Cold Weather Concreting
As Fall becomes Winter, the process by which concrete is placed and finished slows in direct relation to temperature drops. In order to expedite cold weather pours, hot water is used is in the mix process and often times accelerators are also added to the concrete mix itself. The most common accelerators are calcium-chloride, and non-chloride accelerators (NCA’s). NCA’s are most commonly utilized in projects using reinforcing structural steel. Unlike calcium chloride, the accelerating additives in NCA’s will not attack or deteriorate the steel, and therefore will not damage structure or shorten its lifespan.
Cold Weather Placing
Warm the sub-grade, the forms, and any reinforcing steel.
Do not place concrete on a frozen sub-grade. As the frozen ground thaws it may settle, causing cracking.
Use heated enclosures to block wind, keep out the cold, and conserve heat: If combustion heaters are used make sure the exhaust is vented out and away from the concrete to reduce possible damage.
Place concrete as early in the day to utilize the sun’s thermal heat.
Cold Weather Curing
The minimum concrete temperature, as placed and maintained, must exceed 50F. However, caution should be exercised with concrete temperatures above 75F.
Maintain concrete temperature above 50F by insulating or heating for at least three days:
Use insulation blankets to retain internal heat generated by the concrete.
Cover concrete with thermal insulation blankets, or use loose straw (12” deep)
Protect concrete against multiple freeze-thaw cycles until it reaches 3500-psi compressive strength.
Remove the heat protection in a manner that ensures the temperature of the concrete changes slowly, cooling the concrete gradually helps reduce the potential for cracking due to thermal stresses.
Do not seal freshly placed concrete. Sealing retains water in the concrete keeping it saturated during freeze/thaw conditions.
Use a good quality curing compound if you are unable to maintain concrete temperatures above 50° Fahrenheit for three to seven days.
The curing period must extend a minimum of 7 days (maintaining the 50F temperature).