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WORKABILITY OF WORKABILITY OF CONCRETECONCRETE

BYBYDHARAM SINGH H.O.D. DHARAM SINGH H.O.D.

GOVT POLY TECHNIC GOVT POLY TECHNIC COLLEGE COLLEGE

G.T.B GARH ( MOGA)G.T.B GARH ( MOGA)

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WORKABILITY OF CONCRETEWORKABILITY OF CONCRETE

workability is an important property of concrete in its workability is an important property of concrete in its stage. workability in simple terms can be defined as stage. workability in simple terms can be defined as “the ease with which the concrete can be mixed, “the ease with which the concrete can be mixed, transported, placed and compacted”. The workability transported, placed and compacted”. The workability of concrete has also been defined as the amount of of concrete has also been defined as the amount of work required to place concrete and to compact it work required to place concrete and to compact it thoroughlythoroughlyThe workability is associated with the following four The workability is associated with the following four concepts:concepts: 1.Ease of flow (internal friction) 1.Ease of flow (internal friction) 2.Prevention of segregation 2.Prevention of segregation 3.Prevention of harshness 3.Prevention of harshness 4.Prevention of bleeding 4.Prevention of bleeding

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EASE OF FLOW (INTERNAL EASE OF FLOW (INTERNAL FRICTION)FRICTION)

The ease with which the concrete The ease with which the concrete flows depends upon the internal flows depends upon the internal friction between the particles of friction between the particles of concrete. To improve workability, concrete. To improve workability, therefore it is necessary to reduce the therefore it is necessary to reduce the internal friction. The internal friction internal friction. The internal friction can be reduced by the lubrication of can be reduced by the lubrication of the surface of aggregates. The the surface of aggregates. The lubrication can be improved in two lubrication can be improved in two ways:ways:

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..

(a) By increasing the quantity of water in a concrete mix. Greater the quantity of water, more area it can lubricate. But this method is inefficient because it increases water-cement ratio and thereby decreases the strength of cement.(b) The second way is to reduce the total surface area of the aggregates by adopting coarse aggregate. But coarse aggregate should not be used too much otherwise segregation will take place.

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SEGREGATIONSEGREGATION The separation of coarse aggregate The separation of coarse aggregate

from the concrete mix in plastic stage is from the concrete mix in plastic stage is called segregation.called segregation.Concrete is not a homogenous material Concrete is not a homogenous material but rather a mixture of materials of but rather a mixture of materials of different specific gravities. So there is different specific gravities. So there is always a tendency for the coarser and always a tendency for the coarser and heavier particles to settle down and for heavier particles to settle down and for lighter materials to rise to the surface. lighter materials to rise to the surface. Segregation reduces the strength and Segregation reduces the strength and durability of concrete. durability of concrete.

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Causes of segregation. Segregation takes place when: (i) there is too much of water in the mix.(ii) there are badly graded aggregates.(iii) there are too much shocks to a concrete mix due to transport over longer distances, discharge of concrete from a considerable height (more than 1m) , Pumping of concrete.(iv) leakage of mortar from formwork.(v) concreting is done under-water.

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Prevention of segregationPrevention of segregation.. (i) The mix should be designed correctly (i) The mix should be designed correctly and minimum amount of water should be and minimum amount of water should be used for mixing.used for mixing.(ii) The height of free fall of concrete should (ii) The height of free fall of concrete should not exceed 1m in any case.not exceed 1m in any case.(iii) The air-entraining agents should be (iii) The air-entraining agents should be used for reducing segregation as these used for reducing segregation as these reduce the quantity of mixing water.reduce the quantity of mixing water.(iv) The concreting operations should be (iv) The concreting operations should be supervised strictly.supervised strictly.

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HARSHNESSHARSHNESSThe concrete mix which does not give The concrete mix which does not give smooth surface with a certain amount of smooth surface with a certain amount of trowelling is known as harsh mix.trowelling is known as harsh mix.Causes of harshness.Causes of harshness. (i) The cement mortar not sufficient to fill (i) The cement mortar not sufficient to fill the voids in the coarse aggregates.the voids in the coarse aggregates.(ii) The presence of excessive proportion of (ii) The presence of excessive proportion of one particle-size in an aggregate grading.one particle-size in an aggregate grading.Prevention of harshnessPrevention of harshness. Harshness can . Harshness can be prevented if there is sufficient proportion be prevented if there is sufficient proportion of mortar to fill the voids in coarse of mortar to fill the voids in coarse aggregatesaggregates..

