Portland cement concrete paving is also referred to as rigid paving. Concrete pavement is supported by a base layer of compacted aggregates or treated soils, which is in turn placed upon the foundational subgrade soils of the prepared roadbed. If two base layers are utilized, they are referred to as the base and sub-base layers (or courses). Rigid (concrete) pavement differs from flexible (asphalt) pavement in that rigid pavement is designed to carry traffic loads within the pavement layer itself.