Concrete Slump Test
Thirty seconds with a cone, and the first decision of the pour: this test measures consistency, not quality, but it catches a wrong load before it is in the forms.
| Standards | ASTM C143, CSA A23.2 5C |
| The cone | 300 mm tall, 200 mm base, 100 mm top |
| Filling | three equal layers, 25 strokes of the tamping rod each |
| Footings and pavements | 25 to 75 mm |
| General structural work | 75 to 100 mm |
| Pumped walls and columns | 100 to 150 mm |
| Self consolidating mixes | slump flow instead, spread of 550 to 750 mm |
| Common tolerance | plus or minus 30 mm of specified |
How the test works
Fill the cone in three rodded layers on a rigid flat base, strike off the top, then lift the cone straight up in three to five seconds. The concrete subsides, and the slump is the distance from the top of the cone to the displaced original centre of the sample. If the sample shears sideways or collapses rather than slumping evenly, the result does not count: retest, and if it shears again, that behaviour is itself information about the mix.
Where it goes wrong
The expensive mistake happens after the test: water added on site to buy back slump. It works, the concrete flows, and the water cement ratio quietly climbs while strength and durability fall, which is why retempering beyond the batched water needs the producer’s blessing, not the pump operator’s. Sample from the middle of the load, not the first wheelbarrow. And on superplasticized or self consolidating mixes the ordinary slump number stops meaning anything: switch to slump flow and judge the spread.
Related Calculators and Reference
- Concrete Mix Volume and Quantity Estimator: the mix the slump is checking.
- Concrete Cylinder Test: the strength half of the same sampling.
- Unit Weight of Concrete: the density behind the yield check.
- Modulus of Elasticity of Concrete: what the strength eventually buys.
- American Concrete Institute: practice guidance for placement and QC.