Frozen materials should never be used. Concrete should not be cast on a frozen subgrade, and ice must be removed from forms before concreting. Concrete allowed to freeze wet, before or during early curing, may be seriously damaged. Furthermore, temperatures should be kept above 40F for any appreciable curing (strength gain).
Concrete suppliers are equipped to heat materials and to deliver concrete at controlled temperatures in cold weather. These services should be utilized.
In very cold weather, for thin sections used in buildings, the freshly cast concrete must be enclosed and provided with temporary heat. For more massive sections or in moderately cold weather, it is usually less expensive to provide insulated forms or insulated coverings to retain the initial heat and subsequent heat of hydration generated in the concrete during initial curing.
The curing time required depends on the temperature maintained and whether regular or high-early-strength concrete is used. High-early-strength concrete may be achieved with accelerating admixtures (Art. 9.9) or with high-early-strength cement (Types III or IIIA) or by a lower water-cementitious materials ratio, to produce the required 28-day strength in about 7 days.
An important precaution in using heated enclosures is to supply heat without drying the concrete or releasing carbon dioxide fumes. Exposure of fresh concrete to drying or fumes results in chalky surfaces. Another precaution is to avoid rapid temperature changes of the concrete surfaces when heating is discontinued. The heat supply should be reduced gradually, and the enclosure left in place to permit cooling to ambient temperatures gradually, usually over a period of at least 24 h.
(Cold Weather Concreting, ACI 306R; Standard Specification for Cold Weather Concreting, ACI 306.1; and Standard Specifications for Structural Concrete, ACI 301.)
