workability

CONCRETE TECHNOLOGY

UNIT 3

WORKABILITY OF

FRESH CONCRETE

M. Irfaan Mungroo

Civil Engineering Department

JSS ATE, Mauritius

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Unit 3 Workability of Concrete

VTU SYLLABUS

Introduction

Factors Affecting Workability

Measurement of Workability

Slump Test

Flow Test

Compaction Factor Test

Vee-bee Consistometer Test

Segregation and Bleeding

Process of Manufactures of Concrete

i. Batching

ii. Mixing

iii. Transporting

iv. Placing

v. Compacting

vi. Curing

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WORKABILITY

Introduction

The behaviour of fresh concrete from mixing up to compaction on site depends mainly on the

property called ‘the workability of concrete’.

Since the long-term properties of hardened concrete (i.e. its strength/ durability) are seriously

affected by its degree of compaction, it is vital that the workability of the fresh concrete to be such

that the concrete can be properly compacted and also that it can be transported, placed and

finished sufficiently easily without segregation.

Definition

A workable concrete is one which exhibits very little internal friction between particle and particle or

which overcomes the frictional resistance offered by the formwork surface or the reinforcement.

Workability represents the amount of work which needs to be done to achieve maximum compaction

of the fresh concrete in a given mould. A workable mix should not segregate (form layers / settles).

PARTIAL PROPERTIES (CHARACTERISTICS) OF WORKABILITY

There are 4 partial properties of workability:

i. Mixability

ii. Transportability

iii. Mouldability

iv. Compactibility

i. Mixability

It is the ability of the mix to produce a homogeneous green / fresh concrete from the constituent

materials of the batch under the action of the mixing forces. A homogeneous mixture is a type of

mixture in which the composition is uniform and every part of the mixture has the same properties.

A less mixable concrete mix requires more time of mixing to produce a homogeneous and uniform

mix.

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ii. Transportability

Transportability is the capacity of the concrete mix to keep the homogeneous concrete mix from

segregating during a limited time period of transportation of concrete.

iii. Mouldability

It is the ability of the concrete mix to fill completely the forms or moulds without losing continuity

or homogeneity under the available techniques of placing the concrete.

iv. Compactibility

It is the ability of the concrete mix to be compacted into a dense, compact concrete with minimum

voids under the existing means of compaction at the site. The best mix from the point of view of

compactibility should close the voids to an extent of 99% of the original voids present when the

concrete was placed in the moulds.

FACTORS AFFECTING WORKABILITY

The factors that help concrete to have a more lubricating effect to reduce internal friction for

helping easy compaction are given below:

i. Water Content

ii. Size of Aggregates

iii. Surface Texture of Aggregate

iv. Use of Admixtures

v. Mix Proportions

vi. Shape of Aggregates

vii. Grading of Aggregates

viii. Time

ix. Temperature

i. Water Content

The higher the water content per cubic meter of concrete, the concrete will be more fluid and the

inter-particle lubrication will be increased. This is one of the easiest methods to increase

workability of concrete.

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On sites, supervisors who are not well-versed with the practice of making good cement often resort

to adding more water to increase workability. It should be notes that increasing water content is

the last recourse to be taken to improve workability in uncontrolled concrete. For controlled

concrete, one cannot arbitrarily increase the water content. In case all other steps to improve

workability fail, only as last recourse can addition of more water be considered.

If more water than necessary is added, the water cement ratio will increase and as a result, the

strength of the concrete will decrease.

ii. Size of Aggregates

The bigger the size of aggregate, the less the surface are and hence, less amount of water is needed

for wetting the surface and less paste is needed to lubricate the surface to reduce internal friction.

For a given quantity of water and paste, bigger size of aggregates will give higher workability.

iii. Surface Texture of Aggregates

The total surface area of rough textured aggregate is more than the surface area of smooth rounded

aggregate of the same volume. Hence, smooth or glassy textured aggregate will give better

workability than rough textured aggregate because of the following reasons:

a) Smaller surface area of smooth aggregates means less water is needed to wet the surface

and less paste is needed to lubricate the surface to reduce internal friction.

b) Smooth aggregates provide little inter-particle frictional resistance between the aggregates

and hence, contribute to a higher workability.

iv. Use of Admixtures

Of all factors, this is the most important one .admixtures are substances that are added to concrete

during the mixing phase to achieve the desired effect.

To increase the workability of concrete so as to ease placing in inaccessible locations, an admixture

called water- reducer (or plasticizer) is added to the mix.

The principal component of water- reducing admixtures is surface-active agents which are

absorbed on the cement particles, giving them a negative charge. This leads to repulsion of

particles. Air bubbles are repelled as they cannot attach to the cement particles. The negative

charge attracts water molecules to the cement and hence there is greater mobility of particles and

lubrification.

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v. Mix Proportions

Aggregate / cement ration s an important factor influencing workability. When the aggregate /

cement ratio is high, the concrete is said to lean. In a lean concrete, less amount of paste is available

to provide lubrification and hence, the mobility of aggregate is restrained. On the other hand, a rich

concrete, i.e. one with a lower aggregate to cement ratio, more paste is available to make the mix

cohesive and hence, to give better workability.

vi. Shapes of Aggregates

Angular, elongated or flaky aggregates make the concrete very harsh when compared to rounded

aggregates or cubical aggregates. Rounded aggregates contribute to better workability from the fact

that for a given volume, it will have less surface area and less voids than the angular or flaky

aggregate. Moreover, being rounded in shape, the frictional resistance is greatly reduced.

This explains why river sand and gravel provide greater workability to concrete than crushed sand

and aggregate.

vii. Grading of Aggregates

This is one of the factors which will have maximum influence on workability. A well graded

aggregate is one which has least amount of voids in a given volume. Other factors being kept

constant, when the total voids are less, excess paste is available to give better lubricating effect.

With this excess paste, the mixture becomes cohesive and fatty, which prevents segregation of

particles. Aggregate particles will slide past each other with the least amount of compacting efforts.

The better the grading, the less is the void content and the higher is the workability.

viii. Time

Freshly mixed concrete stiffens with time. This should not be confused with setting of cement.