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BLEEDINGBLEEDING The appearance of water along with some The appearance of water along with some

particles of cement and very fine sand on the particles of cement and very fine sand on the surface of freshly placed concrete after compaction surface of freshly placed concrete after compaction is called bleeding. A good concrete should be free is called bleeding. A good concrete should be free from bleeding. The term water gain is also from bleeding. The term water gain is also sometimes used instead of bleeding sometimes used instead of bleeding when water rises to the surface, flow channels are when water rises to the surface, flow channels are formed in concrete mass. Thus concrete with large formed in concrete mass. Thus concrete with large amount of bleeding are permeable. Due to bleeding amount of bleeding are permeable. Due to bleeding watery scum (water+ particles of fine sand and watery scum (water+ particles of fine sand and cement) is formed on the concrete surface. This cement) is formed on the concrete surface. This scum layer is known as laitance. This layer should scum layer is known as laitance. This layer should be removed if a new concrete layer is to be placed be removed if a new concrete layer is to be placed over the old layer.over the old layer.

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Causes of bleedingCauses of bleeding..(i) presence of excess water.(i) presence of excess water.(ii) Deficiency of fine aggregate.(ii) Deficiency of fine aggregate.(iii) Too much finishing.(iii) Too much finishing.Prevention of bleedingPrevention of bleeding. . bleeding can be bleeding can be prevented by:prevented by: (i) controlling the water- content ratio (i) controlling the water- content ratio (ii) providing finer grading of fine aggregates. (ii) providing finer grading of fine aggregates. (iii) using finely ground cement. (iii) using finely ground cement.(iv) controlling compaction.(iv) controlling compaction. (v) The air-entraining agents should be used to (v) The air-entraining agents should be used to prevent bleeding.prevent bleeding.

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FACTORS AFFECTING FACTORS AFFECTING WORKABILITYWORKABILITY

The factors which effect the workability of The factors which effect the workability of concrete are:concrete are: 1. Water content 1. Water content 2. Size of aggregate 2. Size of aggregate 3. Shape of aggregate 3. Shape of aggregate 4. Grading of aggregate 4. Grading of aggregate 5. Porosity and absorption of aggregate 5. Porosity and absorption of aggregate 6. Admixtures 6. Admixtures 7. Mixing time 7. Mixing time 8. Temperature 8. Temperature

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► 1.Water content. 1.Water content. Workability of concrete mix largely Workability of concrete mix largely depends upon its water content. With the increases of depends upon its water content. With the increases of water, the workability also increases. But too much water, the workability also increases. But too much water results into concrete of low strength and poor water results into concrete of low strength and poor durability.durability.2. Size of Aggregate2. Size of Aggregate. Workability increases with the . Workability increases with the increase in the size of the aggregate. Large size increase in the size of the aggregate. Large size particles provide less surface area as compared to particles provide less surface area as compared to surface area given by smaller size particles. Hence for surface area given by smaller size particles. Hence for same degree of workability, less water is required for same degree of workability, less water is required for large size aggregate. Lesser quantity of water used in large size aggregate. Lesser quantity of water used in large size aggregate also reduces the quantity of large size aggregate also reduces the quantity of cement for given water-cement ratio and is therefore cement for given water-cement ratio and is therefore economical. From practical point of view, the maximum economical. From practical point of view, the maximum size of aggregate to be used will depend upon the size of aggregate to be used will depend upon the handling, mixing and placing equipment, thickness of handling, mixing and placing equipment, thickness of section and quantity of reinforcement. section and quantity of reinforcement.

