What is concrete slump?
Concrete slump is the number of millimetres fresh concrete sinks after a filled standard cone is lifted off it. The higher the slump, the more fluid the mix. The test is described in the European standard EN 12350-2 (and in ASTM C143 in the United States). The cone is called the Abrams cone, after the American researcher who developed the method.
How the test works
- The Abrams cone (top diameter 100 mm, bottom diameter 200 mm, height 300 mm) is placed on a flat, damp, non-absorbent base.
- The cone is filled in three layers; each layer is rodded with a tamping rod.
- The top is struck off level and the cone is lifted straight up in a steady movement.
- You measure the difference in height between the cone and the highest point of the slumped concrete. That is the slump.
If the concrete shears off or collapses to one side, the result is not valid and the test must be repeated.
Slump classes
| Class | Slump (mm) |
|---|---|
| S1 | 10-40 |
| S2 | 50-90 |
| S3 | 100-150 |
| S4 | 160-210 |
| S5 | ≥ 220 |
These classes come from the European concrete standard EN 206. Most everyday work falls in S2 to S4. For very fluid and self-compacting concrete, slump flow is measured instead, because the slump no longer tells mixes apart.
Application
- Ordering: ready-mixed concrete is ordered with a consistency class. Choose stiffer for slopes or strip footings, more fluid for congested reinforcement or pumping.
- Site checks: the slump test confirms whether the delivered concrete matches the order.
Practical tips
- Never add water on site to make the mix flow better. It raises the water-cement ratio and reduces strength and durability. If needed, use a plasticiser as advised by the supplier.
- Test promptly: slump decreases as the concrete ages, so test straight after delivery.
Related terms
- Consistency class
- Consistency range
- Slump flow
- Self-compacting concrete
- Superplasticizer
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