Slump test

The slump test has been commonly used on site because of its simplicity. For a given mix of aggregates and cement, slump is governed by the quantity of mixing water used. In general, the higher the slump value, higher the workability of the fresh concrete. The slump is defined as shown in the figure below.

In order to perform the slump test in accordance with AS 1012.3.1:2014, a steel slump cone is placed on a solid, impermeable, level base and filled with the fresh concrete in three equal layers. Each layer is rodded 25 times while holding the cone, to ensure compaction. The strokes should be distributed in a uniform manner over the cross-section of the cone and each layer has to be penetrated when tamping. After the third layer has been rodded, the level of the concrete has to reach the tip of the cone. The cone should be carefully lifted up, leaving a heap of concrete that settles or ‘slumps’ slightly. The upturned slump cone is placed on the base to act as a reference, and the difference in level between its top and the top of the concrete is measured and recorded to the nearest 5 mm to give the slump of the concrete.

When the cone is removed, the slump may take several forms. For a slump to be acceptable the concrete simply subsides, keeping more or less to the shape of the cone. In a shear slump the top portion of the concrete shears off and slips sideways. In a collapse slump, the concrete collapses completely. Only a true slump is of any use in the test. If a shear or collapse slump occurs, a fresh sample should be taken and the test repeated.

Figure: Types of slump