Slump Testing of Fresh Concrete
The workability of a concrete mix is the ease with which it flows and is a function of the water/cement ratio, too little water in the mix means the concrete is stiff and hard to move around. This makes the placement of the mixt hard work, increases the risk of air voids and poor encasement of reinforcement. Too much water will improve the workability but reduce the strength of the cured concrete. The test method is defined in BS EN 12350-2 and involves the use of an open ended cone mould.
The mould is placed on a plate and is filled in three layers with the wet concrete. Each layer is “tamped” with a round rod 25 times. The top of the concrete is struck off level with the top of the mould and the mould is gently removed. The cone is then upturned and placed alongside the slumped concrete. Using a calibrated rule the depth to which the concrete has collapsed from the top of the cone is measured and the value recorded. This is known as the delivered slump and is compared to the designed slump. A mix that is too stiff can be wetted up to meet the required value, however a mix that is too wet should be rejected as it may be too weak. The water/cement ratio may have been compromised due to the excess water.
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