Use of Slump cone
test :
Slump test is most
commonly used method for measuring consistency of concrete.
This is test is a
control test and gives an indication of the uniformity of concrete from batch
to batch.
Apparatus and Equipment
used : Apparatus for conducting
slump test essentially consists of metallic mould in the form of frustum of a
cone having internal dimensions as under:-
Bottom diameter = 20 cm.
Top diameter= 10 cm.
Height = 30 cm.
Thickness of metallic
sheet for mould should not be thinner than 1.62 mm.
Tampering rod diameter = 16 mm. (60 cm long bullet pointed at
lower end).
Steps
for Procedure:
i.The internal surface
of the mould is thoroughly cleaned and applied with a light coat of oil.
ii.The mould is placed
on a smooth, horizontal, rigid and non-absorbent surface.
iii.The mould is then
filled in four layers with freshly mixed concrete, each approximately to
one-fourth of the height of the mould.
iv. Each layer is tamped
25 times by the rounded end of the tamping rod (strokes are distributed evenly
over the cross section).
v. After the top layer
is rodded, the concrete is struck off the level with a trowel.
vi. The mould is removed
from the concrete immediately by raising it slowly in the vertical direction.
vii.The difference in
level between the height of the mould and that of the highest point of the
subsided concrete is measured.
viii. This difference in
height in mm is the slump of the concrete.
Result Recorded:
Slump of concrete= Height of the mould – highest point of the
subsided concrete is measured.
Slump of concrete was
recorded = 85
mm
Degree of workability = Medium (True slump).
Note:
The
reading was taken at site construction and it will be different for other
concrete mixes.
Factors affecting Slump test of fresh
concrete test:
1.
Concrete batching,
mixing and transportation methods and equipment.
2.
Concrete temperature.
3.
Sampling of concrete,
slump-testing technique and condition of test equipment.
4.
The amount of free water
in the concrete.
5.
Mixing time of concrete
at the time of testing.
6.
Physical properties such
as chemistry, fineness, particle size distribution, moisture content and
temperature of cement material. shape, texture, combined grading, cleaniness
and moisture of aggregates.
7.
Chemical mixture dosage-
composition and addition sequence.
Results of Slump Test on Concrete can be one of the following:
Figure a -Results of Slump Test on Concrete.
True Slump - True slump is the only trump that can be
measured in a test. Measurements are taken between the top of the cone and the
top of the concrete after the cone is removed as shown in figure a.
Zero Slump - A zero recession is a sign of a very low water-cement ratio, resulting in a dry mixture. This type of concrete is commonly used for road construction.
Collapsed slump - This is an indication that the water-cement ratio is too high, i.e. the concrete mixture is too wet or it is a high practicality mixture, for which a deceleration test is not suitable.
Shear Slump – The shear slump indicates that the result is incomplete, and concrete to be retested.
SAFETY & PRECAUTIONS:
§ Use hand gloves & shoes while testing.
§ Equipment should be cleaned thoroughly before
testing & after testing.
§ The apparatus should remain free from vibrations
during the test.
§ Petroleum jelly or oiling should be applied to
the mould.
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