entire mould sealed. After 24

hours the cube is removed and cured under water at about 20oC, until tested

at age of 7th, 14

thand 28

thdays.

At the testing day, the cube with size of 150mm x 150mm x 150mm or

100mm x 100mm x 100mm, then place centrally between the platens of a

compression testing machine, trowelled face sideways, and the load is

applied such that the stress increase at a given constant rate until failure.

The maximum load is recorded and the values were divided with the crosssectional area of the cube to obtained the compressive strength of the cube.


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Figure 3.18: Compacting of concrete cube

2) Cylinder Splitting Test (BS 1881: Part 117)

In this test, cylinders which are typically 300mm long and 150mm in diameter,

are loaded in a compression tester with their cylindrical axes horizontal,

stress concentrations being avoided by use the hardboard or plywood strips

about 12mm wide.

The successful operation of the test requires careful alignment of the cylinder

(or use of a jig) and packing strips should be used once only to ensure

uniform bedding, especially in the case of weak concretes, for which plywood

is more suitable material. Except near the packing pieces, a tensile stress is

induced by concrete on the vertical plane and the tensile strengthft at failure

is given by:

Equation 3.1

Where;

W= Load at failure

D= Diameter of cylinder

L= Length of cylinder

Note that, since the failure area is DL, the expression is the same as

Equation 3.2

DL

W

ft p

2=

p

2

_

=

areafailure

loadft


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Figure 3.19: The cylinder splitting test for measurement of the tensile

strength of concrete

b) Non-destructive tests

These tests are useful to: (1) quality control; (2) determination of the time for

form removal; and (3) help assess the soundness of existing concrete

structures. Surface Hardness Methods- One of the oldest nondestructive tests,

developed in Germany in the 1930's. Basically, the surface is

impacted with a mass and the size of the resulting indention is

measured. The accuracy of these types of tests is only 20 to 30%.

Rebound Hardness - The most common nondestructive test is the

rebound test. The test measures the rebound of a hardened steel

hammer impacted on the concrete by a spring. This method has the

same limitations as the surface hardness tests. The results are

affected by: (1) surface finish; (2) moisture content; (3) temperature;(4) rigidity of the member being tested; (5) carbonation of the surface;

and (6) direction of impact (upward, downward, horizontal). Most

useful in checking the uniformity of concrete.

Penetration Resistance- Resistance of concrete to penetration by a

steel probe driven by a given amount of energy is measured. This test

is not affected by surface hardness or carbonation as the above tests,

however, the mix proportions and material properties are still

important.


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Pull-Out Test - Pull-out test determine the force required to pull a

steel insert out of concrete which it was embedded during casting.

This test is a measure of the shear strength of the concrete which can

be correlated with compressive strength. This test is better than thosepreviously discussed, however, the test may be planned in advance

and the assembly embedded in the concrete during casting.

Ultrasonic Pulse Velocity - This test is based o the fact that the

velocity of sound is related to the elastic modulus. The device is

accurate to about + 1%. The position of the testing equipment can

affect the measurement, method A given the best results. There are

several factors which affect this test: (1) surface smoothness; (2)

travel path of the pulse; (3) temperature effects on the pulse velocity;

(4) moisture content; (5) presence of steel reinforcing bars; and (6)age of concrete.

3.6 Properties of Hardened Concrete

3.6.1 Deformation under Load

It is a stress strain relationship under normal loading and under sustained

loading. Under normal loading, the first effect of applying a load to concrete is

to produce an elastic deformation, thus obey Hooke's Law. Concrete iselastoplastic material, therefore as the load increases deformation increases.

Under sustained loading, prolong application of stress causes a slow

deformation, commonly known as creep. The increase of deformation is not

proportional; as the time passes the deformation is lesser. The magnitude of

deformation depends primarily on the stress-strength ratio at the time of

loading, but it is also influence by factors such as the mix proportions, the

size of the specimen and even the climatic conditions. If the load is later

removed, the concrete undergoes an immediate elastic recovery. Creeprecovery is a slower process, and the concrete will in any case not fully

regain its original dimensions. Figure 4.18 shows the graph of deformation of

hardened under load. In this graph, the assumption made is that the stress

strain curve is linear (straight line) and the creep deformation of concrete also

varies linearly with the sustained stress up to a value of0.5fo.


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Figure 3.20: Deformation of hardened concrete under load

3.6.2 Durability of Concrete

Durability is defined as the ability to withstand the damaging effects of the

environment over a long period of time. Therefore it is essential that concreteis designed in such a way that it may be of service without deterioration over

a period of years. Such concrete is said to be durable.

The absence of durability may be caused either by the environment to which

the concrete is exposed i.e. external or by internal cause within the concrete

itself. The external causes can be:

Physical, chemical or mechanical.

Due to weathering. occurrence of extreme e temperature, abrasion,electrolytic action, and

Attack by natural or industrial liquids and gases.

