Workability of Concrete.ppt

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WORKABILITY OF CONCRETE
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Prepaid by: 130670106039
130670106040

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WORKABILITY OF CONCRETE
workability is an important property of concrete in its
stage. workability in simple terms can be defined as “the
ease with which the concrete can be mixed, transported,
placed and compacted”. The workability of concrete has
also been defined as the amount of work required to place
concrete and to compact it thoroughly
The workability is associated with the following four
concepts:
1.Ease of flow (internal friction)
2.Prevention of segregation
3.Prevention of harshness
4.Prevention of bleeding
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EASE OF FLOW (INTERNAL FRICTION)
The ease with which the concrete flows
depends upon the internal friction between the
particles of concrete. To improve workability,
therefore it is necessary to reduce the internal
friction. The internal friction can be reduced by
the lubrication of the surface of aggregates.
The lubrication can be improved in two 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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SEGREGATION
The separation of coarse aggregate from
the concrete mix in plastic stage is called
segregation.
Concrete is not a homogenous material
but rather a mixture of materials of
different specific gravities. So there is
always a tendency for the coarser and
heavier particles to settle down and for
lighter materials to rise to the surface.
Segregation reduces the strength and
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 segregation.
(i) The mix should be designed correctly and
minimum amount of water should be used for
mixing.
(ii) The height of free fall of concrete should not
exceed 1m in any case.
(iii) The air-entraining agents should be used for
reducing segregation as these reduce the
quantity of mixing water.
(iv) The concreting operations should be
supervised strictly.
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HARSHNESS
The concrete mix which does not give smooth
surface with a certain amount of trowelling is
known as harsh mix.
Causes of harshness.
(i) The cement mortar not sufficient to fill the
voids in the coarse aggregates.
(ii) The presence of excessive proportion of one
particle-size in an aggregate grading.
Prevention of harshness. Harshness can be
prevented if there is sufficient proportion of mortar
to fill the voids in coarse aggregates.
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BLEEDING
The appearance of water along with some particles of
cement and very fine sand on the surface of freshly
placed concrete after compaction is called bleeding. A
good concrete should be free from bleeding. The term
water gain is also sometimes used instead of bleeding
when water rises to the surface, flow channels are
formed in concrete mass. Thus concrete with large
amount of bleeding are permeable. Due to bleeding
watery scum (water+ particles of fine sand and cement)
is formed on the concrete surface. This scum layer is
known as laitance. This layer should be removed if a new
concrete layer is to be placed over the old layer.
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Causes of bleeding.
(i) presence of excess water.
(ii) Deficiency of fine aggregate.
(iii) Too much finishing.
Prevention of bleeding. bleeding can be prevented
by:
(i) controlling the water- content ratio
(ii) providing finer grading of fine aggregates.
(iii) using finely ground cement.
(iv) controlling compaction.
(v) The air-entraining agents should be used to
prevent bleeding.
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FACTORS AFFECTING
WORKABILITY
The factors which effect the workability of
concrete are:
1. Water content
2. Size of aggregate
3. Shape of aggregate
4. Grading of aggregate
5. Porosity and absorption of aggregate
6. Admixtures
7. Mixing time
8. Temperature
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► 1.Water content. Workability of concrete mix largely
depends upon its water content. With the increases of
water, the workability also increases. But too much water
results into concrete of low strength and poor durability.
2. Size of Aggregate. Workability increases with the
increase in the size of the aggregate. Large size particles
provide less surface area as compared to surface area
given by smaller size particles. Hence for same degree of
workability, less water is required for large size
aggregate. Lesser quantity of water used in large size
aggregate also reduces the quantity of cement for given
water-cement ratio and is therefore economical. From
practical point of view, the maximum size of aggregate to
be used will depend upon the handling, mixing and
placing equipment, thickness of 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. Admixtures. Workability also increases
with addition of admixture such as air,
entraining agents which produce well
dispersed air bubbles.
7. Mixing Time . with the increases in
mixing time upto certain limit workability
increases.
8. Temperature. The temperature at
which the concrete mix is prepared also
effects its workability. The slump of the
concrete mix decreases as the temperature
of the mix increases.
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MEASUREMENT OF WORKABILITY
The following are the methods of measurements of workability:
1. slump test
2. compacting factors test
3. vee-bee consistometer test
Slum Test The slump test was devised in U.S.A. this test can be
performed in the laboratory or at the site of work.
Apparatus. The slump test apparatus consists of steel mould in
the form of a frustum of a cone. The internal dimensions of a
mould are:
(i) Bottom diameter =200mm
(ii) Top diameter=100mm
(iii) Height=300mm.
The bottom and top ends of the mould are open. Thickness of the
mould should not be less than 1.6 mm. for tamping the wet
concrete, a tamping rod of steel (16mm in diameter, 0.6m long) is
used.
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Procedure. The following procedure is adopted
for determining the slump value of concrete:
1. The internal surface of the mould is
thoroughly cleaned and the mould is placed on
a smooth, horizontal, rigid and non-absorbent
surface.
2.The mould is held firmly in place by standing
on the foot pieces before the concrete is filled
in.
3. Concrete under test is filled in the mould in
four layers. Each layer is tamped (25 times)
with the help of steel rod. The stroke should be
uniformly distributed.
.
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4. After the top layer has been rodded, the
concrete should be struck off level with a trowel.
Any mortar which has leaked out between the
base plate and the mould is cleaned away.
5. The mould is then removed by raising it
slowly and carefully in vertical direction. On the
removal of mould, the concrete subside.
6. Slump is measured immediately by
determining the vertical distance between the
height of the mould and that of highest point of
the specimen being tested
7. If specimen collapses than test may be
repeated
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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 value
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S.No. Slump value in mm Degree of
workability
1.
2.
3.
4.
(compacting factor is suitable)
25-75
50-100
100-150
Very low
Low
Medium
High

