Engineering Surveying,-5th Ed_Construction Materials

7/29/2019 Engineering Surveying,-5th Ed_Construction Materials

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Materials of Construction

(805261)

Prepared by:

Dr. Aslam Al-Omari


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Chapter 5: Fresh Concrete

Introduction

Workability

Factors Affecting Workability

Cohesion and Segregation

Bleeding

Workability Tests


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Chapter 5: Fresh Concrete

Comparison of tests

Workability

Density (Unit Mass or Unit Weight in Air) of

Fresh Concrete


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Introduction

Since the long-term properties of hardened

concrete:strength, volume stability, anddurability

are seriouslyaffectedby its degree ofcompaction, itis vitalthat the consistencyorworkabilityof the

fresh concrete be such that the concrete can be:-properly compacted,

-transported,

-placed, and-finished

sufficientlyeasily without segregation, which wouldbe detrimental (harmful or unfavorable) to such

compaction


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Workability

The strict definition of workability is the amount of

useful internal work necessary to produce fullcompaction (ease of placing and consolidating

freshly mixed concrete)

Wetconcretes are more workable than dry

concretes, butconcretes of the same consistency

(consistency refers to the ease with which theconcrete will flow) may varyin workability


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Workability

Compression is expressed as a density ratio [a ratio

of actual density of a given concrete (partiallycompacted) to the density offullcompaction]

Because the strength of concrete is adverselyand

significantlyaffected by thepresence of voids in the

compacted mass, it is vitalto achieve a maximumdensity. This requires a sufficient workability for

virtually full compaction to be possible using a

reasonable amount of work under the givenconditions

ConcreteCompactedFullyofStrength

ConcreteCompactedPartiallyofStrengthRatioStrength

=


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Workability

It is obvious that thepresence of voids in concrete

reduces the density and greatlyreduces thestrength:

(5% of voids can lower the strength as much as 30%)

Types of voids in hardened concrete:

1) Bubbles of entrapped air

2) Spaces left after excess water has been removed

Voids maybe usefulto preventfreezingandthawing

of water in concrete and theydependon the degree

of compaction


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These factors include:

1. Water Content (most important factor)

2. Aggregate size, type, and grading

3. Aggregate/cement ratio

4. Presence of admixtures

5. Fineness of cement


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The main factor is water contentof the mix since by

simplyaddingwater the interparticle lubrication isincreased

However, to achieve optimum conditions for

minimum voids, or formaximum density with nosegregation, the influence of aggregate type and

gradinghas to be considered.

For example, finerparticles require more water towet theirlargerspecific surface, whilstthe irregular

shape androughertexture of an angular aggregate

demand more water than, say, a roundedaggregate


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The main factor is water contentof the mix since by

simplyaddingwater the interparticle lubrication isincreased

However, to achieve optimum conditions for

minimum voids, or formaximum density with nosegregation, the influence of aggregate type and

gradinghas to be considered.

For example, finerparticles require more water towet theirlargerspecific surface, whilstthe irregular

shape androughertexture of an angular aggregate

demand more water than, say, a roundedaggregate


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Lightweightaggregate tends to lowerthe workability.

In fact, workability is governed by the volumetricproportions of particles of different sizes, so that

when aggregates of varying specific gravity (or

particle density) are used, e.g. semi-lightweightaggregate, the mix proportions should be assessed

on the basis of the absolute volume of each size

fraction

For a constantwater/cement(W/C) ratio, the

workabilityincreases as the aggregate/cementratio

is reducedbecause the amount ofwaterrelative to

the total surface of solids is increased


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A ratherhigh ratio of volumes ofcoarse aggregate to

fine aggregate can result in segregation and in alower workability, so that the mix is harsh and not

easily finished.

Conversely, too manyfines lead to a higher

workabilitybutsuch an over-sanded mix makes less

durable concrete

Air entrainmentreduces the water requirement for a

given workability


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There are other two factors affecting workability;

time and temperature.

Freshly mixed concrete stiffens with time. Some of

the mixing water is absorbedby the aggregate, someis lostby evaporation (particularly if the concrete is

exposed to the sun or wind), and some is removed

by initial chemical reactions.


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The stiffeningof concrete is effectivelymeasuredby

a loss of workability with time, knowing as slumploss, which varies with:

richness of the mix,

type of cement,

temperature of concrete, and

initial workability.

A highertemperature reduces the workability and

increases the slump loss


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In considering the workability of concrete, concrete

should not segregate, and it ought to be cohesive.

The absence of segregation is essential if full

compaction is to be achieved

Segregation can be defined as separation of the

constituents of a heterogeneous mixture so that the

distribution is no longer uniform.


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In the case of concrete, it is the differences in the

size of particles (and sometimes in the specific

gravityof the mix constituents) that are the primary

cause of segregation, but its extentcan be

controlled by:

1) The choice of suitable grading

2) Care in handling


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The actualextent of segregation depends on the

method ofhandlingandplacingof concrete. If theconcrete does not have far to travel and is

transferred directly from the skip or the wheelbarrow

to the final position in the formwork, the danger ofsegregation is small

Segregation forms:

1) The coarserparticles tend to separate out sincethey travel further along a slope or settle more

than the finer particles (occurs mostly in dry mixes)

2) Separation ofgrout(cement + water)(occurs mostly in wet mixes)


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The addition of water wouldimprove the cohesion of

the mix, butwhen the mix becomes too wet thesecondtype of segregation would take place.

Topreventorreduce segregation:1) Good grading

2)Appropriate handling and placing

3) Proper vibration, needed to achieve goodcompaction: neither under nor over vibration

4) Using of air entrainment (reduces segregation)

5) Close specific gravity values for coarse and fineaggregate


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It must be stressed, nevertheless, that concrete

should always be placeddirectin the position inwhich it is to remain and mustnotbe allowedto flow

or be worked along the form

This prohibition includes the use of a vibrator to

spread a heap of concrete over a larger area.

Vibrationprovides a most valuable means ofcompacting concrete, but, because a large amount

of work is being done on the concrete, the dangerof

segregation (in placing, as distinct from handling) isincreasedwith improper use of a vibrator.


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This is particularly so when vibration is allowed to

continue too long: with many mixes, separation ofcoarse aggregate toward the bottom of the form and

of the cement paste toward the top may result.

Segregation is difficultto measure quantitativelybut

is easily detected and can be evaluated by:

a) Measuring mix cohesion by the flow table test, or

b) By vibrating a concrete cube or cylinder for

about 10 minutes and then strip it to observe the

distribution of coarse aggregate

(any segregation will be easily seen)

Engineering Surveying,-5th Ed_Construction Materials

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