In this article, we explained how to determine the workability of freshly mixed concrete by the use of a flow table.
Theory and Scope of flow table test
The test consists of moulding the fresh concrete in the form of a frustum of a cone on the top of the platform of the flow table; The concrete cone is then given a certain amount of shaking. The dispersion of the concrete is measured by increasing the diameter of the concrete pile and expressed as a percentage of the original base diameter of the concrete, which is taken as a measure of the flow or stability of the concrete.
This test explores the stability and mobility aspect of work performance. In general, the test will give satisfactory performance for the stability of the slope cone test. However, it should be noted that the flow test does not measure performance, as concretes with the same flow may differ significantly in their performance.
Apparatus
for Flow Table Test
· Flow
table
· Cone
mould
· Balance
· Tamping
rod
· Calipers.
Description
of Apparatus
The flow table shown in
Fig. below shall conform to IS: 15-1959 and shall be rigidly mounted on a
concrete base having a height of 400 to 500 mm and weighing not less than 140
kg. A motorised flow table (IS:5512) is also available.
The cone mould shall be
made of a smooth metal casting in the form of a frustum of a cone. The internal
dimensions are base diameter 250 mm, top diameter 170 mm, height 120 mm. The
base and the top shall be open at right angles to the axis of the cone. The
mould is provided with handles to facilitate lifting it vertically from the
moulded concrete test specimen.
The tamping rod shall be of
steel or some other suitable metal, 16 mm in diameter 600 mm long and bullet-pointed at one end.
The procedure of Flow Table Test on concrete
Step
1:
Moisten the clean tabletop and inside of the cleaned cone mould; Remove the
excess moisture with a wet cloth before commencing the test.
Step
2:
Centre the mould on the table platform and hold it firmly in place.
Step 3: Fill the mould in two equal layers, each layer being given 25 strokes with the standard tamping rod. The strokes shall be distributed in a uniform manner over the cross-section of the mould and for the second layer shall penetrate into the underlying layer.
Flow table apparatus (all dimensions in mm
Step
4:
After the top layer has been rodded struck off level the surface with a trowel
so that the mould is exactly filled. Remove the excess concrete which has
overflowed the mould and clean the area of the table outside the mould.
Step
5:
Remove the mould immediately by lifting it vertically by a steady upward pull.
Step
6:
Turn the handle 15 times at a rate of 1 revolution per second, such that the
concrete is given a jolt by raising and then dropping it by 12.5 mm.
Step
7:
Measure the diameter of the spread concrete at six equally spaced positions
along the circumference with callipers read to the nearest 5 mm and record the
average.
Step
8:
Obtain the flow or the consistency of concrete by expressing the increase in
diameter of the concrete specimen as the percentage of the original diameter of 250
mm.
Observations and Calculations
|
Original or base diameter, |
d1mm |
250 |
250 |
|
|
Spread diameter, |
d2mm |
|
|
|
|
Increase the diameter, |
d2 – d1mm |
|
|
|
|
Flow, |
d2-d1 |
% |
|
|
|
d1 |
||||
The flow of the concrete is ______ per
cent.
Precautions
1. The surface of the flow table and the inside of the mould should be moistened with any set of concrete and thoroughly cleaned before testing.
2. The stroke of the damping rod should be uniformly distributed over the cross-section of the axis.
3. The mould should be removed very slowly by lifting it upwards so as not to disturb the concrete inside the mould.
Discussion
The flow test provides satisfactory performance for the stabilization of the applicability of the slope test, but the results of the flow test are more reproducible than those given by the slope test. The test is mostly for laboratory use only. The weakness of the flow test is that the flow is uncontrolled and some aggregates are only partially embedded in the concrete, and at the end of the test the mass is dispersed instead of being uniform.
As the concrete is scattered on the flow table, the test with a tendency towards separation suffers from a defect.
FAQs of flow table test
Why
is the flow table test used?
Flow table test or flow
test is a method to determine the workability
of freshly mixed concrete. The flow table test is also used to identify the transportable
moisture limit of solid bulk cargoes.
NATIONAL
STANDARDS
1. IS 1199-1959 (reaffirmed
2008): Methods of Sampling and Analysis of Concrete
2. IS 6461 (Part 10) -1973
(reaffirmed 2011): Glossary of Terms Relating to Cement Concrete; Part 10:
Tests and Testing Apparatus
REFERENCES
BS EN 12350-5:2000, Testing of Fresh Concrete: Flow Table Test
Gambhir, M. L., Concrete Technology, 4th edition, McGraw-Hill Education
(India), 2009
Gambhir, M. L. and Neha Jamwal, Building
Materials: Products, Properties and Systems, McGraw-Hill Education (India),
2011.
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