The hardening of concrete is a chemical reaction that requires certain conditions and temperatures. Ideally, concrete should be poured when the air temperature is between 50 and 90 degrees Fahrenheit. Cold temperatures slow the concrete-setting process, and when temperatures dip below freezing, the delay allows the water in the mix to freeze before the concrete hardens. This creates cracks in the concrete because water expands as it freezes.
How Cold Is Too Cold?
Concrete can be successfully poured at temperatures outside the ideal range, but it must never be poured onto frozen ground or exposed to freezing temperatures before it has set. If you absolutely must pour concrete when the temperature is below 32 degrees Fahrenheit, you will first need to tent the area and warm it with space heaters. Store the concrete powder mix in a heated garage or other warm area until use and mix it with hot water when you are ready. Allow the concrete to stiffen after it has been poured and then cover it with insulated blankets to keep it warm or keep the tent up and the space heater running overnight.