Unit 2 module 2

Fresh concrete
 Flow Test
• This is a laboratory test, which gives an indication of the quality of
concrete with respect to consistency, cohesiveness and the 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 consists of flow table, about 76 cm. in diameter over which
concentric circles are marked. A mould made from smooth metal casting
in the form of a frustum of a cone is used with the following internal
dimensions.
– The base is 25 cm. in diameter, upper surface 17 cm. in
– diameter, and height of the cone is 12 cm. The table top is cleaned of all gritty
material and is wetted. The mould is kept on the centre of the table, firmly held
and is filled in two layers. Each layer is rodded 25 times with a tamping rod 1.6
cm in diameter and 61 cm long rounded at the lower tamping end.
➢ After the top layer is rodded evenly, the excess of concrete which has
overflowed the mould is removed.
➢ The mould is lifted vertically upward and the concrete stands on its own
without support. The table is then raised and dropped 12.5 mm 15 times
in about 15 seconds.
➢ The diameter of the spread concrete is measured in about 6 directions to
the nearest 5 mm 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 per cent. A close look at the
pattern of spread of concrete can also give a good indication of the
characteristics of concrete such as tendency for segregation.
https://www.youtube.com/watch?v=KN-nKeSqDPU
 Kelly Ball Test:
• The test has been devised by Kelly and hence known as Kelly Ball Test.
• The advantages of this test is that it can be performed on the concrete
placed in site and it is claimed that this test can be performed faster with a
greater precision than slump test.