In order to be durable, concrete must be relatively impervious. The salts in

sea water react chemically with concrete. The salts crystallize in the pores of

concrete after the evaporation of water and cause disruption. The corrosion

of reinforcement may lead to rupture of cover concrete. Therefore it is

recommended to the following in order to have durable concrete, and they

are:


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Use of low, water-cement ratio.

Well compacted concrete.

Good workmanship to reduced porosity.

Sufficient cover over reinforcement. The use of aluminious sulphate resisting cement, Portland blast-

furnace or Portland pozzolana cement.

3.6.3 Permeability

Concrete has a tendency to be porous due to the presence of voids formed

during or after placing. To produce concrete of low permeability, full

compaction and proper curing is essential. For a given aggregate, the

permeability of concrete can be reduced by reducing the water content or by

increasing the cement content. Low permeability of concrete is important inincreasing resistant to frost action and chemical attack and in protecting

embedded steel against corrosion.

Therefore the study of permeability of concrete is important in case of

reinforced concrete, ingress of moisture and air will result in corrosion of steel

which leads to an increase in the volume of steel, and to cracking and

spalling of concrete cover.

Factors influencing permeability are: Water-cement ratio.

Workability

Type of structure.

Method of compaction.

Soundness and porosity of the aggregate.

Age - Permeability decreases with age. The decrease being greater

for wet mixes than for drier one.

Grading of aggregate.

Curing.

3.6.4 Shrinkage

Shrinkage is a contraction deformation suffered by concrete even under no

load. The shrinkage of concrete is dependent on the amount of drying that

can take place. It is therefore influenced by the humidity and temperature of

the surrounding air, the rate of air flow over the surface and the proportion of

the surface area to volume of concrete.

The two types of shrinkage strains are:-

Plastic shrinkage. Drying shrinkage.


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Plastic shrinkage is cause due to the hydration of cement which results in

reduction in the volume of the system of cement plus water to an extent of

about 1 percent of the volume of dry cement. This contraction is known as

plastic strain and it is aggravated due to loss of water by evaporation fromthe surface of concrete, particularly under hot climates and high winds. This

can result in surface cracking.

Drying shrinkage is shrinkage which takes place after the concrete has set

and hardened. It takes place in the first few months. Drying shrinkage is

cause due to withdrawal of water from concrete stored in unsaturated air

voids. A part of this shrinkage can be recovered on immersion of concrete in

water.

3.7 Concrete Mixture and Design

Concrete mix design has a number of different approaches such as ACI

(American Concrete Institution) developed in U.S.A. and the most popular

and widely used is the DOE (Department of Environmental) method. It is the

British method of concrete mix design and it is being used in United Kingdom

and other parts of the world. This method is based on extensive laboratory

and field experiments carried out by the Road Research Laboratory U.K.

3.7.1 DOE Method for Normal ConcreteThis method is applicable to normal weight concrete made with Portland

cement only or taken into account ground granulated blastfurnace slag or fly

ash. This method does not cover the flowing concrete, pumped concrete or

lightweight aggregate concrete.

3.7.1.2 Design Mix Stages

The mix design is carried out according to the DOE Method in the following

five stages.

1) Determining the free water/cement ratio

Given the required characteristics strength at a specified age. Use Equation

3.3 to obtain the target mean strength at that age, which is the compressive

strength to be used in the mix design.

Target mean strength = Characteristic strength + Ks Equation 3.3

The constant k is derived from the mathematics of normal distribution and

increases as the proportion of defectives is decreased, thus:
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K for 10% defectives = 1.28

K for 5% defectives = 1.64

K for 2.5% defectives = 1.96

K for 1% defectives = 2.33s = standard deviation of the strength tests. Refer to Table 3.3 for the

typical values.

For design the concrete mix grade 30 with assumption of 5% defectives and

8 N/mm2 standard deviation. The target mean strength is calculated as

follows.

Target mean strength= 30 N/mm2+ 1.64 (8 N/mm2) = 43 N/mm2

Table 3.3: Standard deviation under different condition

Conditions Standard Deviation (N/mm2)

Good control with weight batching, use

of graded aggregates, etc Constant

supervision.

Fair control with weight batching. Use of

two sizes of aggregates. Occasional

supervision.

Poor control. Inaccurate volume batching

of all-in aggregates. No supervision.

3 5

4 7

5 8

Given the type of cement and aggregate, use Table 3.4 to obtain the

compressive strength at the specified ages that correspond to a free water

cement ratio of 0.5.

For example: ordinary Portland cement and crushed aggregate are used.

From the Table 3.4of the compressive strength of49 N/mm2at 28 days(and

36 N/mm2 at 7 days and etc.)

In Figure 3.21 follow the starting line to locate the curve which passes

through the point. (49 N/mm2, w/c=0.5), in this particular case, it is the third

curve from the top of the figure. This curve shows that to obtain our target

mean strength of 43 N/mm2, we need a water/cement ratio of 0.54.


