Curing

Curing means keeping concrete moist and at a suitable temperature for long enough that the cement can hydrate. Hydration needs water. If the surface dries early, the outer few centimetres, the cover zone that protects the steel, never develop their full strength and density, and plastic and drying shrinkage cracks appear. Good curing is cheap, and poor curing undoes good mix design.

Methods fall into two groups. Water-adding methods keep the surface wet: ponding on slabs, sprinkling, wet hessian or sand, and immersion for precast units. Water-retaining methods stop evaporation: plastic sheets, leaving formwork in place and curing compounds sprayed on as a membrane. Accelerated methods such as steam curing are used in precast yards to reach handling strength quickly.

The minimum curing period depends on cement and weather. Ordinary Portland cement concrete is commonly cured for at least seven days, and longer for blended cements with fly ash or slag, which gain strength more slowly, and in hot, dry or windy weather. Hot-weather concreting adds measures such as cooled water, ice, shade and night pours. Cold weather slows hydration and risks freezing of young concrete. Mass concrete needs temperature control because the core heats up while the surface cools, and the difference causes cracking.