Slump test is done to assess the workability of concrete. It does not measure all factors contributing to workability, nor is it always representative of concrete placement. However, it is easily used as a control test and gives an indication of the uniformity of the concrete from batch to batch.
The apparatus for conducting the slump test essentially consists of a metallic mould in the form of a frustum of a cone having internal dimensions as follows:
The internal surface of the mould is thoroughly cleaned and freed from any superfluous moisture. The mould is placed on a smooth, horizontal, rigid and non-absorbent surface. The mould is then filled in four layers, each approximately one-fourth of the height of the mould. For tamping the concrete, a steel tamping rod of 16 mm diameter and 0.6 m length with a bullet end is used. Each layer is tamped 25 times by the tamping rod. After the top layer has been rodded, the concrete is struck off level with a trowel and tamping rod. The mould is removed from the concrete immediately by raising it slowly and carefully in a vertical direction. This allows the concrete to subside. The difference between the height of the mould and that of the subsided concrete is measured in mm and this is referred to as the slump of concrete.
It is seen that the slump test gives good consistent results for a plastic mix. This test is not sensitive for a stiff mix. Despite many limitations, the slump test is very useful on site to check the day-to-day or hour-to-hour variation in the quality of the mix.
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