Q.: I frequently see the terms subbase and subgrade, and they confuse me. Are they the same or different? If different, which one goes on top?

A.: In concrete paving terminology, the subgrade is the soil the pavement is built on. Usually it is treated by compacting it sufficiently to prevent the pavement from settling. Subbase is composed of the layer(s) of material between the subgrade and the concrete pavement. A subbase may not always be required. The subbase may be econocrete, cement-treated subbase, soil-cement, asphalt-treated subbase, crushed stone or other suitable material.