Curing is one of the most important steps in concrete construction because proper curing greatly increases concrete strength and durability. Concrete hardens as a result of hydration: the chemical reaction between cement and water. However, hydration occurs only if water is available and if the concrete's temperature stays within a suitable range. During the curing period, from five to seven days after placement for conventional concrete, the concrete surface needs to be kept moist to permit the hydration process. The most common method of curing (and one of the simplest) is to utilize a liquid membrane, which is sprayed or rolled on the surface of a slab after finishing to prevent premature drying of the surface. Water may be used when you are able to ensure constant wetting for at least 3 days.
Curing in Extreme Weather
Temperature extremes make it difficult to properly cure concrete. On hot days, too much water is lost by evaporation from newly placed concrete. If the temperature drops too close to freezing, hydration slows to nearly a standstill. Under these conditions, concrete ceases to gain strength and other desirable properties. In general, the temperature of new concrete should not be allowed to fall below 50°F during the curing period. Membrane curing does not keep the concrete warm enough in freezing temperature, nor does calcium chloride accelerator prevent concrete from freezing. Only adequate insulation or heating will maintain proper curing temperatures during freezing weather.
Curing Shortcuts to Avoid Compaction
Avoid any curing method that lets the surface dry in a short time. Quick drying stops the hardening process, thus making a weak surface that is likely to scale and/or crack.
After curing, newly placed outdoor concrete needs time to dry in warm air. For best results, plan to place your concrete early enough in the season so that it has one month of temperatures above 40°F for curing and still another month for drying out before hard freezes are expected.