Slump test and types of slump

Think about the cone dimensions, the filling procedure, and what each failure shape suggests about the mix.

Model answer.

Purpose. The slump test is a quick site test for the workability (consistency) of fresh concrete.

Apparatus. A frustum of a cone, 300 mm high, with a 200 mm bottom diameter and a 100 mm top diameter, a 16 mm diameter tamping rod with a rounded end, and a base plate (as per IS 1199).

Procedure

1. Place the cone on a smooth, level, non-absorbent surface and hold it firmly.
2. Fill it in four roughly equal layers, tamping each layer 25 times with the rod.
3. Strike off the top level.
4. Lift the cone vertically and steadily, without twisting.
5. Measure the slump: the difference between the cone height and the highest point of the slumped concrete.

Types of slump

• True slump: The concrete subsides evenly and keeps roughly its shape. This is the only valid result for reporting slump.
• Shear slump: One side shears off and slides down. It suggests a lack of cohesion or a harsh mix. The test should be repeated; if shear slump recurs, the mix needs adjustment.
• Collapse slump: The concrete collapses completely. It indicates a very wet mix, or a high-workability mix such as one with superplasticiser. For flowing or self-compacting concrete, the slump-flow test is used instead.

Typical values (IS 456, clause 7.1). Roughly 25–75 mm for mass concrete and lightly reinforced sections, 50–100 mm for heavily reinforced sections, 75–100 mm for pumped concrete, and 100–150 mm for in-situ piling. The project specification has the final say.

Limitations

• Not suitable for very dry mixes (near-zero slump) or very fluid mixes.
• Results depend on operator technique.

For dry mixes, the compaction factor test or the Vee-Bee consistometer is more sensitive.

Common mistake. Adding water on site to "improve slump." This raises the water-cement ratio and lowers strength and durability. Use an approved plasticiser instead.

Likely follow-up. "When should the test be done?" Promptly after sampling, at the point of placing where possible, because concrete loses slump with time, especially in hot weather.