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If the w/c ratio obtained in previous step exceeds the maximum w/c ratio

specified for durability(Table 3.5 BS8110) then adopt the lower value-

resulting in a concrete having a higher strength than required.

Table 3.4: Approximate compressive strengths of concrete made with a

free water/cement ratio of 0.5 according to the DOE Method

Compressive strength* (MPa(psi))

at the age of (days)Type of

cement

Type of

coarse

aggregate3 7 28 91

Ordinary

Portland

(Type I)

Uncrushed

Crushed

22 (3200)

27 (3900)

30 (3200)

36 (5200)

42 (6100)

49(7100)

49 (7100)

56 (8100)

Rapid-

hardening

Portland

(Type III)

Uncrushed

Crushed

29 (4200)

34 (4900)

37 (5400)

43 (6200)

48 (7000)

55 (8000)

54 (7800)

61 (8900)

Figure 3.21: Relationship between cube compressive strength and free

water/cement ratio


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Table 3.5: Durability requirement(BS8110)

Exposure Condition Nominal cover (mm)

Mild 25 20 20 20

Moderate - 35 30 20Severe - - 40 25

Very severe - - 50 30

Maximum free w/c ratio 0.65 0.60 0.55 0.45

Minimum cement content (kg/m3) 275 300 325 400

Concrete fcu(N/mm2) 30 35 40 50

2) Determining the water content

Given the slump or VB time, determine the water content from Figure 3.21.

Using Table 3.4, when coarse aggregate and fine aggregates of different

types are used, the water content W is estimated as follows:

Water Content (kg/m3) (Equation 3.4)

Wf : water content appropriate to the type of fine aggregate

Wc : water content appropriate to the type of coarse aggregate

The aggregate type in Table 3.4 refers to all types of aggregates.

Table 3.6: Approximate free water contents required for various level of

workability according to the 1988 DOE Method

Aggregate Water content, kg/m3(lb/yd3) for :

Slump

mm (in.)

0 10

(0 )

10 30

(1/2 1)

30 60

(1 2 )

60 80

(2 - 7)Max. Size

mm (in.)

Type

Vebetime, s

> 12 6 12 3 6 0 3

10 (3/8)Uncrushed

Crushed

150 (255)

180 (305)

180 (305)

205 (345)

205 (345)

230 (390)

225 (380)

250 (420)

20 (3/4)Uncrushed

Crushed

135 (230)

170 (285)

160 (270)

190 (320)

180 (305)

210(355)

195 (330)

225 (380)

40 (11/2)Uncrushed

Crushed

115 (195)

155 (260)

140 (235)

175 (295)

160 (270)

190 (320)

175 (295)

205 (345)

cf WW3

1

3

2+=


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3) Determining the cement content

The value given by Equation 3.1 should be checked against any maximum or

minimum cement contents that may have been specified for durability. Refer

Table 3.5.

Cement content (kg/m3) (Equation 3.5)

If the cement content calculated from Equation 3.5 is below a specified

minimum, this minimum must be used - resulting in a reduced water/cement

ratio and hence has a higher strength than the target mean strength. If the

calculated cement content is higher than a specified maximum, the specified

strength and workability cannot be simultaneously met with the selectedmaterials, try to change the type and maximum size of the aggregate.

4) Determining the aggregate content

Having calculated the water content and cement content, the total aggregate

content in practice is obtained from the chart in the DOE document. The

value can be calculated from basic principles. For each cubic meter of fully

compacted fresh concrete,

Volume occupied by aggregate

(Equation 3.6)

Where,

c = 3150 kg/m3is density of cement particles

w= 1000 kg/m3is the density of water

Therefore;

Total aggregate content (kg/m

3

) = a x Volume occupied by aggregate (Equation 3.7)

Where,

a, is the density of the aggregate particles. The DOE recommends that if no

information is available a should be taken as 2600 kg/m3 for uncrushed

aggregates and 2700 kg/m3

for crushed aggregate.

5) Determining of the fine and coarse aggregate contents

Total aggregate content consists of fine aggregate will depends on the

grading zone 1, 2, 3 and 4 (see Table 3.5). The general principle in mixdesign is the finer the grading of the fine aggregate. The larger its structure

ratiocementwater

ntwaterconte=

wc

ntwaterconteentcementcontgg

--= 1


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area per unit weight, the lower will be the proportion expressed as a

percentage of the total aggregate required to produce a concrete.

For a given slump and w/c ratio, the proportion of fine aggregate can bedetermined from Figure 3.22in which the grading zones are those ofTable

3.7.

