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.

How to choose

"Air-entrained" is a word to look for, not infer

It's a mix specification that must be ordered. It won't appear by accident on a low-bid quote.

Higher slump is not better

It means wetter. NRMCA's 3 to 5 inch guidance exists because wetter concrete bleeds, segregates, and scales.

Distinguish scaling from spalling in conversation

Scaling is surface flaking from freeze-thaw and salt. Deeper loss around reinforcement is a different diagnosis with a different fix.

Contraction joints and expansion joints are not the same thing

One controls where cracks go; the other separates the slab from things that restrain it. A good plan has both.

Ask for ASTM references on curing products

Curing compounds should conform to ASTM C309 or C1315; plastic sheeting to ASTM C171, at least 4 mils thick.

"Fiber mesh" is not "rebar"

NRMCA lists replacement of structural steel reinforcement among the things synthetic fibers should not be used for.