Stiffening of freshly mixed concrete may occur when:

a) Some mixing water is absorbed by the aggregates.

b) Some water is lost by evaporation ( especially when the concrete is exposed to the sun or

wind)

c) Or some water is removed by initial chemical reactions.

The stiffness of concrete is effectively measured by a loss of workability with time, known as Slump

Loss. Since we are interested with the workability of concrete at the time of placing (i.e. some time

after mixing), it is preferable to delay the appropriate test until, say 15 minutes after mixing.

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ix. Temperature

A higher temperature reduced the workability and increases the slump loss. In practice, when

working condition is hot, it is better to make actual site tests in order to determine the workability

of the mix.

WORKING TESTS

There is no acceptable test which will measure directly the workability of fresh concrete. The

following methods give a measure of workability which is applicable only with reference to that

particular method.

There are 4 common tests of workability:

i. Slump Test

ii. Flow Test

iii. Compaction Factor Test

iv. Vee-Bee Consistometer Test

THE SLUMP TEST

Objective:

To determine the consistency of concrete mix of given proportions

Background:

Unsupported fresh concrete flows to the sides and a sinking in height takes place. This vertical

settlement is known as slump.

In this test, fresh concrete is filled into a mould of specified shape and dimensions and the

settlement or slump is measured when the mould is removed.

Note:

i. Slump increases as water-content is increased.

ii. The slump is a measure of the workability of the concrete.

iii. By this test, we can determine the water content to give specified slump value.

iv. In this test, water content is varied and in each case, the slump value is measured till we

arrive at the water content giving the required slump value.

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Apparatus:

Iron pan to mix concrete, slump cone, spatula, trowels, tamping rod and graduated cylinder

Test procedures:

Four mixes are to be prepared with water – cement ratio (by mass) of 0.50, 0.60, 0.70 and 0.80

respectively and for each mix, take 10Kg of coarse aggregates, 5Kg of sand and 2.5Kg of cement.

With each mix, proceed as follows:

i. Mix the dry constituents thoroughly to get a uniform colour and then, add water.

ii. Place the mixed concrete in the cleaned slump cone mould in 4 layers, each approximately

14 of the height of the mould.

iii. Tamper each layer 25 ties with a tamping rod distributing the strokes in a uniform manner

over the cross-section of the mould. For the second and subsequent layers, the tamping rod

should penetrate into the underlying layer.

iv. Strike off the top with a trowel or tampering rod so that the mould is exactly filled.

v. During the entire operation, the mould must be held against its base. This can be done by

using handles or foot-rests brazed to the mould.

vi. Remove the cone immediately, raising it slowly and carefully in the vertical direction.

vii. As soon as the concrete settlement stops, measure the decrease in the height of the concrete

in mm.

Note:

i. Slump test is adopted in laboratories or during the progress of work on site to determine

the consistency (firmness) of concrete where maximum size of aggregate does not exceed

40mm.

ii. In order to reduce the influence of surface friction on the slump, the inside of the mould and

its base should be moistened at the beginning of every test.

iii. Prior to lifting of the mould, the area immediately around the base of the cone should be

cleaned from concrete which may have been dropped accidentally.

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TYPICAL MOULD FOR SLUMP TEST

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SLUMP: TRUE, SHEAR AND COLLAPSE

The results are tabulated as shown below:

Water-Cement Ratio Slump / mm

Standard values:

Name of Works Slump (mm) Water-Cement Ratio

1 Concrete for roads and mass concrete 25-50 0.70

2 Concrete for R.C.C beams and slabs 50-100 0.55

3 Columns and retaining walls 75-125 0.45

4 Mass concrete in foundation 25-50 0.75

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THE FLOW TEST

This is a lab test which gives an indication of the quality of concrete with respect to consistency,

cohesiveness and proneness to segregation.

In this test, a standard mass of concrete is subjected to jolting. The spread or the flow of the

concrete is measured and this flow is related to workability.

The Apparatus

Procedures:

i. The table top is cleaned of all gritty material and is wetted.

ii. The mould is kept on the centre of the table, firmly held and is filled in two layers.

iii. Each layer is rodded 25 times with a tamping rod, 1.6cm in diameter and 61cm long,

rounded at the lower tamping end.

iv. The top layer is rodded evenly and the excess of concrete which has overflowed the mould

is removed.

v. The mould is lifted vertically and the concrete stands on its own without support.

vi. The table is then raised and dropped 12.5mm 15 times in about 15 seconds. The diameter of

the spread concrete is measured in about 6 directions to the nearest 5mm and the average

spread is noted.

The flow of concrete is the percentage increase in the average diameter of the spread concrete over

the base diameter of the mould. The value could range anything from 0 to 150%.

Flow = 𝑺𝒑𝒓𝒆𝒂𝒅 𝒅𝒊𝒂𝒎𝒆𝒕𝒆𝒓 𝒊𝒏 𝒄𝒎 − 𝟐𝟓

𝟐𝟓 𝒙 𝟏𝟎𝟎%

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THE FLOW TABLE TEST

This is a modification of the standard flow test.

The apparatus consists mainly of a wooden board covered by a steel plate with a total mass of 16Kg.

The board is hinged along one side to a base board. Both boards are 700mm square. The upper

board can be lifted up to a stop so that the free edge rises 40mm. There are appropriate markings to

indicate where the concrete is to be deposited on the upper board.

Procedures:

i. The table top is moistened.

ii. Concrete is then placed on the table top using a mould 200mm high with a bottom diameter

of 200mm and a top diameter of 130mm.

iii. The concrete is tampered by a wooden tamper in a prescribed manner.

iv. Before lifting the mould, excess concrete is removed and the surrounding table top is

cleaned.

v. After an interval of 30 seconds, the mould is slowly removed.

vi. The table top is lifted and allowed to drop, avoiding a significant force against the stop,

15times, each cycle taking ≅ 4s.

Results:

Consequently, the concrete spreads and the maximum spread is measured as explained in the

standard flow test and the flow is calculated similarly.

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COMPACTION FACTOR TEST

Objective:

To determine the workability of a concrete mix of given proportions by the compaction factor test.