Table 3.7: Grading limits for DOE mix design procedure

Percentage by weight passing standard sieves

Standard Sieve sizes Grading

Zone 1

Grading

Zone 2

Grading

Zone 3

Grading

Zone 4

10 mm 100 100 100 100

5 mm 90 100 90 100 90 100 95 100No. 7 (2.36 mm) 60 95 75 100 85 100 95 100

No. 14 (1.18 mm) 30 70 55 90 75 100 90 100

No. 25 (600 m) 15 34 35 59 60 79 80 100

No. 52 (300 m) 5 20 8 30 12 40 15 50

No. 100 (150 m) 0 10 0 10 0 10 0 15

Figure 3.22: Proportions of fine aggregates for grading zones 1,2,3,4

(See Table 3.5) for use with 20 mm nominal maximum size coarse

aggregate


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Figure 3.23: Estimate wet density of fully compacted concrete

3.7.2 ACI Method for Normal ConcreteThe American Concrete Institute (ACI) mix design method is one of basic

concrete mix design methods available. This section summarizes the ACI

absolute volume method because it is widely accepted in the U.S. and

continually updated by the ACI.

3.7.2.2 Design Mix Stages

The standard ACI mix design procedure can be divided up into 8 basic steps:

1) Choice of SlumpThe choice of slump is actually a choice of mix workability. Workability can

be described as a combination of several different, but related, PCC

properties related to its rheology:

Ease of mixing

Ease of placing

Ease of compaction

Ease of finishing


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Generally, mixes of the stiffest consistency that can still be placed adequately

should be used (ACI, 2000). Typically slump is specified, but Table 3.6

shows general slump ranges for specific applications. Slump specifications

are different for fixed form paving and slip form paving. Table 3.7 showstypical and extreme state DOT slump ranges.

Table 3.6: Slump ranges for specific applications (after ACI, 2000)

SlumpType of Construction

(mm) (inches)

Reinforced foundation walls and footings 25 - 75 1 - 3

Plain footings, caissons and substructure

walls

25 - 75 1 - 3

Beams and reinforced walls 25 - 100 1 - 4

Building columns 25 - 100 1 - 4

Pavements and slabs 25 - 75 1 - 3

Mass concrete 25 - 50 1 - 2

Table 3.7: Typical state DOT slump specifications

Fixed Form Slip FormSpecifications

(mm) (inches) (mm) (inches)

Typical 25 - 75 1 - 3 0 - 75 0 - 3

Extremes

as low as 25

as high as

175

as low as 1

as high as 7

as low as 0

as high as

125

as low as 0

as high as 5

2) Maximum Aggregate Size

Maximum aggregate size will affect such PCC parameters as amount of

cement paste, workability and strength. In general, ACI recommends that

maximum aggregate size be limited to 1/3 of the slab depth and 3/4 of the

minimum clear space between reinforcing bars. Aggregate larger than these

dimensions may be difficult to consolidate and compact resulting in a

honeycombed structure or large air pockets. Pavement PCC maximum

aggregate sizes are on the order of 25 mm (1 inch) to 37.5 mm (1.5 inches).

3) Mixing Water and Air Content Estimation

Slump is dependent upon nominal maximum aggregate size, particle shape,

aggregate gradation, PCC temperature, the amount of entrained air and
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certain chemical admixtures. It is not generally affected by the amount of

cementitious material. Therefore, ACI provides a table relating nominal

maximum aggregate size, air entrainment and desired slump to the desired

mixing water quantity. Table 3.8 is a partial reproduction of ACI Table 6.3.3(keep in mind that pavement PCC is almost always air-entrained so air-

entrained values are most appropriate). Typically, state agencies specify

between about 4 and 8 percent air by total volume (based on data from

ACPA, 2001).

4) Water-Cement Ratio

The water-cement ratio is a convenient measurement whose value is well

correlated with PCC strength and durability. In general, lower water-cement

ratios produce stronger, more durable PCC. If natural pozzolans are used inthe mix (such as fly ash) then the ratio becomes a water-cementitious

material ratio (cementitious material = portland cement + pozzolonic

material). The ACI method bases the water-cement ratio selection on

desired compressive strength and then calculates the required cement

content based on the selected water-cement ratio. Table 3.9 is a general

estimate of 28-day compressive strength vs. water-cement ratio (or water-

cementitious ratio). Values in this table tend to be conservative (ACI,

2000). Most state DOTs tend to set a maximum water-cement ratio between

0.40 - 0.50

5) Cement Content

Cement content is determined by comparing the following two items:

The calculated amount based on the selected mixing water content

and water-cement ratio.

The specified minimum cement content, if applicable. Most state

DOTs specify minimum cement contents in the range of 300 - 360

kg/m3

(500 - 600 lbs/yd3

).
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Table 3.8: Approximate Mixing Water and Air Content Requirements for Different Slumps and Maximum Aggregate Sizes

Mixing Water Quantity in kg/m3(lb/yd3) for the listed Nominal Maximum Aggregate Size

Slump9.5 mm

(0.375 in.)

12.5 mm

(0.5 in.)

19 mm

(0.75 in.)

25 mm

(1 in.)

37.5 mm

(1.5 in.)

50 mm

(2 in.)

75 mm

(3 in.)

100 mm

(4 in.)

