Question

A concrete slab can be described in terms of flatness and levelness. What is the difference, and if a problem occurs with one or the other, is it likely due to the finishing or the screeding of the concrete?

Answer

Flatness in concrete is thought of as smoothness — how bumpy or smooth the slab is. The flatness is determined by the concrete finisher. The levelness is a little different in that it is the average change in elevation over a longer distance, for example one-half inch in ten feet (one centimeter in three meters). The levelness of the slab is determined by how well the concrete is screeded. Screeding is the process of raking the wet, newly poured concrete to the level of the desired elevation. Finishing is the smoothing/ troweling process that occurs as the concrete is setting up and hardening.