Curing and Drying

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.

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.

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