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Table -2
Recommended slumps for placement in various conditions
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S.NO Slump in mm Degree of
workability
Placing Conditions
1. (compacting factor
is suitable)
Very low (i) Blinding concrete
(ii) Shallow sections
(iii) Pavements using pavers
2. 25-75
Low
(i) Mass concrete
(ii) Lightly reinforced
(iii) Floors
(iv) Canal linings
(v) Strip footings
3. 50-100
Medium
(i) Heavily reinforced sections in
slabs,beams,walls,columns
(ii) Pumped concrete
(iii) Slip form work
4. 100-150 High (i) Trench fill
(ii) in-situ piling

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COMPACTING FACTOR TEST
This test was developed by Road Research
Laboratory . It is mainly used in laboratory to determine
workability of concrete.
Principle. The workability of concrete has also been defined
as the amount of work required to place the concrete and
to compact it thoroughly. It is not easy to measure the
amount of work required to fully compact it. But it is
simpler to apply a standard amount of work to the concrete
and to measure its degree of compaction. Work is applied
by dropping the concrete through a standard height into a
cylindrical container. The compaction in the cylindrical
container is produced by the destruction of kinetic energy
of the falling concrete. The degree of compaction is termed
as compaction factor.
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Apparatus. The apparatus used for this test . The apparatus consists of
two hoppers in line vertically above the cylindrical container. The
dimensions of the hoppers and cylinder are as follows:
Upper hopper (A) :
(i) Top internal diameter =254mm
(ii) Bottom internal diameter =127mm
(iii) Internal height =239mm
Lower hopper (B):
(i) Top internal diameter =229mm
(ii) Bottom internal diameter =127mm
(iii) Internal height =229mm
Cylinder (C)
(i) Internal diameter =152mm
(ii) Internal height =305mm
Distance between bottom of upper hopper and top of lower
hopper = 203 mm
Distance between bottom of lower hopper and top of Cylinder
=203mm
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Procedure.
The following procedure is adopted for performing the
compacting factor test.
1. The concrete sample is placed in the upper hopper A.
2.Then the door of hopper A is opened. The sample
drops into lower hopper B filling it to overflowing.
3. The trap door of the lower hopper is then opened and
the sample falls into the cylinder which is also filled to
overflowing.
4. The surplus cement is removed from the top of the
cylinder with the help of a trowel.
5. The outside surface of cylinder is wiped and 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 = wt of partially compacted conc
wt of fully compacted conc.
Degree of workability w.r.t. compacting factor
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S.NO. Compacting factor Degree of workability
1.
2.
3.
4.
0.75to 0.80
0.87
0.935
0.96
Very low
Low
Medium
High

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VEE-BEE CONSISTOMETER TEST
►The name vee-bee is derived from the
initials of V. Bahrner of Sweden who
developed the test. Sometimes this test is
called V-B consist meter test. Vee-bee is
a good laboratory test, particularly for
very dry mixes.
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Procedure. The following procedure is adopted
for performing the vee-bee consistometer test.
1. Sheet metal cone is filled with concrete.
2. The glass plate rider is then brought into
position so that it is touching the top of the
concrete.
3. The concrete cone is then subjected to
vibrating action by starting the vibrator. The
stop watch is started as soon as the vibrator
is switched on. The concrete is allowed to
spread out in the cylindrical container.
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4. The vibration is until the concrete
surface becomes horizontal. The time
taken to attain horizontal surface is noted.
The time recorded in seconds gives the
degree of workability.
Suitability. This method is suitable for dry
concrete having very low workability.
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Degree of workability w.r.t. Vee-Bee
Seconds
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S.NO. Time in seconds Degree of
workability
1.
2.
3.
4.
5.
20-40
10-20
7-10
3-7
1-3
Very low
Low
Medium
High
Very High

Workability of Concrete.ppt

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