Concrete glossary

Compressive strength (psi) - how much load the hardened concrete can take. NRMCA points to 4,000 psi for exterior slabs facing freezing and deicers, 3,500 psi otherwise. Air entrainment - deliberately mixed-in microscopic air bubbles that give freezing water room to expand; NRMCA recommends 6 percent total air for typical 3/4-inch or 1-inch aggregate mixes. Slump - a measure of how wet and flowable the mix is; NRMCA recommends 3 to 5 inches for most slab work.

Contraction (control) joint - a deliberately weakened plane, tooled or sawcut, that decides where the concrete cracks. Isolation (expansion) joint - a full-depth separation between the slab and walls, footings, columns, or adjoining slabs so they can move independently. Construction joint - the surface where two placements of concrete meet, typically at the end of a day's work.

Curing - keeping the new concrete moist and warm enough for hydration to continue, typically 3 to 7 days. Bleed water - water rising to the surface after placement; finishing before it has dissipated causes surface defects. Plastic shrinkage cracks - surface cracks forming within hours, roughly parallel and 1 to 3 feet apart.

Scaling - local flaking or peeling of the hardened surface from freeze-thaw cycles, with moderate cases losing 1/8 to 3/8 inch of surface mortar. Crazing - a fine network of shallow surface cracks; cosmetic. Curling - edges lifting as the top of the slab dries faster than the bottom. Subgrade - the compacted soil or granular fill the slab sits on.