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3.Shape of Aggregate. The particle shape also effect the workability. Workability increases with round and smooth surface aggregates. Crushed or angular aggregates has less workability because of higher and rough surface area.4. Grading of Aggregate. The grading of aggregate has a considerable effect on workability of concrete. Grading is more important when lean mixes of high workability are required than rich mixes. For lean mixes, the grading should be continuous, whereas for rich mixes the grading should be coarse. 5. Porosity and Absorption of Aggregate. Porous and non-saturated aggregate will require more water than a non-porous and saturated aggregate. For same degree or workability, the latter will require less quantity of water.

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6. Admixtures6. Admixtures.. Workability also Workability also increases with addition of admixture increases with addition of admixture such as air, entraining agents which such as air, entraining agents which produce well dispersed air bubbles.produce well dispersed air bubbles.7. Mixing Time 7. Mixing Time . . with the increases in with the increases in mixing time upto certain limit mixing time upto certain limit workability increases.workability increases.8. Temperature.8. Temperature. The temperature at The temperature at which the concrete mix is prepared which the concrete mix is prepared also effects its workability. The slump also effects its workability. The slump of the concrete mix decreases as the of the concrete mix decreases as the temperature of the mix increases.temperature of the mix increases.

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MEASUREMENT OF WORKABILITYMEASUREMENT OF WORKABILITY

The following are the methods of measurements of The following are the methods of measurements of workability:workability: 1. slump test 1. slump test 2. compacting factors test 2. compacting factors test 3. vee-bee consistometer test 3. vee-bee consistometer test Slum TestSlum Test The slump test was devised in U.S.A. this test The slump test was devised in U.S.A. this test can be performed in the laboratory or at the site of work.can be performed in the laboratory or at the site of work.Apparatus.Apparatus. The slump test apparatus consists of steel The slump test apparatus consists of steel mould in the form of a frustum of a cone. The internal mould in the form of a frustum of a cone. The internal dimensions of a mould are:dimensions of a mould are: (i) Bottom diameter =200mm (i) Bottom diameter =200mm (ii) Top diameter=100mm (ii) Top diameter=100mm (iii) Height=300mm. (iii) Height=300mm.The bottom and top ends of the mould are open. Thickness The bottom and top ends of the mould are open. Thickness of the mould should not be less than 1.6 mm. for tamping of the mould should not be less than 1.6 mm. for tamping the wet concrete, a tamping rod of steel (16mm in diameter, the wet concrete, a tamping rod of steel (16mm in diameter, 0.6m long) is used.0.6m long) is used.

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Procedure. Procedure. The following procedure is The following procedure is adopted for determining the slump value adopted for determining the slump value of concrete: of concrete: 1.1. The internal The internal surface of the mould is thoroughly surface of the mould is thoroughly cleaned and the mould is placed on a cleaned and the mould is placed on a smooth, horizontal, rigid and non-smooth, horizontal, rigid and non-absorbent surface.absorbent surface.2.2.The mould is held firmly in place by The mould is held firmly in place by standing on the foot pieces before the standing on the foot pieces before the concrete is filled in.concrete is filled in.3.3. Concrete under test is filled in the Concrete under test is filled in the mould in four layers. Each layer is mould in four layers. Each layer is tamped (25 times) with the help of steel tamped (25 times) with the help of steel rod. The stroke should be uniformly rod. The stroke should be uniformly distributed.distributed.. .

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4. 4. After the top layer has been rodded, After the top layer has been rodded, the concrete should be struck off level with the concrete should be struck off level with a trowel. Any mortar which has leaked out a trowel. Any mortar which has leaked out between the base plate and the mould is between the base plate and the mould is cleaned away.cleaned away.55. The mould is then removed by raising it . The mould is then removed by raising it slowly and carefully in vertical direction. slowly and carefully in vertical direction. On the removal of mould, the concrete On the removal of mould, the concrete subside. subside. 66. Slump is measured immediately by . Slump is measured immediately by determining the vertical distance between determining the vertical distance between the height of the mould and that of highest the height of the mould and that of highest point of the specimen being tested point of the specimen being tested 77. If specimen collapses than test may be . If specimen collapses than test may be repeatedrepeated

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The slump which is the subsidence of the concrete cone after the mould is lifted up can give the following three results:(a) True slump------ If the concrete subside evenly, it is known as true slump.(b) shear slump----- If one half of the concrete slides down as soon as the mould is removed.(c) collapse slump----- If the concrete slides down as soon as the mould is removed , it is known as collapse slump. It is very difficult to measure. It occurs in very wet mixes.

