Slump is the quickest check that the concrete arriving on site has the workability the pour needs. A slump far above specification usually means water was added, and that costs strength and durability.
The Procedure
- Use a standard slump cone: 300 mm high, 200 mm base diameter, 100 mm top diameter, on a flat, non-absorbent base.
- Fill in three layers of roughly equal height. Tamp each layer 25 times with a 16 mm rod.
- Strike off the top and lift the cone vertically and steadily, in about 5 to 10 seconds.
- Measure the drop from the top of the cone to the highest point of the concrete.
Reading the shape: a true slump subsides evenly and is valid. A shear slump (one side slides off) means repeat the test. A collapse means the mix is far too wet, unless it is a designed flowing concrete.
Workability Ranges (IS 456, Clause 7.1)
| Placing condition | Degree of workability | Slump (mm) |
|---|---|---|
| Blinding concrete, shallow sections, pavements using pavers | Very low | See IS 456 clause 7.1.1 |
| Mass concrete, lightly reinforced slabs, beams, walls, columns, floors, strip footings | Low | 25 – 75 |
| Heavily reinforced slabs, beams, walls, columns | Medium | 50 – 100 |
| Slipform work, pumped concrete | Medium | 75 – 100 |
| Trench fill, in-situ piling | High | 100 – 150 |
| Tremie concrete | Very high | See IS 456 clause 7.1.1 |
For bored cast-in-situ piles, IS 2911 (Part 1/Sec 2) specifies a tremie concrete slump of 150 to 180 mm at the time of pouring. See our pile foundation guide.
Site Rules We Enforce
- The specified slump is written on the pour card before the pour starts
- Slump is tested for the first load and at regular intervals, recorded against the cube samples
- No water is added at site to raise slump. If workability is needed, it comes from the mix design and admixtures.
Reference
- IS 456:2000, clause 7 (Workability of concrete)
- IS 1199, Methods of sampling and analysis of concrete (slump test)
- IS 2911 (Part 1/Sec 2):2010, clause 7.3
Summary for site use. Always work from the current edition of the standard and your project specification. Where they differ, the structural engineer’s specification governs.