When it’s cold outside, placing, finishing and curing concrete calls for extra care. In cold weather, concrete sets up more slowly. Worst of all, there’s a risk that the fresh concrete will freeze—and if it does, the damage done by ice formation can reduce its final compressive strength by as much as 50 percent.
When the water in fresh concrete freezes, the ice takes up more space than water. Ice crystal growth causes an overall volume expansion of the concrete. This expansion weakens the concrete by creating void spaces and disrupting the bond between the cement paste and the aggregate. Later, when the concrete thaws, voids left by melting ice makes the concrete porous and can cut concrete’s 28-day compressive strength in half.
Before you place concrete in cold weather, you need to make sure your forms and subgrade are ready. Otherwise, the subgrade will likely thaw unevenly, resulting in uneven settlement. Thaw the subgrade out first.
Never place concrete on snow and ice. Clear the ice and snow from both forms and subgrade before you pour. The subgrade should be at least 35 degrees F before you place the concrete.
Place concrete in the late afternoon, after the sun has heated the subgrade and forms.
If your concrete freezes, it can be disastrous—the strength loss may be unacceptable, and tearing out concrete and doing it over is not light work.
In addition to thawing the substrate and pouring the concrete at the right temperature, in cold weather it’s important to protect fresh concrete from freezing. The most common and easiest way to protect it is by insulating. During the first three days after placement, the chemical reaction between cement and water generates heat, called the “heat of hydration.” If you can keep that heat in, you often won’t need to supply extra heat.
Six inches of straw held in place with tarps or polyethylene sheeting will keep heat from escaping. But straw has disadvantages: It’s bulky, it’s flammable, and if it gets wet, it loses its insulating value. For light construction, insulating or curing blankets are the most practical way to maintain heat and moisture during cold weather.
Your goal should be to keep the concrete from freezing until it reaches a strength of 500 psi and most of the water is gone. This takes longer if the concrete gets cold. To be on the safe side, protect all fresh concrete from freezing for three days. After that, protect it from rain, snow or any other source of water for a few days before exposing it to freezing temperatures.
Whether you insulate or tent out, do not allow the concrete to cool too fast at the end of the protection period. Sudden drops in temperature will cause thermal shrinkage cracking. Remove the protection in a way that lets the concrete cool gradually—no faster than a 50 degree drop in 24 hours.