Slump Test of Concrete: Procedure (IS 1199), Types and Recommended Slump Values
Short answer: IS 456 recommends a slump of 25–75 mm for lightly reinforced slabs, beams, walls, columns, footings and floors, 50–100 mm for heavily reinforced sections, 75–100 mm for pumped concrete and slipform work, and 100–150 mm for trench fill and in-situ piling. Slump is measured with a 300 mm high cone as described in IS 1199.
This page covers the test procedure, how to read the result, the full IS 456 workability table and what to do on site when the slump is wrong.
What the slump test tells you
Slump is a quick measure of the consistency (workability) of fresh concrete. It does not measure strength, but a slump that suddenly changes from batch to batch is a strong sign that water, grading or mix proportions have changed. That is why the test is done at the point of placing, for every truck or at regular intervals for site-mixed concrete.
Apparatus
- Slump cone: a metal frustum, 200 mm diameter at the bottom, 100 mm at the top and 300 mm high, with foot pieces and handles.
- Tamping rod: steel, 16 mm diameter, 600 mm long, with a rounded (bullet-shaped) end.
- A smooth, rigid, level, non-absorbent base plate, a scoop, a trowel and a steel rule.
Procedure (IS 1199)
- Take a representative sample of concrete at the point of placing. Clean the cone and base plate and wipe them damp (not wet).
- Place the cone on the plate, narrow end up, and hold it down firmly by standing on the foot pieces.
- Fill the cone in four layers, each about one quarter of the height.
- Tamp each layer with 25 strokes of the rod, spread evenly over the cross-section. For the second and later layers, the rod should just penetrate into the layer below.
- After tamping the top layer, strike off the surface level with the top of the cone using the rod or trowel. Clean away any concrete that has fallen around the base.
- Lift the cone slowly and carefully straight up, without twisting or jerking, in one steady movement taking a few seconds.
- Immediately measure the slump: the difference between the height of the cone (300 mm) and the highest point of the slumped concrete. Record the value in millimetres along with the type of slump.
Complete the whole test quickly after sampling, because concrete loses slump with time, especially in hot weather. Some newer specifications and the international method fill the cone in three layers instead of four; check which edition of the standard your contract refers to.
Worked example: reading a slump
After lifting the cone, the highest point of the concrete is measured at 215 mm above the base plate. Slump = 300 − 215 = 85 mm. The concrete subsided evenly and kept its shape, so it is a true slump of 85 mm. For a lightly reinforced slab (IS 456 range 25–75 mm) this is on the wet side; for heavily reinforced columns (50–100 mm) it is acceptable.
Types of slump
| Type | What you see | What it means |
|---|---|---|
| True slump | Concrete subsides evenly and keeps roughly its cone shape | Valid result; record the value |
| Shear slump | One side shears off and slides down at an angle | Result not valid; repeat with a fresh sample. If shear recurs, the mix lacks cohesion (often too little fines or harsh grading) |
| Collapse slump | Concrete collapses and spreads out flat | Mix is too wet, or it is a very high workability / flowing concrete for which slump is not the right test |
| Zero slump | Concrete stays almost exactly in cone shape | Very stiff mix (e.g. for road bases); use compacting factor or Vee-Bee test instead |
Recommended slump values (IS 456)
IS 456:2000 gives the following guidance on workability for different placing conditions:
| Placing condition | Degree of workability | Slump (mm) |
|---|---|---|
| Blinding concrete; shallow sections; pavements using pavers | Very low | Use compacting factor (0.75–0.80) rather than slump |
| Mass concrete; lightly reinforced slabs, beams, walls, columns; floors; hand-placed pavements; canal lining; 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 | Use flow test rather than slump |
What to do when the slump is off
Slump too low (stiff mix)
- Check the time since batching and the temperature; slump loss is normal over time, faster in summer.
- For site-mixed concrete, check that the aggregates were not unusually dry or absorbent and that the water measure is correct.
- For ready-mixed concrete, ask the supplier's representative. Re-dosing with superplasticiser at site is acceptable only if the supplier's approved mix design allows it; mix thoroughly and retest before placing.
- Do not let workers add water from a hose or bucket. That is the most common cause of weak concrete on Indian sites.
Slump too high (wet mix)
- Look for bleeding and segregation: water on the surface, aggregate separating from mortar.
- Check moisture correction: if sand is wet and no correction was made, the mix gets extra water. Reduce added water for the next batch accordingly.
- For RMC, reject the load if the slump is outside the agreed tolerance. Specifications commonly allow about ±25 mm on the specified slump, but use the tolerance stated in your contract.
- Never "fix" a wet batch by throwing in loose cement on site without measurement; it leads to lumps and unpredictable strength.
Worked example: what adding water at site really does
A 6 m³ transit mixer carries M25 designed with 380 kg/m³ cement and w/c 0.45, so water = 0.45 × 380 = 171 L/m³. To make placing easier, the crew adds 60 litres to the drum.
- Extra water per m³ = 60 / 6 = 10 L/m³
- New water content = 171 + 10 = 181 L/m³
- New w/c = 181 / 380 = 0.476
That small-looking addition raised the water-cement ratio by about 6%, past the 0.45 the mix was designed for, and the strength and durability drop accordingly. More on this in water-cement ratio.
Site tips
- Specify the slump on the order (e.g. "M25, 100 ± 25 mm at site") so there is a clear acceptance criterion.
- Test at the point of placing, not only at the plant; transit time changes slump.
- Make cube samples from the same batch on which you do the slump test, and record both together.
- Keep the cone clean and undented; a bent cone gives wrong readings.
- Cure properly after placing; see curing of concrete. Practise these test values for exams in our PSC mock tests.
Frequently asked questions
What are the dimensions of a slump cone?
The slump cone is 300 mm high with a bottom diameter of 200 mm and a top diameter of 100 mm. The tamping rod is 16 mm in diameter and 600 mm long.
What is the slump value for RCC slabs and beams?
IS 456 recommends 25–75 mm for lightly reinforced slabs, beams, walls and columns and 50–100 mm for heavily reinforced sections, assuming vibration.
What slump is used for pumped concrete?
IS 456 suggests 75–100 mm for pumped concrete. In practice, ready-mixed pumped concrete is often supplied at higher slump using superplasticisers, as set in the mix design.
What does a shear slump mean?
A shear slump, where one side of the concrete slides off, makes the test invalid and it should be repeated. If it happens again, the mix lacks cohesion.
How many layers and blows are used in the slump test?
Under IS 1199 the cone is filled in four layers, each tamped with 25 strokes of the 16 mm rod. Some newer specifications use three layers, so check the edition your contract cites.
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