The slump test measures concrete workability and uses a 12-inch high truncated cone mold, narrowing from eight inches at the base to four inches at the top, securely attached to a flat base before the concrete is filled.
The concrete is added to the dampened mold in three layers, each compacted 25 times using a steel tamping rod with a five-eighths-inch diameter and rounded end. Finally, the top surface is struck off.
After filling, the cone is gently lifted, and the concrete slumps. This decrease in its height, called a slump, is measured to the nearest quarter inch.
A uniform slump is called a true slump, while concrete that slides down the mold indicates a shear slump, which requires retesting. Persistent shear slump suggests the mix lacks cohesion.
Sometimes, the concrete may collapse for lean mixes, which highlights the slump test's unreliability for lean mixes.
Slump, either too high or too low, serves as an immediate alert, prompting correction of the issue with the mix in bulk concreting. Recommended slump values for different workabilities are predetermined.