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Degree of workability w.r.t. slump Degree of workability w.r.t. slump valuevalue

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S.No.S.No. Slump value in mmSlump value in mm Degree of Degree of workabilityworkability

1.1.

2.2.

3.3.

4.4.

(compacting factor is (compacting factor is suitable)suitable)

25-7525-75

50-10050-100

100-150100-150

Very Very low low

Low Low

Medium Medium

HighHigh

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Table -2Table -2Recommended slumps for placement in various Recommended slumps for placement in various

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S.NOS.NO Slump in mmSlump in mm Degree of Degree of workabilitworkabilityy

Placing ConditionsPlacing Conditions

1.1. (compacting (compacting factor is factor is suitable)suitable)

Very lowVery low (i)(i) Blinding concreteBlinding concrete

(ii)(ii) Shallow sectionsShallow sections

(iii) Pavements using pavers(iii) Pavements using pavers

2.2. 25-7525-75

LowLow

(i)(i) Mass concreteMass concrete

(ii)(ii) Lightly reinforced Lightly reinforced

(iii) Floors (iii) Floors

(iv) Canal linings(iv) Canal linings

(v) Strip footings(v) Strip footings

3.3. 50-10050-100

MediumMedium

(i)(i) Heavily reinforced sections in Heavily reinforced sections in slabs,beams,walls,columnsslabs,beams,walls,columns

(ii)(ii) Pumped concretePumped concrete

(iii) Slip form work(iii) Slip form work

4.4. 100-150100-150 HighHigh (i)(i) Trench fillTrench fill

(ii) in-situ piling(ii) in-situ piling

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COMPACTING FACTOR TESTCOMPACTING FACTOR TEST This test was developed by Road Research This test was developed by Road Research

Laboratory . It is mainly used in laboratory to Laboratory . It is mainly used in laboratory to determine workability of concrete.determine workability of concrete.Principle. Principle. The workability of concrete has also been The workability of concrete has also been defined as the amount of work required to place the defined as the amount of work required to place the concrete and to compact it thoroughly. It is not easy concrete and to compact it thoroughly. It is not easy to measure the amount of work required to fully to measure the amount of work required to fully compact it. But it is simpler to apply a standard compact it. But it is simpler to apply a standard amount of work to the concrete and to measure its amount of work to the concrete and to measure its degree of compaction. Work is applied by dropping degree of compaction. Work is applied by dropping the concrete through a standard height into a the concrete through a standard height into a cylindrical container. The compaction in the cylindrical container. The compaction in the cylindrical container is produced by the destruction cylindrical container is produced by the destruction of kinetic energy of the falling concrete. The degree of kinetic energy of the falling concrete. The degree of compaction is termed as compaction factor.of compaction is termed as compaction factor.

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Apparatus.Apparatus. The apparatus used for this test . The apparatus The apparatus used for this test . The apparatus consists of two hoppers in line vertically above the cylindrical consists of two hoppers in line vertically above the cylindrical container. The dimensions of the hoppers and cylinder are as container. The dimensions of the hoppers and cylinder are as follows:follows:Upper hopper (A) :Upper hopper (A) : (i) Top internal diameter =254mm (i) Top internal diameter =254mm (ii) Bottom internal diameter =127mm (ii) Bottom internal diameter =127mm (iii) Internal height =239mm (iii) Internal height =239mmLower hopper (B):Lower hopper (B): (i) Top internal diameter =229mm (i) Top internal diameter =229mm (ii) Bottom internal diameter =127mm (ii) Bottom internal diameter =127mm (iii) Internal height =229mm (iii) Internal height =229mmCylinder (C)Cylinder (C) (i) Internal diameter =152mm (i) Internal diameter =152mm (ii) Internal height =305mm (ii) Internal height =305mm Distance between bottom of upper hopper and top Distance between bottom of upper hopper and top of lower of lower hopper = 203 mm hopper = 203 mm Distance between bottom Distance between bottom of lower hopper and top of of lower hopper and top of CylinderCylinder =203mm =203mm