Background:

The compaction factor test is used to determine the workability of concrete where nominal size of

aggregate does not exceed 40mm and is primarily used in laboratory. It is based on the definition

that workability is that property of the concrete which determines the amount of work needed to

produce full compaction.

Probability the best test available yet, the compaction factor test uses the inverse approach, i.e. the

degree of compaction achieved by a standard amount of work is determined. Hence, the test

consists mainly of applying a standard amount of work to a standard quantity of concrete and

measuring the resulting compaction.

The compaction factor test is more sensitive and precise than the slump test and is particularly

useful for concrete mixes of low workability. It is able to determine small variations in workability

over a wide range.

Apparatus:

Compaction factor apparatus, trowels, graduated cylinder, balance, tamping rod and iron buckets.

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Essential dimension of compacting factor Apparatus with aggregate not exceeding 40mm.

i. Distance between bottom of upper hopper and top of lower hopper = 20.3cm

ii. Distance between bottom of lower hopper and top of cylinder = 20.3cm

Procedures:

i. Keep the compaction factor apparatus on a level ground and apply grease on the inner

surface of the hoppers and cylinder. Then fasten the flap doors.

ii. Weight the empty cylinder accurately and note down the mass as W1.

iii. Fix the cylinder on the base with fly nuts and bolts in such a way that the cylinder is exactly

centered below the two hoppers.

iv. Four mixes are prepared with water-cement ration (by mass) 0.5, 0.6, 0.7 and 0.8

respectively. For each mix, use 9Kg of aggregate, 4.5Kg of sand and 2.25Kg of cement. With

each mix, proceed as follows:

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a) Mix the sand and cement until a uniform colour is obtained. Now, mix the coarse

aggregate until it is uniformly distributed throughout the batch.

b) Add the required amount of water to the above mixture and mix it thoroughly until

the concrete is homogeneous.

v. Fill the concrete in the upper hopper gently and carefully so as no work is done on the

concrete to produce compaction. Do not tamper or compact he concrete.

vi. After 2 mins, release the trap door so that the concrete may fall into the lower hopper

bringing the concrete in partial compaction.

vii. As soon as the concrete has come to ret in the second hopper, its trap door is opened,

allowing the concrete to fall into the cylinder bringing the concrete in even more partial

compaction.

viii. Remove the excess concrete above the top of the cylinder by cutting across the top of the

cylinder. DO NOT REMOVE CONCRETE FROM THE INSIDE OF THE CYLINDER.

ix. Clean the cylinder from all external sides properly. Note down the weight of the partially

compacted concrete in the cylinder, W2.

x. Empty the cylinder and refill it with the same sample of concrete in approximately 50mm

layers.

xi. Each layer is to be heavily rammed or vibrated to obtain full compaction by expelling all the

air.

xii. Scrape off the top of the cylinder and weigh the cylinder filled with fully compacted

concrete. Let this mass be W3.

Compacting Factor = 𝐖𝐞𝐢𝐠𝐡𝐭 𝐨𝐟 𝐏𝐚𝐫𝐭𝐢𝐚𝐥𝐥𝐲 𝐂𝐨𝐦𝐩𝐚𝐜𝐭𝐞𝐝 𝐂𝐨𝐧𝐜𝐫𝐞𝐭𝐞

𝐖𝐞𝐢𝐠𝐡𝐭 𝐨𝐟 𝐅𝐮𝐥𝐥𝐲 𝐂𝐨𝐦𝐩𝐚𝐜𝐭𝐞𝐝 𝐂𝐨𝐧𝐜𝐫𝐞𝐭𝐞

C.F = 𝐖𝟐− 𝐖𝟏

𝑾𝟑− 𝑾𝟏

Note:

i. The test is more sensitive at the low workability end of the scale than at high workability.

ii. Very dry mix tends to stick in one or both hoppers and hence, the concrete has to be eased

gently by poking with a steel rod.

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Standard Values:

Degree of

Workability

Compacting Factor

Use for concrete suitable in: Small Apparatus

[Agg < 20mm]

Large Apparatus

[Agg < 40mm]

Very Low 0.78 0.80 Roads vibrated by power-operated

machines

Low 0.85 0.87 Road vibrated by hand-operated

machines

Medium 0.92 0.932

Manually compacted flat slabs using

crushed aggregate. Normal

reinforced concrete manually

compacted

High 0.95 0.96

For sections with congested

reinforcement where it is not

suitable to apply vibration

THE VEE-BEE CONSISTOMETER

Objective:

To determine the workability of freshly mixed concrete by use of Vee-Bee consistometer

Scope:

This test gives an indication of the mobility and to some extent, the compactibility of freshly mixed

concrete.

The test measures the relative effort required to change a mass of concrete from one shape to

anther (i.e. from conical to cylindrical) by means of vibration. The amount of effort (called the

remoulding effort) is taken as the time in seconds required to complete the change.

The time required for complete remoulding in seconds is considered as a measure of workability

and is expressed as the number of Vee-Bee seconds.

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Apparatus:

Cylindrical container, Vee-Bee apparatus (consisting of vibrating table, slump cone), standard iron

rod, weight balance and trowels.

Procedure:

i. Place the slump cone in the cylindrical container of the consistometer.

ii. Fill the cone in 4 layers, tampering each layer 25 times with a rounded end of the tampering

tod.

iii. After the top layer has been rodded, struck off level the concrete with a trowel so that the

cone is exactly filled.

iv. Move the glass disc attached to the swivel arm and place it just on the top of the slump cone

in the cylindrical container.

v. Note the initial reading on the graduated rod. This gives the initial height of concrete, x1.

vi. Remove the glass disc and take off the cone from the concrete immediately after by raising

it slowly and vertically.

vii. Allow the concrete to settle (just like in the slump test) and then, lower the disc on the

concrete top.

viii. Note down the slump reading on the graduated rod, x2.

ix. Calculate the slump difference, x2 – x1

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x. Switch on the electrical vibrations and start the stop watch simultaneously. Allow the

concrete to remould in the cylindrical container.

xi. The vibrations are continued until the concrete is completely remoulded, i.e. the surface

becomes horizontal.

xii. Record the time required for complete remoulding. This gives a measure of the workability

and is expressed as no. Of Vee-Bee seconds.

