The slump test is the quickest check you can make on concrete before it goes into your slab. It takes two minutes, needs one inexpensive cone, and catches the most common cause of weak concrete on Indian sites: water added at the mixer.
The Equipment
- A slump cone: 300 mm high, 200 mm base diameter, 100 mm top diameter.
- A tamping rod: 16 mm diameter, 600 mm long, rounded end.
- A flat, non-absorbent base plate and a ruler.
How to Do It (IS 1199)
- Place the cone on the level, damp base plate and hold it firmly by the foot rests.
- Fill in four equal layers, rodding each layer 25 times evenly over its area.
- Strike off the top level.
- Lift the cone straight up, smoothly, in about 5–10 seconds.
- Measure the drop from the top of the cone to the highest point of the concrete — that is the slump in mm.
- Do it within a couple of minutes of sampling.
Typical Slump Values (IS 456 guidance)
| Placing condition | Slump |
|---|---|
| Mass concrete, lightly reinforced slabs, beams, walls, columns (vibrated) | 25 – 75 mm |
| Heavily reinforced slabs, beams, walls, columns | 50 – 100 mm |
| Pumped concrete | 75 – 100 mm |
| Trench fill, in-situ piling | 100 – 150 mm |
Agree a target slump with your engineer or ready-mix supplier and check every batch against it.
Reading the Result
| Shape | Meaning |
|---|---|
| True slump — even subsidence | Normal; measure it |
| Shear slump — one side slides | Lacks cohesion; repeat; if repeated, the mix needs adjusting |
| Collapse — concrete flattens | Far too wet or poorly proportioned; reject |
What to Do With a High Slump
A slump well above the target in site-mixed concrete almost always means extra water. Extra water means more pores, lower strength and easier rusting of steel.
- Do not accept "it will be fine".
- Re-batch with the correct water, accounting for wet sand.
- If the concrete genuinely needs more flow to place around steel, use a plasticiser.
The Short Version
Cone, four layers, 25 blows each, lift, measure. Aim for about 25–75 mm on vibrated house work, and treat a slump far above target as a warning about water, not a convenience.
