Slump is vertical settlement of the
concrete after the mould has been withdrawn and the vertical difference between
height of mould and the highest point of subsided concrete.
Following slump test apparatus is
used to do slump test.
The apparatus for determining the slump
includes following items
1- Slump Cone ( Steel mould in form of a
truncated cone).
2- Tamping Rod (Is used to tamp the slump cone
and for Roding of concrete
3-Measuring Tape (measure vertical displacement
of concrete after removal of cone)
4-Smooth Surface Sheet (Provides smooth surface
to keep slump cone vertical).
5-Trowel. (Used to Fill the concrete in cone).
6-Hand Level (after removal of cone Hand level
is placed horizontally over the Slump cone to measure vertical displacement of
concrete.)
Testing Procedure
Place the slump cone on leveled sheet and fill the cone with concrete in 3" consecutive layers with Roding to compact concrete until it fills up to the top of cone. During filling of concrete cone should be in hold with placing of feet on lower pedestals and may be lifted above holding the hooks fixed in the middle of the cone. Now after removal of cone place the cone near settled concrete and place the hand level on the top of cone extending up to the top of concrete and then measure vertical displacement with measuring tape.
What should be the slump of concrete.
Although the slump test is not entirely
satisfactory since it gives widely varying results and also does not give a
true measure of workability but it is of value in the field as a control test
and is useful in comparing the consistence of successive batches of concrete
made with the same ingredients ,and is one of the simplest tests to carry out at
site. Provided no change is made in the aggregate or it's grading ,slump tests
will indicate whether correct water and cement contents are being maintained. For
a given slump and aggregate grading,
water required for the unit volume of concrete is constant irrespective of the
change of the cement content.
The amount of the slump depends not only on the amount of water in the mix but also on the nature of aggregates ; rounded stones give a greater slump than angular stones for the same mixture.
|
Recommended
values for slump in Millimeters. |
|||
|
S No |
Types
of Work |
With Vibrations
|
Without
Vibrations |
|
1 |
Mass Concrete, Large sections, roads
|
10 to
25 |
50 to
75 |
|
2 |
Foundations ,Footings ,Sub-Structures,
Walls and other heavy sections. |
26 to
50 |
40 to
115 |
|
3 |
Thin Sections such as slabs
,columns, beams with congested reinforcements |
40 to
80 |
100
to 175 |
How slump is recommended ?
This is very important to understand about the practical
recommendations of slump and here is the details which includes Selections of
materials and proceeding with job mix formula with number of hit and trial
tests to obtained required strength of concrete. During each test of job mix
slump test has been conducted and recorded in laboratory and when that slump is
recommended on which designed strength of concrete has been obtained.
Types of Slump
There are three types of Slumps .
True
slump refers to common drop of the concrete mass evenly all around without collapse.
Shear
slump indicates that the concrete lacks cohesion. It may undergo segregation
and bleeding and thus is undesirable for the durability of concrete.
Collapse slump indicates
that concrete mix is too wet and the mix is regarded as harsh and lean.
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