Procedure

The procedure for performing the test is as follows-

• The internal surfaces of the mould shall be thoroughly cleaned and freed from superfluous and any set concrete before commencing the test.

• The mould shall be placed on a smooth, horizontal, rigid and non-absorbent surface, such as a metal base plate properly cleaned can be used.

• The mix prepared shall be brought near the testing assembly, the mould should be filled with concrete in three layers each one-third of the height and each layer must be tamped thoroughly in circular motion by giving 25 blows each with the tamping rod.

• After the top layer has been compacted, the surface should be levelled off using a trowel or the tamping rod itself.

• The mould shall be removed from concrete immediately by raising it slowly and carefully in the vertical direction.

• This will allow the concrete to subside, the slump height in mm then can be measured with the help of a graduated scale by finding the difference between the height of the inverted cone & highest point of slump formed.

• Any slump specimen which collapses or shears off laterally gives incorrect result and if this occurs, the test shall be repeated with another sample, if in repeat test also again this happens, the slump shall be measured and the fact that specimen sheared, shall be recorded.

Workability Classification

Types of Slump

The slump can be of the following four types-

• TRUE SLUMP refers to a general drop of concrete mass evenly all around without disintegration.

• 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 the concrete mix is too wet and the mix is regarded as harsh and lean.

• Concrete with slump height 0 is referred to as ZERO SLUMP concrete.

In short, higher w/c ratio leads to more slump height and hence more & better workability, reverse is the case with low w/c ratio concrete.