Standard Values:

Workability Vee-Bee time / s

Extremely Dry 32-18

Very Stiff 18-10

Stiff 10-5

Stiff Plastic 5-3

Plastic 3-0

Flowing .......

Note:

i. This method is suitable for dry mix.

ii. For concrete of slump is excess of 50mm, the remoulding is so quick that the time cannot be

measured.

SEGREGATION

Segregation is defined as the separation of the constituents of a homogeneous mixture of concrete

so that their distribution is no longer uniform.

Causes:

1) It is caused by the differences in sizes and weights of the constituent particles.

2) In relatively lean and dry mixes (lower cement content), segregation can be caused by the

coarser particles separating out as they settle more or travel farther along the slope than finer

particles.

3) In very wet mixes, there is separation of the cement-water paste.

4) Segregation can also be caused by poor handling, such as dropping wet concrete from a

considerable height, passing long chutes along a slope or discharging concrete carelessly

against some firm obstruction.

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5) It can also be caused by the vibration of concrete. Although vibration provided a useful way of

compaction, over-vibration leads to segregation of the coarse aggregates from the mix

especially when the vibration is allowed to continue for a long time. Aggregates settle at the

bottom and the cement-water paste moves to the top in the form of a scum.

Measurement:

Segregation is difficult to measure but its occurrence is easily detected. The flow test can indicate

the susceptibility of a mix that is likely to be segregated.

In dry mixes, heavier particles move away and occupy the edges of the flow table.

In wet mixes, the cement paste tends to move away from the middle and the centre of the flow table

is left only with coarser particles.

BLEEDING

Bleeding is also known as water gain. It is the accumulation f water at the surface. This is caused by

the inability of the solid constituent of the mix to hold all of the mixing water when they settle

downwards due to gravity and the water moves up.

Bleeding is expressed quantitatively as the total settlement per unit height of concrete. It can also

be expressed as the percentage of mixing water. In extreme cases, this can be nearly 20%.

Bleeding is a function of:

i. Air Velocity

ii. Temperature

iii. Humidity

If the rate of bleeding is roughly equal to the rate of evaporation, then bleeding will not cause any

problem. If bleeding is less than the rate of evaporation, then the surface becomes dry and as a

result, cracks appear on it.

The evaporation of water from the surface of the concrete depends on:

i. Relative Humidity

ii. Ambient Temperature

iii. Velocity of Wind

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Effects of Ingredients on Bleeding:

Every ingredients and their amount have a major effect on the bleeding of concrete.

1. Water content or Water-Cement Ratio

Any increase in the amount of water or water to cement ratio results in more water available for

bleeding. An 1 5 increase in water content of a normal concrete can increase the rate of bleeding by

more than 2 1 2 times.

2. Cement

The type, content and fineness of cement can affect bleeding. As the fineness of cement increases,

bleeding decreases. When cement content increases, water-cement ratio decreases and therefore,

bleeding decreases.

3. Supplementary Cementing Materials

Fly-ash, slag, silica fume can all reduce bleeding by their inherent properties and by increasing the

amount of cemeatitious materials in a mixture.

4. Aggregates

Aggregates containing a high amount of silt, clay or other material passing the 75μm sieve can

reduce bleeding as they hold the water molecules to themselves.

5. Chemical Admixtures

Air-entraining agents can be used as the air-bubbles formed appear to keep the solid particles in

suspension (hence, avoiding settlement).

Water reducers also reduce the amount of bleeding as they release trapped water molecules and

dispersing them into the cement.

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PROCESS OF MANUFACTURES OF CONCRETE

Great care has to be taken at every stage of manufacture of concrete. It is interesting to know that

the ingredients of good concrete and bad concrete are the same. The difference is whether care has

been taken and rules have been observed.

The various stages of manufacture of concrete are:

i. Batching

ii. Mixing

iii. Transporting

iv. Placing

v. Compacting

vi. Curing

1. Batching

The measurement of material for making concrete is known as batching. There are two types of

batching:

i. Volume Batching

ii. Weigh Batching

Volume Batching

It is the measurement of materials by volume. This is not a good method for proportioning the

material because it is difficult to measure granular or irregular materials by volume.

For example, the volume of moist sand in a loose condition weighs much less than the same volume

of dry compacted sand. Volume batching can only be used for unimportant concrete jobs.

Weigh Batching

Measurement of materials should be done by weigh batching only. The use of weigh batching

system facilitates accuracy, flexibility and simplicity. Different types of weigh batchers are available

depending on the different nature of the job.

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Types of Weigh Batchers:

i. In smaller works, the weighing arrangement consists of two weighing bucket, each connected

through a system of levers to spring-loaded dials which indicate the load. The weighing buckets

are mounted on a central spindle about which they can rotate. Thus, one can be loaded while

the other is being discharged into the mixer. A simple spring balance can also be used for small

jobs.

ii. On large work sites, the weigh bucket type of weighing equipments is used. Materials are fed

from a large overhead storage hopper and it discharges by gravity, straight into the mixer. The

weighing is done through a lever-arm system and two interlinked beams and jockey weights.

iii. Automatic batching plants are available in small or large capacity. In this type, the operator

press one or two buttons to put into motion the weighing of all the different materials. The flow

of each material is cut off when the correct weight is reached.

Note:

In small jobs, cement is not weighed and it is added in bags, with one bag assumed to be 50Kg. In

reality, though the weight of cement bag is 50Kg, cement is lost due to transportation or handling.

Sometimes, the loss of weight becomes more than 5Kg. This is one major source of error.

Measurement of Water:

When weigh batching is done, the measurement of water must be done accurately. Spillage of water

must be prevented at all cost.

It is of good practice to have a tank of water fitted to the mixer. These tanks are filled after every

batch. The filling of water is designed to have a control on the amount of water. Sometimes, water-

meters are fitted in the main water supply to the mixer to know the exact amount used.

In modern technology plants, sophisticated automatic microprocessor, controlling weigh batchers,

not only measures the constituent materials, but also the moisture content of aggregates.