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Procedure.Procedure.The following procedure is adopted for performing The following procedure is adopted for performing the compacting factor test. the compacting factor test. 1. The concrete sample is placed in the upper 1. The concrete sample is placed in the upper hopper A.hopper A. 2.Then the door of hopper A is opened. The 2.Then the door of hopper A is opened. The sample drops into lower sample drops into lower hopper B filling it to overflowing. hopper B filling it to overflowing. 3. The trap door of the lower hopper is then 3. The trap door of the lower hopper is then opened and the sample falls into the cylinder which opened and the sample falls into the cylinder which is also filled to overflowing.is also filled to overflowing. 4. The surplus cement is removed from the top 4. The surplus cement is removed from the top of the cylinder with the help of a trowel.of the cylinder with the help of a trowel. 5. The outside surface of cylinder is wiped and 5. The outside surface of cylinder is wiped and cleaned.cleaned.

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6. The cylinder is then weighed and it is recorded as weight of partially compacted concrete.

7. The cylinder is again filled with concrete in layers not exceeding 50 mm in thickness. Each layer is fully compacted with tamping rod. 8.The cylinder is again weighed after wiping and cleaning the outside surface of cylinder. This weight is recorded as the weight of fully compacted concrete.

9. The compacting factor is then calculated from the formula:

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Compacting factor Compacting factor = = wt of partially compacted wt of partially compacted concconc wt of fully compacted wt of fully compacted conc.conc. Degree of workability w.r.t. compacting factorDegree of workability w.r.t. compacting factor

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S.NO.S.NO. Compacting factor Compacting factor Degree of Degree of workabilityworkability

1.1.

2.2.

3.3.

4.4.

0.75to 0.800.75to 0.80

0.870.87

0.9350.935

0.960.96

Very lowVery low

LowLow

MediumMedium

HighHigh

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VEE-BEE CONSISTOMETER TESTVEE-BEE CONSISTOMETER TEST

►The name vee-bee is derived from The name vee-bee is derived from the initials of V. Bahrner of Sweden the initials of V. Bahrner of Sweden who developed the test. Sometimes who developed the test. Sometimes this test is called V-B consist meter this test is called V-B consist meter test. Vee-bee is a good laboratory test. Vee-bee is a good laboratory test, particularly for very dry mixes.test, particularly for very dry mixes.

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ProcedureProcedure. . The following procedure is The following procedure is adopted for performing the vee-bee adopted for performing the vee-bee consistometer test.consistometer test.

1. Sheet metal cone is filled with 1. Sheet metal cone is filled with concrete. concrete.

2. The glass plate rider is then brought 2. The glass plate rider is then brought into into position so that it is touching the top position so that it is touching the top of the of the concrete. concrete.

3. The concrete cone is then subjected to3. The concrete cone is then subjected to vibrating action by starting the vibrating action by starting the vibrator. Thevibrator. The stop watch is started as soon as the stop watch is started as soon as the vibrator vibrator is switched on. The concrete is allowed is switched on. The concrete is allowed toto spread out in the cylindrical container. spread out in the cylindrical container.

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4. The vibration is until the concrete 4. The vibration is until the concrete surface becomes horizontal. The time surface becomes horizontal. The time taken to attain horizontal surface is taken to attain horizontal surface is noted. The time recorded in seconds noted. The time recorded in seconds gives the degree of workability.gives the degree of workability.

SuitabilitySuitability.. This method is suitable This method is suitable for dry concrete having very low for dry concrete having very low workability. workability.

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Degree of workability w.r.t. Vee-Bee Degree of workability w.r.t. Vee-Bee SecondsSeconds

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S.NOS.NO..

Time in Time in secondsseconds

Degree of Degree of workabilityworkability

1.1.

2.2.

3.3.

4.4.

5.5.

20-4020-40

10-2010-20

7-107-10

3-73-7

1-31-3

Very low Very low

Low Low

MediumMedium

HighHigh

Very HighVery High

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