The slump test of concrete measures the workability of fresh concrete. You fill a standard cone (300 mm high, 200 mm base, 100 mm top) with concrete in layers, tamp each layer 25 times with a 16 mm rod, lift the cone straight up and measure how far the concrete drops. That drop in millimetres is the slump. For ordinary beams, slabs and columns with light reinforcement, IS 456 suggests a slump of about 25 to 75 mm.
The test takes under five minutes, needs cheap equipment and catches the most common site problem: too much water added to make the concrete easy to place. It is used in labs, on ready-mix deliveries and on house construction sites across Nepal. This post covers the apparatus, the procedure, how to read the result and which slump values suit which work.
Apparatus for the slump test
| Item | Size / detail |
|---|---|
| Slump cone (mould) | Frustum of a cone: 300 mm high, 200 mm bottom diameter, 100 mm top diameter, made of metal, with foot pieces and handles |
| Tamping rod | Steel rod, 16 mm diameter, 600 mm long, with a rounded end |
| Base plate | Rigid, flat, non-absorbent surface such as a steel plate |
| Measuring scale | Steel rule or tape reading in mm |
| Others | Scoop, trowel, oil or damp cloth to wet the mould |
Slump test procedure step by step
- Clean the inside of the cone and dampen it. Place it on the base plate, which should be level and firm.
- Stand on the foot pieces to hold the cone down firmly.
- Fill the cone in three layers of roughly equal height. Rod each layer 25 times, spreading the blows evenly over the area. For the second and third layers, let the rod just enter the layer below.
- Heap concrete over the top while rodding the last layer, then strike off the surface level with the top of the cone using the rod or a trowel.
- Clean spilled concrete from around the base.
- Lift the cone vertically, slowly and steadily, in about 5 to 10 seconds, without twisting or jerking.
- Place the empty cone upside down beside the concrete, lay the rod across its top, and measure from the underside of the rod down to the highest point of the slumped concrete. This is the slump in mm.
- Complete the whole test from start of filling to measuring within about 2.5 minutes.
Note on layers: the current version of IS 1199 (Part 2), like ASTM C143 and EN 12350-2, uses three layers. The old 1959 edition of IS 1199 used four layers of 25 strokes each, and some lab manuals in Nepal still show that. If your institution or contract names a specific standard, follow it.
Worked example: reading the slump
A site engineer tests concrete for a house roof slab. The cone height is 300 mm. After lifting the cone, the highest point of the concrete is 235 mm above the base.
Slump = 300 − 235 = 65 mm.
The slab is lightly reinforced and placed by hand with a needle vibrator, so the target range is 25 to 75 mm. A 65 mm slump is acceptable.
Suppose the next batch shows only 120 mm of concrete height remaining. The slump is then 300 − 120 = 180 mm, far above the target. That almost always means extra water was added at the mixer. Do not accept it for the slab without checking with the engineer.
Why extra water is a problem
Strength depends heavily on the water-cement ratio. If a mix is designed with 25 litres of water per 50 kg bag of cement, the ratio is 25/50 = 0.50. Adding just 5 more litres per bag to make placing easier raises it to 30/50 = 0.60. That change can cut strength noticeably and increase shrinkage cracks. If more workability is needed, the correct fix is a plasticiser or a revised mix, not a bucket of water. The concrete calculator helps you work out cement, sand and aggregate per batch so the water can be controlled per bag.
Types of slump
- True slump: the concrete subsides evenly and keeps roughly its shape. This is the only shape that gives a valid reading.
- Shear slump: one side slides off along a slope. Repeat the test with a fresh sample. If shear happens again, the mix lacks cohesion, often from poor grading or too little fines, and should be reported.
- Collapse slump: the concrete falls flat and spreads. The mix is either far too wet or is a high-workability mix where a flow test is more suitable than a slump test.
- Zero slump: the concrete hardly moves. Typical of very stiff mixes for road bases or precast blocks; workability for these is better judged with the compacting factor or Vee-Bee test.
Slump values for different works (IS 456)
IS 456 gives these workability guidelines based on placing conditions:
| Placing condition | Degree of workability | Slump (mm) |
|---|---|---|
| Blinding concrete, shallow sections, pavements laid with pavers | Very low | Use compacting factor instead |
| Mass concrete, lightly reinforced slabs, beams, walls, columns, floors, hand-placed pavements, canal lining, strip footings | Low | 25 to 75 |
| Heavily reinforced slabs, beams, walls, columns | Medium | 50 to 100 |
| Slipform work, pumped concrete | Medium | 75 to 100 |
| Trench fill, in-situ piling | High | 100 to 150 |
| Tremie concrete | Very high | Use flow test |
For a typical two or three storey RCC house in Nepal with columns and beams that have congested bars at joints, 50 to 100 mm is often more practical than 25 mm, as long as the extra workability comes from a proper mix and not extra water. Ready-mix suppliers in Kathmandu commonly deliver pumped concrete in the higher ranges, so specify the slump in your order.
Limitations of the slump test
- It is not suitable for very dry or very fluid concrete.
- It does not work well for concrete with coarse aggregate larger than about 40 mm.
- Two mixes with the same slump can behave quite differently under vibration, so it is a control test, not a full measure of quality.
- It says nothing directly about strength. Cube tests are still needed. If you are preparing for licence or loksewa exams, slump and cube test questions come up often; practise them with the engineering MCQ practice tool.
Keeping a simple slump record for each pour, along with the cube samples, gives the owner and the engineer something solid to check later. To see how concrete quality and quantity fit into a full budget, try the house construction cost estimator.
Frequently asked questions
What is a good slump for RCC slabs and beams?
For lightly reinforced slabs and beams, 25 to 75 mm as per IS 456. For heavily reinforced members, 50 to 100 mm. Pumped concrete is usually 75 to 100 mm or more, as per the mix design.
How often should the slump test be done on site?
Test the first batch of each pour and then whenever the concrete looks different, the mixer operator changes or water is adjusted. On larger pours, test each truck of ready-mix concrete on arrival.
Can I use a plastic slump cone?
Standards specify a rigid, non-absorbent mould, normally metal. Plastic cones are sold, but check that they meet the required dimensions and are stiff enough not to deform.
Is slump the same as workability?
Slump is one measure of workability, specifically of consistency. Other tests such as compacting factor, Vee-Bee and flow table measure it in different ways for different kinds of mix.
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