The moisture content is automatically measured by sensor probes and corrective action is taken to

deduct that much quantity of water from the total quantity of water.

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2. Mixing

Thorough mixing is essential for the production of uniform concrete. The mixing should ensure that

the mass becomes homogeneous, uniform in colour and consistent.

There are 2 methods of mixing:

i. Hand Mixing

ii. Machine Mixing

Hand Mixing

Hand mixing is practised for small scale unimportant concrete works. As the mixing cannot be

thorough and efficient, it is desirable to add 10% more cement to cater for the inferior concrete

produced.

Method:

i. Hand mixing should be done over an impervious concrete or brick floor.

ii. Spread out the measured amount of coarse and fine aggregate in alternative layers.

iii. Pour the cement onto it and mix them dry by shovel until uniformity of colour is achieved.

iv. The dry mixture is then spread out in thickness of about 20cm and water is sprinkled over the

mixture and simultaneously turned over. It is vital that water is not poured but only

sprinkled.

v. Mixing continues until a uniform, homogeneous concrete is obtained.

Machine Mixing

This is carried out by machine for reinforced concrete work and for medium or large scale mass

concrete work. Machine mixing is not only efficient but also economical when the quantity of

concrete to be produced is large.

Many types of mixers exist. They can be classified as:

i. Batch Mixers

ii. Continuous Mixers

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i. Batch Mixers

They produce concrete, batch by batch with time interval. They are used in normal concrete works.

They can be of pan type or drum type.

The drum type can be further classified as tilting, non-tilting, reversing or forced action type. Pam

mixers with revolving star of blades are more efficient especially to prepare stiff and lean concrete.

Concerning the drum mixer, the shape of the drum, the angle and size of the blades, the angle at

which the drum is held affect the efficiency of mixing.

Types of Batch Mixers

A tilting drum mixer is one whose drum in which mixing takes place is tilted for discharging. The

drum is conical or bowl-shaped and the discharge is raped and unsegregated.

A non-tilting drum mixer is one in which the axis of the mixer is always horizontal and discharge

takes place by inserting a chute into the drum.

A reversing drum mixer is similar to the non-tilting drum mixer except that discharge of concrete

takes place by reversing the direction or rotation of the drum.

A pan-type mixer is a forced-action mixer which relies on the free fall of concrete inside the drum.

It consists of a circular pan rotating about its axis or two stars of paddles rotating about a vertical

axis.

Method of mixing concrete in drum mixer:

i. About 25% of the total mixing water is introduced into the mixing drum to wet the drum and to

prevent any cement sticking to the blades or at the bottom of the drum.

ii. Add 1 2 the quantity of coarse aggregate in the mixer.

iii. Add 1 2 the amount of fine aggregate over the coarse aggregate.

iv. On that, add the full amount of cement (e.g. 1 bag).

v. Now, add the remaining portion of the coarse and fine aggregate in sequence. This prevents

spilling of cement and also the blowing away of cement in windy weather.

vi. Immediately after, add the remaining 75% of water into the drum. The time is counted from the

moment all the materials; especially the complete amount of water is added into the drum.

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Note:

When admixtures like plasticizer or super plasticizer is used, the usual procedure s followed except

that 1L of water is held back.

Calculated amount of plasticizer is mixed with that 1L of water which is then added to the mixer

drum after about 1 min of mixing.

ii. Continuous Mixers

They produce concrete continuously without stoppage till such time the plant is working. The

materials are fed continuously by screw feeders and the materials are continuously mixed and

discharged.

Mixing Time:

Concrete mixers are generally designed to run at a speed of 15 to 20 revolutions per minutes.

If the mixing time is reduced, the quality of concrete becomes poor. On the other hand, if the

concrete is mixed for longer time, it is uneconomical in respect of fuel consumption and the amount

of concrete produced. Hence, it is vital to mix concrete for such a duration which will be of optimum

benefits.

From experiments, it is seen that the compressive strength of concrete increases with increase time

of mixing but for mixing time beyond two minutes, the improvement in compressive strength is not

very significant.

Generally, mixing time is related to the capacity of the mixer. The mixing time varies between 1 to 2

mins. Bigger the capacity of the drum more is the mixing time. However, modern mixer may take

less time.

Long-time mixing of concrete will generally result in increase of compressive strength of concrete

within limits.

Due to mixing over long periods, the effective water/ cement ratios gets reduced, as water is

absorbed by aggregates or lost through evaporation.

It is also possible that the increase in strength may be due to an improvement in workability.

Coarse aggregates become rounded from the abrasion and attrition that takes place between

themselves, resulting in the formation of excess fine particles.

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3. Transportation of Concrete

Concrete can be transported by various methods. The precaution that has to be taken while

transporting concrete is that the homogeneity obtained at the time of mixing should be maintained

while being transported to the final place of deposition.

The methods used to transport concrete are:

i. Mortar Pan

ii. Wheel Barrow

iii. Crane, Bucket and Rope Way

iv. Truck Mixer and Dumpers

v. Belt Conveyors

vi. Chute

vii. Skip and Hoist

viii. Transit Mixer

Mortar Pan

One of the most common methods used.

However, it is labour intensive.

In this case, concrete is carried in small quantities.

Can be used at ground level, below or above easily.

Wheel Barrow

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Used to transport cement at ground level.

This method is mainly used to transport concrete for comparatively longer distance.

However, if the concrete is transported over a long distance, ON ROUGH GROUND, it is likely

that the concrete will segregate.

To prevent this, the wheel barrow is equipped with a wheel to reduce vibration.

A wooden plank road is also provided to reduce vibration and hence, segregation.

Crane, Bucket and Rope Way

Used to carry concrete above ground level.

Cranes are fast and versatile to move concrete horizontally as well as vertically along the

high rise construction projects allowing the placement of concrete at the exact point.

Cranes carry skips or buckets containing concrete. Skips have discharge doors at the bottom

whereas buckets have to be tilted for emptying.

Rope way and bucket of various sizes are used to transport concrete to places where simple

method of transport is not feasible (valley, river, dam).

Since the size of the bucket is considerably large and concrete is not that much exposed to

sun and wind, there would not be much change in the state of the concrete or workability.

