Theory:
A concrete is said to be workable if it can be easily mixed, placed, compacted and finished. A
workable concrete should not show any segregation or bleeding. Segregation is said to occur
when coarse aggregate tries to separate out from the finer material and a concentration of
coarse aggregate at one place occurs. This results in large voids, less durability and strength.
Bleeding of concrete is said to occur when excess water comes up at the surface of concrete.
This causes small pores through the mass of concrete and is undesirable.
Unsupported fresh concrete flows to the sides and a sinking in height takes place. This
vertical settlement is known as slump. The slump is a measure indicating the consistency or workability of cement concrete. It gives an idea of water content needed for concrete to be used for different works. To measure the slump value, the test fresh concrete is filled into a mould of specified shape and dimensions, and the settlement or slump is measured when supporting mould is removed. The slump increases as water-content is increased. For different works different slump values have been recommended. The following table indicates the relationship between degree of workability and slump value
Slump test is adopted in the laboratory or during the progress of the work in the field for determining consistency of concrete where nominal max., size of aggregates does not exceed 40 mm. The pattern of slump indicates the characteristics of concrete in addition to the slump value. If the concrete slumps evenly it is called true slump. If one half of the cone slides
down, it is called shear slump. In case of a shear slump, the slump value is measured as the difference in height between the height of the mould and the average value of the subsidence.
Shear slump also indicates that the concrete is non-cohesive and shows the characteristic of segregation. Any slump specimen, which collapses or shears off laterally gives incorrect results and at this juncture the test is repeated only true slump should be measured.
Although, slump test is popular due to the simplicity of apparatus used and simple
procedure, unfortunately, the simplicity is also often allows a wide variability and many time it could not provide true guide to workability. For example, a harsh mix cannot be said to have same workability as one with a large proportion of sand even though they may have the same slump.
The slump cone experiment is conducted in an apparatus called slump cone (Figure 6).
This apparatus essentially consists of a metallic mould in the form of a frustum of a cone having the internal dimensions as under: Bottom diameter: 20 cm, Top diameter: 10 cm,
Height: 30 cm and the thickness of the metallic sheet for the mould should not be thinner than 1.6 mm.
Objective:
To determine the workability of freshly mixed concrete by the use of slump
test.
Reference:
IS: 7320-1974, IS: 1199-1959, SP: 23-1982.
Apparatus:
Slump cone, tamping rod, metallic sheet, weighing machine and scale.
Material:
Cement, sand, aggregate and water
Procedure:
1. Clean the internal surface of the mould thoroughly and place it on a smooth
horizontal, rigid and non-absorbent surf ace, such as of a metal plate.
2. Consider a W/C ratio of 0.5 to 0.6 and design mix of proportion about 1:2:4 (it
is presumed that a mix is designed already for the test). Weigh the quantity of cement, sand, aggregate and water correctly. Mix thoroughly. Use this freshly prepared concrete for the test.
3. Fill the mould to about one fourth of its height with concrete. While filling,hold the mould firmly in position
4. Tamp the layer with the round end of the tamping rod with 25 strokes disturbing the strokes uniformly over the cross section.
5. Fill the mould further in 3 layers each time by 1/4th height and tamping evenly
each layer as above. After completion of rodding of the topmost layer strike of
the concrete with a trowel or tamping bar, level with the top of mould.
6. Lift the mould vertically slowly and remove it.
7. The concrete will subside. Measure the height of the specimen of concrete after
subsidence.
8. The slump of concrete is the subsidence, i.e. difference in original height and height up to the topmost point of the subsided concrete in millimetres.
Observation:
Result:
The slump of concrete is: ________ mm.
Conclusions:
The slump value indicates that the concrete has Very low/ Low/ Medium/ High degree of workability.
Precautions:
(Discuss about the precautions to be taken while conducting this experiment)
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Questions:
1. What is meaning of consistency in concrete?
2. What is slump of concrete?
3. What is the significance of shear slump?
4. What is segregation?