Non-Air-Entrained PCC

25 50 (1 - 2) 207 (350) 199 (335) 190 (315) 179 (300) 166 (275) 154 (260) 130 (220) 113 (190)

75 100 (3 - 4) 228 (385) 216 (365) 205 (340) 193 (325) 181 (300) 169 (285) 145 (245) 124 (210)

150 175 (6 - 7) 243 (410) 228 (385) 216 (360) 202 (340) 190 (315) 178 (300) 160 (270) -

Typical entrapped air (percent) 3 2.5 2 1.5 1 0.5 0.3 0.2

Air-Entrained PCC

25 50 (1 - 2) 181 (305) 175 (295) 168 (280) 160 (270) 148 (250) 142 (240) 122 (205) 107 (180)

75 100 (3 - 4) 202 (340) 193 (325) 184 (305) 175 (295) 165 (275) 157 (265) 133 (225) 119 (200)

150 175 (6 - 7) 216 (365) 205 (345) 197 (325) 184 (310) 174 (290) 166 (280) 154 (260) -

Recommended Air Content (percent)

Mild Exposure 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0

Moderate Exposure 6.0 5.5 5.0 4.5 4.5 4.0 3.5 3.0

Severe Exposure 7.5 7.0 6.0 6.0 5.5 5.0 4.5 4.0
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Table 3.9: Water-Cement Ratio and Compressive Strength Relationship

Water-cement ratio by weight28-Day Compressive

Strength in MPa (psi)Non-Air-Entrained Air-Entrained

41.4 (6000) 0.41 -

34.5 (5000) 0.48 0.40

27.6 (4000) 0.57 0.48

20.7 (3000) 0.68 0.59

13.8 (2000) 0.82 0.74

An older practice used to be to specify the cement content in terms of the

number of 94 lb. sacks of portland cement per cubic yard of PCC. This

resulted in specifications such as a "6 sack mix" or a "5 sack mix". While

these specifications are quite logical to a small contractor or individual who

buys portland cement in 94 lb. sacks, they do not have much meaning to the

typical pavement contractor or batching plant who buys portland cement in

bulk. As such, specifying cement content by the number of sacks should be

avoided.

6) Coarse Aggregate Content

Selection of coarse aggregate content is empirically based on mixture

workability. ACI recommends the percentage (by unit volume) of coarse

aggregate based on nominal maximum aggregate size and fine aggregate

fineness modulus. This recommendation is based on empirical relationships

to produce PCC with a degree of workability suitable for usual reinforced

construction (ACI, 2000). Since pavement PCC should, in general, be more

stiff and less workable, ACI allows increasing their recommended values by

up to about 10 percent. Table 3.10 shows ACI recommended values.

Table 3.10: Volume of Coarse Aggregate per Unit Volume of PCC for

Different Fine aggregate Fineness Modulus for Pavement PCC.

Fine Aggregate Fineness ModulusNominal Maximum Aggregate Size

2.40 2.60 2.80 3.00

9.5 mm (0.375 inches) 0.50 0.48 0.46 0.44

12.5 mm (0.5 inches) 0.59 0.57 0.55 0.53

19 mm (0.75 inches) 0.66 0.64 0.62 0.60

25 mm (1 inches) 0.71 0.69 0.67 0.65

37.5 mm (1.5 inches) 0.75 0.73 0.71 0.69

50 mm (2 inches) 0.78 0.76 0.74 0.72


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Notes:

1. These values can be increased by up to about 10 percent for

pavement applications.

2. Coarse aggregate volumes are based on oven-dry-rodded weightsobtained in accordance with ASTM C 29.

7) Fine Aggregate Content

At this point, all other constituent volumes have been specified (water,

portland cement, air and coarse aggregate). Thus, the fine aggregate

volume is just the remaining volume:

Unit volume (1 m3or yd3)

- Volume of mixing water- Volume of air

- Volume of portland cement

- Volume of coarse aggregate

Volume of fine aggregate

8) Adjustments for Aggregate Moisture

Unlike HMA, PCC batching does not require dried aggregate. Therefore,

aggregate moisture content must be accounted for. Aggregate moisture

affects the following parameters:

1. Aggregate weights. Aggregate volumes are calculated based on oven

dry unit weights, but aggregate is typically batched based on actual

weight. Therefore, any moisture in the aggregate will increase its

weight and stockpiled aggregates almost always contain some

moisture. Without correcting for this, the batched aggregate volumes

will be incorrect.

2. Amount of mixing water. If the batched aggregate is anything but

saturated surface dry it will absorb water (if oven dry or air dry) or give

up water (if wet) to the cement paste. This causes a net change in the

amount of water available in the mix and must be compensated for by

adjusting the amount of mixing water added.

3.8 Types Of Concrete

3.8.1 Reinforced Cement Concrete

Reinforced cement concrete is a composite material made up of cement

concrete and reinforcement in which the concrete resists compression with
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reinforcement resisting 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.

