6.5 : Slump Test

The slump test is a widely used method to measure the workability of concrete. It employs a 12-inch high truncated cone mold that tapers from eight inches at the base to four inches at the top. Before testing, the mold is securely attached to a flat base and dampened.

Concrete is poured into the mold in three layers to conduct the test. Each layer is compacted 25 times with a steel tamping rod, which has a five-eighths-inch diameter and a rounded end, to ensure even distribution and eliminate air pockets. After compacting, the top surface of the concrete is leveled off.

Once the mold is filled, it is gently lifted upward, allowing the concrete to slump downward due to gravity. The decrease in height of the concrete, referred to as the slump, is then measured to the nearest quarter inch. A proper slump, where the concrete maintains its conical shape, is a true slump. If the concrete collapses or spreads out, indicating a shear slump, the test may need to be repeated as it suggests the mix lacks cohesion and could be problematic for structural applications.

Excessively high or low slump values are indicators of potential issues in the concrete mix, requiring adjustments. The slump test is particularly effective for identifying the immediate need for corrections in bulk concreting operations, although it may be less reliable for lean mixes where the concrete tends to collapse. Recommended slump values are predetermined based on the specific requirements for different types of workability.