For discharging, the bucket may be tilted or the concrete is discharged with the help of a

hinged bottom.

It should be practised that concrete is discharged from the smallest height possible and

should not be made to fall freely from great height.

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Truck Mixer and Dumpers

For large concrete works, particularly for concrete to be placed at ground level, trucks and

dumpers are used.

The advantage is that they can travel to any part of the work.

Dumpers are usually 2m3 – 3m3 whereas that of a truck mixer can be 4m3 or more (standard

being 6m3).

Before loading the concrete, the inside of the body should be wetted with water.

In the case of dumpers, tarpolins or other covers can be used to cover the wet concrete

during transit to prevent evaporation.

Belt Conveyors

Belt conveyors can be used to transport concrete but is very limited in use.

The main objection is the tendency of the concrete to segregate on steep inclines, at transfer

points or change of direction and at the points where the belt passes over the rollers.

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Another disadvantage is that the concrete is exposed over long stretches causing drying and

stiffening, particularly in hot, dry and windy weather.

Segregation also takes place due to the vibration of rubber belt. It is necessary that the

concrete should be remixed at the end of delivery before placing on the final position.

Modern belt conveyors can have adjustable reach, travelling diverter and variable speed

both forward and reverse.

Chutes

Chutes are normally used to transport concrete from ground level to a lower level.

The sections of the chute should be made of or lined with metal and all the runs shall have

approximately the same slope.

The slope must not be less than 1: 2.5.

The layout must be made in such a way that the concrete will slide evenly without any

separation or segregation.

However, this is not a good method of transporting concrete.

Skip and Hoist

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One of the most widely used methods of transporting concrete vertically up.

At the ground level, the mixer directly feed the skip and the skip travels over the rails upto

the level where concrete is required.

At that point, the concrete is discharged manually or automatically.

The quality of concrete at the time of placing will depend on the extent of travel and rolling

over the rails.

If the concrete has travelled considerable height, the concrete should be turned over before

placing.

Transit Mixers

Most popular method used. They are truck mounted having a capacity of 4 to 7m3.

They are of 2 types:

a) In one, mixed concrete is transported to the site by keeping it agitated all along the

way at a speed between 2-4 revolutions per min.

b) Here, the concrete is batched at the central batching plant and mixing is done in

the truck mixer either in transit or prior to discharging of the concrete on site.

They allow longer haul and are less vulnerable in case of delay. Water need not be added till

such a time that the mixing will start.

There is also the twin fin process mixer, which is more efficient. They have both outer

spirals and two opposed inner spirals.

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The outer spirals convey the mix materials towards the bottom of the drum while the

opposed inner spirals push the mix towards the feed opening.

Sometime, there is also a small concrete pump mounted on the truck carrying the transit

mixer. The pump is used to discharge concrete.

4. Placing of Concrete

It is not enough that a concrete mixed is correctly designed, batched, mixed and transported. It is of

utmost importance that the concrete is placed in a systematic manner to yield optimum results.

The precautions to be taken and methods adopted in various situations are listed below:

a. Placing the concrete within earth mould (E.g. foundation for wall or column)

Before placing concrete in the foundation, all loose earth must be removed from the bed.

Any root tree passing through the foundation must be cut, charred or tarred to prevent its

further growth and piercing the concrete at a later date.

The surface of the earth, if dry, must be made damp so that the earth does not absorb the water

from concrete.

On the other hand, if the foundation bed is too wet and rain-soaked, the water must be removed

completely to expose firm bed before placing the concrete.

If there is any seepage of water in to the foundation, the flow of water must be diverted before

concrete is placed.

b. Placing the concrete with large earth mould or timber plank formwork (e.g. roads

slab and airfield slab)

For the construction of road slabs, airfield slabs and ground floor slabs in buildings, concrete is

placed in bays.

The ground surface must be free from loose earth, pool of water and other organic matters like

grass.

The earth must be properly compacted and made sufficiently damp to prevent absorption of

water from the concrete. If this is not done, the bottom portion of the concrete will become

weak.

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Sometimes, to prevent the earth from absorbing moisture from concrete, especially in the case

of thin road slabs, polyethylene film is used between concrete and ground.

It must be ensured that concrete must be placed in the required thickness.

The practice of placing concrete in a heap at one place and then dragging it should be avoided.

When concrete is laid in great thickness, (e.g. concrete raft), the concrete is placed in layers. In

reinforced concrete, concrete is placed in layers of 30cm.

Before placing the next layer of concrete, the surface of the previous layer must be cleaned with

water and scrubbed with wire brush.

When concrete is laid in layers, it is better to leave the top of the previous layer rough so that

the succeeding layer can have a good bond with the previous layers.

c. Placing the concrete within formworks

Formwork shall be designed and constricted so as to remain sufficiently rigid during placing

and compaction of concrete.

The joints are plugged to prevent the loss of cement paste from concrete.

When placing concrete within the formworks, certain rules must be followed:

i. Reinforcements must be correctly tied and placed.

ii. Appropriate cover must be present.

iii. The joints between planks must be effectively plugged to prevent loss of the cement

paste when concrete is vibrated.

iv. Reinforcement must be clean and free from oil.

v. When reinforcement in placed in a congested formwork, the concrete must be

placed carefully, little by little so that it does not block the entry of subsequent

concrete.

d. Placing the concrete from a greater height

When concrete is poured from a height, it is likely to segregate or block the space to prevent

further entry of concrete.

To avoid this, concrete is directed by drop chute within the reinforcement and ties.

Sometimes, when the formwork is too narrow or reinforcement is too congested, it is not

possible to use a chute.

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5. Compaction of Concrete

Compaction of concrete is the process adopted to expel the entrapped air from the concrete.

In the process of mixing, transporting ad placing of concrete, air is likely to get entrapped in the

concrete. The lower the workability, higher is the amount of air trapped. Hence, stiff concrete mix

would need higher compaction efforts than high workable one. If the air is not fully removed, the

concrete loses strength considerably.