Figure 3.24: Reinforced concrete slab

3.8.2 Prestressed Concrete

A prestressed concrete may be defined as a concrete in which stresses ofsuitable magnitude and distribution are introduced to counteract to a desired

degree the stresses resulting from external loads. The 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 smaller cross-section of member results in smaller self weight.

2. High bearing stresses are generated in anchorage zones.

3. The shrinkage cracks are reduced, with higher modulus of elasticity and

smaller creep strain.

The loss of prestress at the initial stages is very high and for it high strength

steel is required. Prestressing is achieved by either pretensioning or post-

tensioning. In the former the wires or cables are anchored, tensioned and

concrete is cast in the moulds. 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

the prestressing force is applied to the steel bars or cables, after the concrete

has hardened sufficiently. After applying the full prestress the cable passagesare grouted.


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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.

Figure 3.25: Prestressed concrete girder

3.8.3 Polymer ConcreteThe strength of concrete is greatly affected by porosity and attempts to

reduce it by vibration, pressure application, spinning, etc. They are of little

help in reducing the water voids and the inherent porosity of gel which is

about 28 per cent. The impregnation of manomer and subsequent

polymerisation reduces the inherent porosity of the concrete. Polymers-

polyvinyl acetate, homopolymer emulsions and vinyl acetate copolymer

emulsions - are added to increase strength, resistance to oil, grease, and

abrasion. They also improve bond between new and old concrete and are

useful for prefabricated structural elements, prestressed concrete, marineworks, nuclear power plants, water proofing and ferrocement products. The

disadvantages are that they are very brittle and expensive.

3.8.4 Fiber Reinforced Concrete

Conventional concrete is modified by random dispersal of short discrete fine

fibers of asbestos, steel, sisal, glass, carbon, Poly-propylene, glass, nylon,

etc. Asbestos cement fibers so far have proved to be commercially

successful. The improvement in structural performance depends on the

strength characteristics, volume, spacing, dispersion and orientation, shapeand their aspect ratio (ratio of length to diameter) of fibers.


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The pull out resistance of fibers depends upon the bond between the fibers

and matrix, the number of fibers crossing the crack, and the aspect ratio.

The advantages of fiber reinforced concrete are:- Strength of concrete increase

Fibers help to reduce cracking and permit the use of thin concrete

section.

Ductility, impact resistance, tensile and bending strength are

improved.

The disadvantages of fiber reinforced concrete are:-

Fibers reduce the workability of a mix and may cause the entrainment

of air. Steel fibers tend to intermesh and form ball during mixing of concrete.

Fiber reinforced concrete is useful hydraulic structures, airfield pavements,

highway, bridge decks, and heavy duty floors.

3.8.5 Lightweight 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 is most suitable for such construction works. It is bestproduced 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 furnance 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. Theconcrete produced is known as cellular concrete which is further

classified in the following:

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 furnance slag or fly ash).


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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 sulphonapthene compounds andhydrolysed slaughter blood. This is very suitable for heat insulation purposes.

Gas concrete is also known as aeratedconcrete. It is obtained by expanding

the binding material paste by gas forming substances such as aluminium

powder. It is used for same purposes as that the foam concrete. However, it

is better than foam concrete.

The basic considerations in choosing the proportion of light weight concrete

are economy consistent with placability and adequate strength, andattainment of specified bulk density with the lowest consumption of cement.

The characteristics of light weight concrete can be described as follows:-

Density: the density of L.W.C varies from 300-1200 kg/m3.

Strength: It has high compressive strength in relation to density. The

tensile strength is about 1/5th of its compressive strength.

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.

Reparability 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.

The applications of light weight concrete are:-

Low density cellular concrete is used for precast floor and roofing

units.

Load bearing walls using cellular concrete blocks.

As insulation cladding to exterior walls of structures.


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Figure 3.26: Lightweight concrete

3.9 Admixture for Concrete

Admixtures are ingredients other than portland cement, water, and aggre-

gates that may be added to concrete to impart a specific quality to either the

plastic (fresh) mix or the hardened concrete. Some admixtures are charged

into the mix as solutions. In such cases the liquid should be considered partof the mixing water. If admixtures cannot be added in solution, they are either

weighed or measured by volume as recommended by the manufacturer.

Admixtures are classified by the following chemical and functional physical

characteristics:

Air entrainers

Water reducers

Retarders

Hydration controller admixtures Accelerators

Supplementary cementitious admixtures

The Portland Cement Association (PCA) identifies four major reasons for

using admixtures:

to reduce the cost of concrete construction.

to achieve certain properties in concrete more effectively than by other

means.


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to ensure quality of concrete during the stages of mixing, transporting,

placing, and curing in adverse weather conditions.

to overcome certain emergencies during concrete operations.

