There are varying types of slab on grade construction, but often it refers to a slab that is a single layer of concrete and several inches thick. There are three basic / common types used depending on its application:
- Floating slab – can take various forms: (a) a concrete pad that floats on soil or gravel, perhaps for use as a patio or a shed to rest on. Generally, these have little or no structural value as a result. (b) more commonly the other style floating concrete slab rests between the foundation walls of a house with crushed stone beneath it and might act as a basement or garage floor.
- Supported slab – is similar to a floating slab, however it rests on a ledge created in the foundation wall (see right picture). It too rests on gravel.
- Monolithic slab – is a continuous pour of concrete that is thicker at the edges to form a footing (with steel reinforcing rods used to strengthen them). The slab rests on a bed of crushed gravel to improve drainage.
Slabs are often built or move out of level, and can contain cracks with no significant vertical or horizontal displacement and not regarded as a concern. When designed correctly the slab should evacuate any water that might potentially accumulate.