The following methods are adopted for compacting concrete:

1) Hand Compaction

i. Rodding

ii. Ramming

iii. Tamping

2) Compaction by Vibration

i. Internal Vibrator (Needle Vibrator)

ii. Formwork Vibrator (External Vibrator)

iii. Table Vibrator

iv. Platform Vibration

v. Surface Vibrator (Screed Vibrator)

vi. Vibratory Roller

3) Compaction by Presence and Jolting

4) Compaction by Spinning

1. Hand Compaction

Adopted for unimportant concrete work.

May also be used in cases when a large quantity of reinforcement is used and which cannot be

compacted by mechanical means.

When hand compaction is adopted, the consistency (relative fluidity) of concrete is maintained

at a higher level.

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1.1 Rodding

Rodding is poking with a 2m long, 16mm ϕ rod to pack the concrete between reinforcement

and sharp corners and edges.

Rodding is done continuously over the complete area to effectively pack the concrete and

drive away entrapped air.

Sometimes, instead of iron rod, bamboos or can is used.

1.2 Ramming

Ramming is done with a long ramming pole.

Ramming should be done with care.

Light ramming can be allowed in unreinforced foundation concrete but should be prevented

reinforced concrete work especially those placed in formwork supported on struts.

If this is done, the position of the reinforcement may be disturbed or the formwork may fail.

1.3 Tamping

Tamping consists of beating the top surface by wooden cross beam of section 10 X 10 cm.

It is adopted in compacting roof or floor slab or road pavement where the thickness of

concrete is less and the surface to be finished is smooth and level.

2. Compaction by Vibration

When high strength is required, it is vital that stiff concrete, with low water/cement ratio is

used. To compact such concrete, hand compaction cannot be used. Only mechanically

operated vibratory equipment must be used.

Modern high frequency vibrators make it possible to place economically concrete which is

impracticable to place by hand.

A concrete with 4cm slump can be placed and compacted fully in a closely spaced reinforced

concrete work, while for hand compaction, 12cm slump may be needed.

Vibration may be necessary if the available aggregates are of such poor shape and texture

which would provide a concrete of poor workability.

Vibration is adopted to improve the compaction and consequently the durability of

structures.

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2.1 Internal Vibrator

Most commonly used one.

Also called “Needle Vibrator”, “Immersion Vibrator” or “Poker Vibrator”.

Consists essentially of a power unit, a flexible shaft and a needle.

The power unit may be electrically driven or operated by petrol engine or air compressor.

The frequency of vibration varies up to 12 000 cycles of vibration per min.

The needle diameter varies from 20mm to 75mm and its length varies from 25cm to 90cm.

In the presence of congested reinforcement, the needle can be replaced by a blade of

approximately the same length. When the needle would not go in, this blade can effectively

vibrate.

Internal vibrators are portable and can also be used in difficult situations.

2.2 Formwork Vibrator (External Vibrator)

Used for concreting columns, thin walls or in the casting of precast units.

The machine is clamped on to the external wall surface of the formwork.

The vibration is given to the formwork so that the concrete in the vicinity of the shutter gets

vibrated.

This method is useful in cases of congested reinforcement.

Use of formwork vibrator will produce good finishes to the concrete surface.

External vibrators consume more power as vibrator is given to the formworks, which have

higher surface area.

They are less efficient then the internal vibrator.

2.3 Table Vibrator

This is a special case of formwork vibrator, where the vibrator is clamped to a table or to

springs which hold the table.

Everything placed on the table gets vibrated.

They are commonly used for vibrating concrete cubes in laboratories.

2.4 Platform Vibrator

It is a larger table vibrator.

It is used in the manufacture of large prefabricated concrete elements such as electric poles.

Sometimes, the platform vibrator is coupled with a shock giving arrangement for thorough

vibration.

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2.5 Surface Vibrator

Also known as “Screed Board Vibrators”.

A small vibrator placed on the screed board gives an effective method of compacting and

levelling of thin concrete members, such as floor slabs and roof slabs.

Floor slabs and roof slabs are so thin that internal vibrator cannot be easily employed.

Surface vibratos are not effective beyond 15cm.

2.6 Vibratory Roller

Used to compact very dry and lean concrete.

Concrete compacted by a vibratory roller is called Roller Compacted Concrete.

Used mainly in construction of dams and pavements.

Heavy rollers which vibrate while rolling is used to compact dry lean concrete.

3. Compaction by Pressure and Jolting

One of the most effective methods to compact very dry mix.

Used to compact hollow blocks, cavity blocks and solid concrete blocks.

The stiff concrete is vibrated, pressed and given jolts.

With the combined action of the jolts vibrations and pressure, the stiff concrete gets

compacted to a dense form to give good strength and volume stability.

By applying great pressure, a concrete of very low water/cement ratio can be compacted to

yield very high strength.

4. Compaction by Spinning

One of the most recent methods of compaction.

Used during the fabrication of concrete pipes.

The concrete when spun at a very high speed, gets well compacted by centrifugal force.

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6. Curing of Concrete

Definition:

Concrete derives its strength by the hydration of cement particles. The hydration of cement

is not a momentary action but a process continuing for long time.

Even though a higher water/cement ratio is used, since the concrete is open to atmosphere,

the water used in the concrete evaporates and the water available in the concrete will not

be sufficient for effective hydration to take place particularly in the top layer. If the

hydration is to continue unabated, extra water must be added to replenish the loss of water

on account of absorption and evaporation.

Curing can be described as the process of maintaining satisfactory moisture content

and a favourable temperature in concrete during the period immediately following

placement, so that hydration of cement may continue until the desired properties are

developed to a sufficient degree to meet the requirement of service.

A concrete laid in the afternoon of a hot summer day in a dry climatic region, is apt to dry

out quickly. The surface layer of concrete exposed to acute drying condition, with the

combined effect of hot sun and drying wind is likely to be made up of poorly hydrated

cement with inferior gel structure which does not give the desirable bond and strength

characteristics. The dried concrete naturally being weak, cannot withstand these stresses

with the result that innumerable cracks develop at the surface.

The quick surface drying of concrete results in the movement of moisture from the interior

to the surface. This steep moisture gradient cause high internal stresses which are also

responsible for internal micro cracks in the semi-plastic concrete.

