Quick answer: The slump value of concrete is the drop in height, in millimetres, of a cone of fresh concrete when the 300 mm slump cone is lifted off it. It shows how workable the mix is. IS 456 gives 25–75 mm for lightly reinforced work, 50–100 mm for heavily reinforced sections, 75–100 mm for pumped concrete and 100–150 mm for in-situ piling.
What is the slump value of concrete?
Fresh concrete has to be placed in the formwork, worked around the steel and compacted before it starts to set. How easily that can be done, without the stone separating from the mortar and without water bleeding to the top, is called workability. A mix with poor workability leaves voids and air pockets, and those reduce the strength, durability and finish of the member.
The slump test is the usual site check for workability because it needs very little equipment and gives a result in a few minutes. A steel cone is filled with fresh concrete and lifted away. The concrete settles, and the distance by which its top drops is the slump value, measured in millimetres. A stiff mix barely moves. A wet mix drops a long way.
Why the slump test is done
- To check workability: it shows whether the concrete suits the work it is meant for.
- To spot a change in water content: a sudden rise in slump from one batch to the next usually means extra water has gone in.
- To judge how the mix will mould: it indicates how easily the concrete can be placed and compacted.
- To verify the mix design: it confirms that the concrete delivered matches what was specified.
Slump value by type of work (IS 456)
IS 456, the Indian code for plain and reinforced concrete, links the slump to the placing conditions. The harder it is to get concrete into place and compact it, the higher the slump needed:
| Type of work | Degree of workability | Slump |
|---|---|---|
| Mass concrete; lightly reinforced slabs, beams, walls and columns; floors; hand-placed pavements; canal lining; strip footings | Low | 25–75 mm |
| Heavily reinforced concrete sections in slabs, beams, walls and columns | Medium | 50–100 mm |
| Slipform work; pumped concrete | Medium | 75–100 mm |
| Trench fill; in-situ piling | High | 100–150 mm |
The code also lists two cases at the extremes where slump is not the right measure: very stiff mixes such as blinding concrete, and very fluid tremie concrete. Those are checked by other tests.
Treat the table as a guide. The slump for your pour is the one written in the mix design or on the ready-mix order, and the site test checks the delivery against that figure. Ready-mix concrete for pumping is often ordered at a higher slump, achieved with plasticisers and not with extra water.
Slump cone test equipment
- Slump cone: a steel mould 300 mm high, with a bottom diameter of 200 mm and a top diameter of 100 mm, open at both ends.
- Tamping rod: a standard steel rod with a rounded end.
- Base plate: flat, rigid and non-porous.
- Measuring tape or scale.
Slump test procedure
- Clean the inside of the cone and apply a thin coat of oil.
- Place the cone on the base plate on level ground and hold it down firmly.
- Take a sample of freshly mixed concrete and fill the cone in three equal layers.
- Tamp each layer 25 times with the rod, spreading the strokes evenly over the surface. For the second and third layers, let the rod just enter the layer below.
- Strike off the top level with the rim of the cone and clear away any spilled concrete around the base.
- Lift the cone straight up, slowly and steadily, without twisting or jerking it.
- Set the cone beside the concrete and measure the difference between the height of the cone and the height of the slumped concrete. That difference, in millimetres, is the slump.
Do the test as soon as the sample is taken. Concrete stiffens with time, and a late test gives a lower reading than the concrete had when it arrived.
Types of slump
How the cone of concrete settles matters as much as the number.
True slump
The concrete settles evenly and keeps roughly its cone shape. This is the only type that gives a valid reading, and it is what a well-proportioned, cohesive mix does.
Shear slump
One side of the cone shears off and slides down. It points to a mix that lacks cohesion. Repeat the test with a fresh sample. If it shears again, the mix needs to be looked at.
Collapse slump
The concrete flattens out completely. The mix is too wet, or it is a high-workability mix for which the slump test is not suitable.
Zero slump
The concrete holds the shape of the cone with no measurable drop. This is normal for very stiff, dry mixes and a warning sign for anything meant to be placed around reinforcement.
What to do if the slump is off
- Slump too low: the concrete will be hard to place and compact, which leads to voids and honeycombing. Do not add water on site to loosen it. Extra water raises the water-cement ratio and lowers the strength. Ask the supplier or the site engineer to correct the mix.
- Slump too high: unless an admixture explains it, the mix probably has too much water. Expect segregation, bleeding, lower strength and a higher risk of shrinkage cracks. A load well outside the specified slump should be rejected, not poured.
Factors affecting the slump value
- Water content: the water-cement ratio has the largest effect.
- Cement type: different cements need different amounts of water for the same workability.
- Aggregate size and shape: rounded and larger aggregates give more slump than angular, flaky or finer ones for the same water.
- Admixtures: plasticisers increase slump without changing the water content.
- Temperature and time: concrete loses slump as it begins to set, and loses it faster in hot weather.
Limitations of the slump test
- It does not measure strength or durability. Those need cube tests and the other concrete testing methods.
- The result depends on the operator: how the cone is filled, tamped and lifted.
- It does not suit every mix. Very stiff and very fluid concretes fall outside its useful range.
Related reading
Slump is only one of the checks on a concrete pour. See the mix proportions for M15 concrete and M25 concrete, and the types of concrete curing that follow the pour. If you want the concrete on your site specified, tested and supervised, see our construction service.
FAQs
What is a good slump value for concrete?
It depends on the work. IS 456 gives 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 trench fill and in-situ piling. The figure in your mix design is the one to follow.
What slump is used for slabs, beams and columns?
IS 456 gives 25–75 mm where the slab, beam or column is lightly reinforced and 50–100 mm where it is heavily reinforced, because congested steel needs a more workable mix to compact properly.
What is the slump of pumped concrete?
IS 456 gives 75–100 mm for pumped concrete. Ready-mix suppliers often deliver pump mixes at a higher slump using plasticisers, so check the delivery against the slump stated in the mix design.
What are the types of slump?
True slump, where the concrete settles evenly; shear slump, where one side slides away; collapse slump, where the concrete flattens out; and zero slump, where it does not drop at all. Only a true slump gives a valid reading.
How many times is each layer tamped in the slump test?
The cone is filled in three equal layers and each layer is tamped 25 times with the tamping rod, with the strokes spread evenly across the surface.
Does a higher slump mean weaker concrete?
Not by itself. If the slump is high because extra water was added, the concrete will be weaker. If it is high because a plasticiser was used, the strength is not affected. The slump test measures workability, not strength.