3.9.1 Air Entrainers

Air entrainers produce tiny air bubbles in the hardened concrete to provide

space for water to expand upon freezing. Internal stresses reduce the

durability of hardened concrete, especially when cycles of freeze and thaw

are repeated many times. The impact of each of these mechanisms is

mitigated by providing a network of tiny air voids in the hardened concrete

using air entrainers. In the late 1930s. the introduction of air entrainment in

concrete represented a major advance in concrete technology. Air

entrainment is recommended for all concrete exposed to freezing.

All concrete contains entrapped air voids, which have diameters of 1 mm or

larger and which represent approximately 0.2% to 3% of the concrete

volume. Entrained air voids have diameters that range from 0.01 mm to 1

mm, with the majority being less than 0.1 mm. The entrained air voids are not

connected and have a total volume between 1% and 7.5% of the concrete

volume. Concrete mixed for severe frost conditions should contain ap-

proximately 14 billion bubbles per cubic meter. Frost resistance improves

with decreasing void size, and small voids reduce strength less than largeones.

In addition to improving durability, air entrainment provides other important

benefits to both freshly mixed and hardened concrete. Air entrainment

improves concrete's resistance to several destructive factors, including

freeze-thaw cycles, deicers and salts, sulfates, and alkali-silica reactivity. Air

entrainment also increases the workability of fresh concrete. Air entrainment

decreases the strength of concrete however; this effect can be reduced for

moderate-strength concrete by lowering the water-cement ratio andincreasing the cement factor. High strength is difficult to attain with air-

entrained concrete.

3.9.2 Water Reducer

Workability of fresh or plastic concrete requires more water than is needed

for hydration. The excess water, beyond the hydration requirements, is

detrimental to all desirable properties of hardened concrete. Therefore,

waterreducing admixtures have been developed to gain workability and, at

the same time, maintain quality. Water reducers increase the mobility of thecement particles in the plastic mix, allowing workability to be achieved at


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lower water contents. Water reducers are produced with different levels of

effectiveness: conventional, mid-range, and high-range. The high-range

water reducer is typically called superplasticizer.

Water reducing admixtures can be used indirectly to gain strength. Since the

water-reducing admixture increases workability, we can take advantage of

this phenomenon to decrease the mixing water, which in turn reduces the

water-cementitious materials ratio and increases strength.

Superplasticizers, or high-range water reducers, can either greatly increase

the flow of the fresh concrete or reduce the amount of water required for a

given consistency. For example, adding a superplasticizer to a concrete with

a 75-mm (3 in.) slump can increase the slump to 230 mm (9 in.), or theoriginal slump can be maintained by reducing the water content 12% to 30%.

Reducing the amount of mixing water reduces the water-cementitious

materials ratio, which in turn, increases the strength of hardened concrete. In

fact, the use of superplasticizers has resulted in a major breakthrough in the

concrete industry. Now, high-strength concrete in the order of 70-80 MPa

compressive strength or more can be produced when superplasticizers are

used. Superplasticizers can be used in the following cases:

Low water-cementitious materials ratio is beneficial (e.g., high-strengthconcrete, early strength gain, and reduced porosity) 2. placing thin

sections

Placing concrete around tightly spaced reinforcing steel

Placing cement underwater

Placing concrete by pumping

Consolidating the concrete is difficult

When superplasticizers are used, the fresh concrete stays workable for a

short time, 30 min to 60 min, and is followed by rapid loss in workability.Superplasticizers are usually added at the plant to ensure consistency of the

concrete. In critical situations, they can be added at the jobsite, but the con-

crete should be thoroughly mixed following the addition of the admixture. The

setting time varies with the type of agents, the amount used, and the in-

teractions with other admixtures used in the concrete.

3.9.3 Retarders

Some construction conditions require that the time between mixing and

placing or finishing the concrete be increased. In such cases, retarders canbe used to delay the initial set of concrete. Retarders are used for several


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reasons, such as the following:

offsetting the effect of hot weather

allowing for unusual placement or long haul distances providing time for special finishes (e.g.. exposed aggregate)

Retarders can reduce the strength of concrete at early ages (e.g., one to

three days). In addition, some retarders entrain air and improve workability.

Other retarders increase the time required for the initial set but reduce the

time between the initial and final set. The properties of retarders vary with the

materials used in the mix and with job conditions. Thus, the use and effect of

retarders must be evaluated experimentally during the mix design process.

3.9.4 Hydration-Control Admixture

These admixtures have the ability to stop and reactivate the hydration

process of concrete. They consist of two parts: a stabilizer and an activator.

Adding the stabilizer completely stops the hydration of the cementing ma-

terials for up to 72 hours, while adding the activator to the stabilized concrete

reestablishes normal hydration and setting. These admixtures are very useful

in extending the use of ready-mixed concrete when the work at the jobsite is

stopped for various reasons. They are also useful when concrete is being

hauled for a long time.

3.9.5 Accelerators

Accelerators are used to develop early strength of concrete at a faster rate

than that developed in normal concrete. The ultimate strength, however, of

high early strength concrete is about the same as that of normal concrete.

