Most local building codes require that decks rest upon concrete footings. They are piers created by placing forms or mold in the ground and pouring cement in the form. The codes typically set a minimum diameter and a depth for the footings. In cold climates, the code may also require that the footings extend below the frost line.
The hardening process involving concrete is referred to as curing rather than drying. Concrete does not harden simply because the water inside of it evaporates. It hardens because of a complex process that changes its chemical makeup. It's best if the concrete remains wet during the early stages of the process for proper curing. For this reason, water is sometimes sprayed onto concrete as it cures keeping it moist.
The amount of time needed for a chemical reaction to take place often depends on the temperature. Curing concrete is no exception. Warm weather usually leads to faster curing times than do cold conditions. However, extremely hot weather makes the concrete lose too much water too quickly. Humidity also plays a role in the time it takes for a concrete footing or foundation to cure. It's better to pour deck footings in warm, humid conditions than it is in cold and dry weather.
The curing rate is accelerated by adding certain chemicals into the concrete mix. Two frequently used compounds for quickening curing are calcium chloride and sodium chloride, which are salts. Rapid curing affects the concrete’s ultimate strength after the curing process is completed. Concrete that cures by an accelerated process remains slightly weaker than naturally cured concrete.
No single answer exists for how long a deck footing needs cure before building the deck on top of it. The Portland Cement Association advises that conventional concrete cures for five to seven days for best results. Some manufacturers make rapid-curing concrete for deck footings that cure in 24 hours. Check with the product manufacturer or your local distributor for more information.