Workability tests

Workability is how easily fresh concrete can be mixed, placed and compacted without segregating. It is checked on site on every pour because a wrong water content is the commonest cause of weak or honeycombed concrete.

The slump test is the everyday method. A truncated cone (300 mm high, 200 mm base, 100 mm top) is filled in layers, each rodded a set number of times, then lifted straight up. The fall of the concrete is the slump. A true slump keeps its shape and sinks; a shear slump means the mix lacks cohesion; a collapse means it is very wet or a flowing mix. IS 1199 (Part 2), ASTM C143, EN 12350-2 and JIS A 1101 describe the method; each has its own details and we summarise in our own words only.

For very stiff mixes the slump is near zero and tells little, so the compaction factor or Vee-Bee time is used. For very fluid and self-compacting concretes, the slump-flow spread is measured instead.

Slump is not a measure of strength. Two mixes can have the same slump and different strengths if one has more cement or a superplasticiser.