If it's below 40°F, then use cold weather curing techniques. Options for cold weather concrete curing include blankets, additives and heaters.
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Powerblanket MD0320 electric concrete curing blanket: Powerblanket has changed the conventional method of transferring heat to concrete. The revolutionary new design provides uniform heat across entire blanket, eliminating any hot and cold spots. Powerblanket products provide a maintenance-free heating solution, which eliminates the hassle of dealing with cold weather. If an effective heating solution is applied during the curing process, concrete will cure faster and stronger.
3' x 20' Concrete Curing Blanket - Powerblanket MD0320