Like most chemical reactions, hydration of concrete slows considerably in cold weather. Below 14 Fahrenheit degrees, it halts completely. Working with concrete in low temperatures requires careful planning and changes in basic operations. There are three issues that should be considered when placing concrete in cold weather. First, protect the concrete against freezing for about 24 hours. Second, the concrete must set quickly so it can be finished in a reasonable time. Third the concrete must be cured at temperatures that allow it to develop required strength.
Protecting concrete from freezing can be accomplished by heating mix water and/or heating aggregate, combined with insulation of fresh concrete. Heating the fresh concrete also speeds the set time, as does use of a Type III cement, higher cement content, and chemical accelerators. Maintaining adequate curing temperatures can be accomplished by insulating the concrete and taking advantage of the heat caused by hydration.
Use care when exposing new concrete to cold. A quick change in temperature can cause thermal stressing, and result in cracks. It is a good practice to gradually remove insulation and cool heating enclosures.