Accelerators are used to

Reduce the amount of time before finishing operations begin.

Reduce curing time. Increase rate of strength gain.

Plug leaks under hydraulic pressure efficiently.

The first three reasons are particularly applicable to concrete work placed

during cold temperatures. The increased strength gained helps to protect the

concrete from freezing and the rapid rate of hydration generates heat that

can reduce the risk of freezing.

Calcium chloride, CaCl2, is the most widely used accelerator (ASTM D98).Both initial and final set times are reduced with calcium chloride. The initial


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set time of 3 hours for a typical concrete can be reduced to 1.5 hours by

adding an amount of calcium chloride equal to 1% of the cement weight; 2%

reduces the initial set time to 1 hour. Typical final set times are 6 hours, 3

hours, and 2 hours for 0%, 1%, and 2% calcium chloride. Concrete withCaCI2, develops higher early strength compared with plain concrete cured at

the same temperature.

The PCA recommends against using calcium chloride under the following

conditions:

concrete is prestressed

concrete contains embedded aluminum such as conduits, especially if

the aluminum is in contact with steel concrete is subjected to alkali-aggregate reaction

concrete is in contact with water or soils containing sulfates

concrete is placed during hot weather

mass applications of concrete

3.9.6 Supplementary Cementitious Admixtures

Several byproducts of other industries have been used in concrete as

supplementary cementitious admixtures since the 1970s. These materials

have been used to improve some properties of concrete and to reduce theproblem of discarding them. Since these materials are cementitious, they can

be used in addition to or as a partial replacement for portland cement. In fact,

two or more of these supplementary cementitious additives have been used

together to enhance concrete properties. These supplementary cementitious

materials include fly ash, ground granulated blast furnace slag, silica fume,

and natural pozzolans.

a. Fly Ash

Fly ash is the most commonly used pozzolan in civil engineeringstructures. Fly ash is a by-product of the coal industry. Combusting

pulverized coal in an electric power plant burns off the carbon and

most volatile materials. However, depending on the source and type of

coal, a significant amount of impurities passes through the combustion

chamber.

The carbon contents of common coals ranges from 70 to 100 percent.

The noncarbon percentages are impurities (e.g., clay, feldspar, quartz,

and shale), which fuse as they pass through the combustion chamber.Exhaust gas carries the fused material, fly ash, out of the combustion


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chamber. The fly ash cools into spheres, which may be solid, hollow

(cenospheres), or hollow and filled with other spheres (plerospheres).

Particle diameters range from 1 m to more than 0.1 mm, with an

average of 0.015 mm to 0.020 mm, and are 70% to 90% smaller than0.045 mm. Fly ash is primarily a silica glass composed of silica (SiO2),

alumina (Al2O3), iron oxide (Fe203), and lime (Ca0).

The spherical shape of fly ash increases the workability of the fresh

concrete. In addition, fly ash extends the hydration process, allowing a

greater strength development and reduced porosity. Studies have

shown that concrete containing more than 20% fly ash by weight of

cement has a much smaller pore size distribution than portland

cement concrete without fly ash. The lower heat of hydration reducesthe early strength of the concrete. The extended reaction permits a

continuous gaining of strength beyond what can be accomplished with

plain portland cement.

b. Ground Granulated Blast Furnace Slag

Ground granulated blast furnace slag (GGBF slag) is made from iron

blast furnace slag. It is a nonmetallic hydraulic cement consisting

basically of silicates and aluminosilicates of calcium, which is

developed in a molten condition simultaneously with iron in a blastfurnace. The molten slag is rapidly chilled by quenching in water to

form a glassy sandlike granulated material. The material is then

ground to less than 45 microns. The specific gravity of GGBF slag is in

the range of 2.85 to 2.95.

The rough and angular-shaped ground slag in the presence of water

and an activator, NaOH or CaOH, supplied by portland cement,

hydrates and sets in a manner similar to portland cement.

Ground slag has been used as a cementitious material in concrete

since the beginning of the 1900s. Ground granulated blast furnace

slag commonly constitutes between 30% and 45% of the cementing

material in the mix. Some slag concretes have a slag component of

70% or more of the cementitious material. ASTM C 989 (AASHTO M

302) classifies slag by its increasing level of reactivity as Grade 80,

100, or 120.


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c. Silica Fume

Silica fume is a byproduct of the production of silicon metal or

ferrosilicon alloys. One of the most beneficial uses for silica fume is as

a mineral admixture in concrete. Because of its chemical and physicalproperties, it is a very reactive pozzolan. Concrete containing silica

fume can have very high strength and can be very durable. Silica fume

is available from suppliers of concrete admixtures and, when

specified, is simply added during concrete production either in wet or

dry forms. Placing, finishing, and curing silica fume concrete require

special attention on the part of the concrete contractor.

Silicon metal and alloys are produced in electric furnaces. The raw

materials are quartz, coal, and woodchips. The smoke that resultsfrom furnace opera

chapter 3 concrete

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