Though builders would prefer not to pour concrete in cold weather, sometimes that can’t be helped. A structure needs to go up, or a walkway or pavement needs to be repaired even if the temperatures are bone-chilling. How can concrete be poured effectively in cold weather?
How Concrete Works
Concrete is made up of Portland cement, a conglomerate and water, and it’s the water that makes it problematic to pour concrete in cold weather. Water allows concrete to harden through hydration, which is when the ingredients in cement form chemical bonds with the molecules of water and turn into hydrates. For this to happen, the pour needs to be higher than 50 degrees Fahrenheit for at least 48 hours.
Cold Weather and Concrete
There are two serious problems with trying to pour concrete when it’s cold out. First, it can freeze before it cures, and cold temperatures prolongs the curing process in the first place. If the temperature of the concrete drops beneath 40 degrees Fahrenheit, hydration can’t happen. This can cause costly delays. The one good thing is that the curing temperature is the temperature of the concrete and not the air. If concrete is strong enough, it can handle cold air. This strength is expressed as at least 500 pounds per square inch strength. At that level, there is not enough water left in the concrete to freeze and damage it, even if the air is frigid.
Ways to Protect Concrete in Cold Weather
There are several ways to help concrete achieve 500 psi strength even in cold weather. One way is to add concrete blankets. These are insulated blankets that help the concrete to become strong enough to cure even when the air is cold. They are made out of a polyethylene outer fabric filled with water resistant microfoam. The tops of the outer shells are black to absorb as much heat from the sun as possible, while their bottoms are pale. They come in different thicknesses, widths and lengths and can be heated.
Other ways to protect a concrete pour in cold weather is to use hot water in the concrete mix. This can raise the temperature of the concrete to at least 65 degrees F. Other contractors use accelerators. This is usually the addition of 2 percent calcium chloride, though there’s a risk of this accelerator corroding rebar and other steel structures in the concrete. It can also mottle dyed or stained concrete. Other contractors use accelerators that do not contain calcium chloride. They’re more expensive, but they don’t affect the color of the concrete.
Some contractors add extra cement to make the concrete a bit warmer and to speed up the hydration process. If the concrete doesn’t have additives, the contractor can add air to the concrete to create tiny bubbles in the mix. This is called air entrainment and helps the mix resist freezing and makes the concrete more durable. It also cuts down on bleeding, which is when water rises to the surface of the just-poured concrete.