Concrete, while hydrating, releases high heat of hydration. This heat is harmful from the

point of view of volume stability. If the heat generated is removed by some means, the

adverse effect due to the generation of heat can be reduced. This can be done by a thorough

water curing.

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Curing Methods:

Curing methods may be divided broadly into four categories:

i. Water curing

ii. Membrane curing

iii. Application of heat

iv. Miscellaneous

i. Water Curing:

This is by far the best method of curing as it satisfies all the requirements of curing, namely,

promotion of hydration, elimination of shrinkage and absorption of the heat of hydration. It

is pointed out that even if the membrane method is adopted, it is desirable that a certain

extent of water curing is done before the concrete is covered with membranes.

Water curing can be done in the following ways:

a. Immersion

b. Ponding

c. Spraying or Fogging

d. Wet covering

The precast concrete items are normally immersed in curing tanks for certain duration.

Pavement slabs, roof slab etc. are covered under water by making small ponds.

Vertical retaining wall or plastered surfaces or concrete columns etc. are cured by spraying

water. In some cases, wet coverings such as wet gunny bags, hessian cloth, jute matting,

straw etc., are wrapped to vertical surface for keeping the concrete wet.

ii. Membrane Curing:

Sometimes, concrete works are carried out in places where there is acute shortage of

water. Curing does not mean only application of water; it means also creation of conditions

for promotion of uninterrupted and progressive hydration. Moreover, the quantity of

water, normally mixed for making concrete is more than sufficient to hydrate the cement,

provided this water is not allowed to go out from the body of concrete. For this reason,

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concrete could be covered with a membrane which will effectively seal off the evaporation

of water from concrete.

Large numbers of sealing compounds have been developed in recent years. The idea is to

obtain a continuous seal over the concrete surface by means of a firm impervious film to

prevent moisture in concrete from escaping by evaporation. Some of the materials that can

be used for this purpose are bituminous compounds, polyethylene or polyester film,

waterproof paper, rubber compounds etc.

To achieve best results, membrane is applied after one or two days' of actual wet curing.

Since no replenishing of water is done after the membrane has been applied it should be

ensured that the membrane is of good quality and it is applied effectively. Two or three

coats may be required for effective sealing of the surface to prevent the evaporation of

water.

Increase in volume of construction, shortage of water and need for conservation of water,

increase in cost of labour and availability of effective curing compounds have encouraged

the use of curing compounds in concrete construction.

When waterproofing paper or polyethylene film are used as membrane, care must be taken

to see that these are not punctured anywhere and also see whether adequate laping is

given at the junction and this lap is effectively sealed.

iii. Application of Heat:

The development of strength of concrete is a function of not only time but also that of

temperature. When concrete is subjected to higher temperature it accelerates the

hydration process resulting in faster development of strength. Concrete cannot be

subjected to dry heat to accelerate the hydration process as the presence of moisture is also

an essential requisite.

Therefore, subjecting the concrete to higher temperature and maintaining the required

wetness can be achieved by subjecting the concrete to steam curing.

The exposure of concrete to higher temperature is done in the following manner:

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i. Steam curing at ordinary pressure.

ii. Steam curing at high pressure.

iii. Curing by Infra-red radiation.

iv. Electrical curing.

i. Steam curing at ordinary pressure:

Steam curing at ordinary pressure is applied mostly on prefabricated elements stored in a

chamber. The chamber should be big enough to hold a day's production. The door is closed

and steam is applied. The steam may be applied either continuously or intermittently.

An accelerated hydration takes place at this higher temperature and the concrete products

attain the 28 days strength of normal concrete in about 3 days.

It is interesting to note that concrete subjected to higher temperature at the early period of

hydration is found to lose some of the strength gained at a later age. Such concrete is said to

undergo "Retrogression of Strength".

ii. High Pressure Steam Curing:

The high pressure steam curing is something different from ordinary steam curing, in that

the curing is carried out at high pressure in a closed chamber. The superheated steam at

high pressure and high temperature is applied on the concrete. This process is also called

"Autoclaving". The autoclaving process is practised in curing precast concrete products in

the factory, particularly, for the lightweight concrete products.

The following advantages are derived from high pressure steam curing process:

High pressure steam cured concrete develops in one day or less the strength as

much as the 28 days' strength of normally cured concrete.

High pressure steam cured concrete exhibits higher resistance to sulphate attack,

freezing and thawing action and chemical action. It also shows less efflorescence.

High pressure steam cured concrete exhibits lower drying shrinkage, and moisture

movement. (The higher rate of development of strength is attributed to the higher

temperature to which a concrete is subjected.)

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iii. Curing by Infra-red Radiation:

Curing of concrete by Infra-red Radiation has been practised in very cold climatic regions

e.g Russia. It is claimed that much more rapid gain of strength can be obtained than with

steam curing and that rapid initial temperature does not cause a decrease in the ultimate

strength as in the case of steam curing at ordinary pressure. The system is very often

adopted for the curing of hollow concrete products. The normal operative temperature is

kept at 90°C.

iv. Electrical Curing:

Another method of curing concrete, which is applicable mostly to very cold climatic

regions, is the use of electricity. This method is not likely to find much application in

ordinary climate owing to economic reasons.

Concrete can be cured electrically by passing an alternating current through the concrete

itself between two electrodes either buried in or applied to the surface of the concrete. Care

must be taken to prevent the moisture from going out leaving the concrete completely dry.

Miscellaneous Methods of Curing:

Calcium chloride is used either as a surface coating or as an admixture. It has been used

satisfactorily as a curing medium. Both these methods are based on the fact that calcium

chloride being a salt shows affinity for moisture. The salt not only absorbs moisture from

atmosphere but also retains it at the surface. This moisture held at the surface prevents the

mixing water from evaporation and thereby keeps the concrete wet for a long time to

promote hydration.

Formwork prevents escaping of moisture from the concrete, particularly, in the case of

beams and columns. Keeping the formwork intact and sealing the joint with wax or any

other sealing compound prevents the evaporation of moisture from the concrete. This

procedure of promoting hydration can be considered as one of the miscellaneous methods

of curing.