What is slump and why is it important to the finisher?

• A. Slump measures the color of concrete
• B. Slump measures concrete workability; it affects placement, consolidation, and the ability to finish without segregation or laitance
• C. Slump is the time until initial set
• D. Slump is the amount of water added after placement

Answer: (B)

Slump is a measure of the concrete’s workability—the ease with which fresh concrete can be placed, consolidated, and finished. It’s determined by a slump test, where a sample of fresh concrete is placed in a conical mold, the mold is lifted, and the amount the concrete “slumps” or subsides is measured. A higher slump means the mix is more workable and fluid; a lower slump means it’s stiffer and harder to work with.

For the finisher, this matters because workability directly affects placement around forms, filling joints, and achieving a proper finish with vibration. If the slump is too high, extra water can cause segregation and the formation of laitance (a weak, milky surface layer) during finishing. If the slump is too low, the mix is hard to place and consolidate, making finishing difficult and increasing the risk of surface defects. The goal is to have the specified slump so finishing operations can proceed smoothly, producing a clean, durable surface without segregation or laitance.