• The disadvantages are that it requires a large sample of concrete and it

• The minimum depth of concrete must be at least 20 cm and the minimum

• The surface of the concrete is struck off level and the ball is lowered
gradually on the surface of the concrete. The depth of penetration is read
immediately on the stem to the nearest 6 mm. The test can be performed
in about 15 seconds.
https://www.youtube.com/watch?v=G81u8ApH71w
 Vee Bee Consistometer Test:
• This is a good laboratory test to measure indirectly the workability of concrete.
This test consists of a vibrating table, a metal pot, a sheet metal cone, a
standard iron rod.
• Slump test is performed by placing the slump cone inside the sheet metal
cylindrical pot of the consistometer.
• The glass disc attached to the swivel arm is turned and placed on the top of
the concrete in the pot.
• The electrical vibrator is then switched on and simultaneously a stop watch
started. The vibration is continued till such a time as the conical shape of the
concrete disappears and the concrete assumes a cylindrical shape.
• This can be judged by observing the glass disc from the top for disappearance
of transparency. Immediately when the concrete fully assumes a cylindrical
shape, the stop watch is switched off.
• The time required for the shape of concrete to change from slump cone shape
to cylindrical shape in seconds is known as Vee Bee Degree.
• This method is suitable for very dry concrete whose slump value cannot be
measured by Slump Test.
 Segregation:
• Segregation can be defined as the separation of the constituent materials
of concrete.
• A good concrete is one in which all the ingredients are properly distributed
to make a homogeneous mixture.
• If a sample of concrete exhibits a tendency for separation of say, coarse
aggregate from the rest of the ingredients, then, that sample is said to be
showing the tendency for segregation.
• Such concrete is not only going to be weak; lack of homogeneity is also
going to induce all undesirable properties in the hardened concrete.
• Segregation may be of three types: firstly, the coarse aggregate separating
out or settling down from the rest of the matrix,
• secondly, the paste or matrix separating away from coarse aggregate and
• thirdly, water separating out from the rest of the material being a material
of lowest specific gravity.
• The conditions favorable for segregation are the badly proportioned mix
where sufficient matrix is not there to bind and contain the aggregates.
Insufficiently mixed concrete with excess water content shows a higher
tendency for segregation.
• Dropping of concrete from heights as in the case of placing concrete in
column concreting will result in segregation.
• Vibration of concrete is one of the important methods of compaction.
Only comparatively dry mix should be vibrated. If too wet a mix is
excessively vibrated, it is likely that the concrete gets segregated. It should
also be remembered that vibration is continued just for required time for
optimum results.
• If the vibration is continued for a long time, particularly, in too wet a mix,
it is likely to result in segregation of concrete due to settlement of coarse
aggregate in matrix.
• Tendency for segregation can be remedied by correctly proportioning the
mix, by proper handling, transporting, placing, compacting and finishing.
• At any stage, if segregation is observed, remixing for a short time would
make the concrete again homogeneous.
• Use of certain workability agents and pozzolanic materials greatly help in
reducing segregation.
• The use of air entraining agent appreciably reduces segregation.
• The pattern of subsidence of concrete in slump test or the pattern of
spread in the flow test gives a fair idea of the quality of concrete with
respect to segregation
 Bleeding:
• It is a particular form of segregation, in which some of the water
from the concrete comes out to the surface of the concrete, being
of the lowest specific gravity among all the ingredients of concrete.
• Bleeding is predominantly observed in a highly wet mix, badly
proportioned and insufficiently mixed concrete. In thin members
like roof slab or road slabs and when concrete is placed in sunny
weather show excessive bleeding.
• Due to bleeding, water comes up and accumulates at the surface.
Sometimes, along with this water, certain quantity of cement also
comes to the surface. When the surface is worked up with the
trowel and floats, the aggregate goes down and the cement and
water come up to the top surface. This formation of cement paste
at the surface is known as “Laitance”
• In such a case, the top surface of slabs and pavements will not have good
wearing quality. This laitance formed on roads produces dust in summer
and mud in rainy season.
• If laitance is formed on a particular lift, a plane of weakness would form
and the bond with the next lift would be poor. This could be avoided by
removing the laitance fully before the next lift is poured.
• Bleeding can be reduced by proper proportioning and complete mixing.
Use of finely divided pozzolanic materials reduces bleeding by creating a
longer path for the water to traverse.
• Use of air-entraining agent is very effective in reducing the bleeding. It is
also reported that the bleeding can be reduced by the use of finer cement
or cement with low alkali content. Rich mixes are less susceptible to
bleeding than lean mixes.
 Setting time of concrete:
• Setting time of concrete differs widely from setting time of cement.
• Setting time of concrete does not coincide with the setting time of cement
with which the concrete is made.
• The setting time of concrete depends upon the w/c ratio, temperature
conditions, type of cement, use of mineral admixture, use of plasticizers–
in particular retarding plasticizer.
• The setting parameter of concrete is more of practical significance for site
engineers than setting time of cement.
• When retarding plasticizers are used, the increase in setting time, the
duration upto which concrete remains in plastic condition is of special
interest
• The setting time of concrete is found by penetrometer test. This method
of test is covered by IS8142 of 1976 and ASTM C – 403.
• The procedure given below may also be applied to prepared mortar and
grouts.
• The apparatus consist of a container which should have minimum lateral
dimension of 150 mm and minimum depth of 150 mm
• There are six penetration needles with bearing areas of 645, 323, 161, 65,
32 and 16 mm2. Each needle stem is scribed circumferentially at a
distance of 25 mm from the bearing area.
• A device is provided to measure the force required to cause penetration of
the needle.
• The test procedure involves collection of representative sample of
concrete in sufficient quantity and sieves it through 4.75 mm sieve and the
resulting mortar is filled in the container.
• Compact the mortar by rodding, tapping, rocking or by vibrating. Level the
surface and keep it covered to prevent the loss of moisture.
• Remove bleeding water, if any, by means of pipette.
• Insert a needle of appropriate size, depending upon the degree of setting
of the mortar in the following manner. Bring the bearing surface of needle
in contact with the mortar surface.
• Gradually and uniformly apply a vertical force downwards on the
apparatus until the needle penetrates to a depth of 25 ± 1.5 mm, as
indicated by the scribe mark.
• The time taken to penetrate 25 mm depth could be about 10 seconds.
Record the force required to produce 25 mm penetration and the time of
inserting from the time water is added to cement.
• Calculate the penetration resistance by dividing the recorded force by the
bearing area of the needle. This is the penetration resistance.
• For the subsequent penetration avoid the area where the mortar has been
disturbed. The clear distance should be two times the diameter of the
bearing area. Needle is inserted at least 25 mm away from the wall of
container.
• Plot a graph of penetration resistance as ordinate and elapsed time as
abscissa.
• Not less than six penetration resistance determination is made. Continue
the tests until one penetration resistance of at least 27.6 MPa is reached.
Connect various points by a smooth curve.
• From penetration resistance equal to 3.5 MPa, draw a horizontal line. The
point of intersection of this with the smooth curve, is read on the x-axis
which gives the initial setting time.
• Similarly a horizontal line is drawn from the penetration resistance of 27.6
MPa and point it cuts the smooth curve is read on the x-axis which gives
the final set.
Rheology of concrete