Pouring concrete in cold weather may not be advisable. But this raises the question: how cold is too cold? And also, can any measures be taken to mitigate cold conditions? The answers can be had by understanding a little bit about concrete and how it works, and then evaluating how the concrete, and conditions where the concrete will be poured, can be manipulated.
Concrete is composed of 60 to 75 per cent sand, gravel and rock, 10 to 15 per cent cement and 15 to 20 per cent water. In a process called hydration, the chemicals in cement react with water to harden and bind the aggregate into a solid form. The precise ratios of aggregate, cement and water affect ultimate concrete strength. Generally, using less water and more cement results in stronger concrete.
Temperature also affects concrete strength, though indirectly. The hydration reaction takes place more slowly at lower temperatures. Slower hydration allows water to react more thoroughly with the cement, so less water can be used in the concrete mix, so the concrete is stronger. However, temperatures cannot be too low, or the concrete will freeze.
Liquid, or "plastic" concrete freezes when it cools to -3.89 degrees Celsius. When freezing occurs, the ultimate strength of the concrete is reduced by at least 50%. This failure of strength makes the finished concrete structure unsound. Because weather is not entirely predictable, experienced contractors know that forecasted temperatures of 4.44 degrees Celsius or lower for three days or more, constitute risk of concrete freezing.
Pouring concrete in weather conditions between 25 and 40 degrees Fahrenheit is possible with cold weather concrete placement techniques. Ice and snow must be removed from the placement surface and the surface warmed. The finished concrete should be insulated to retain heat from the exothermic hydration reaction of curing, and the concrete should be maintained at a temperature of at least 4.44 degrees Celsius. Freezing and thawing must be prevented. When the concrete reaches 3500 psi compression strength, insulation should be removed gradually to prevent cracking from rapid temperature change.
For cold weather placement, the concrete mix should be air-entrained, which infuses the concrete with microscopic air bubbles that absorb expansion and contraction from freezing and thawing. An accelerator like calcium chloride can be added to the concrete mix, and fast curing cement can be used, to speed curing time to reduce freezing risk. The ready mix supplier should be informed of the cold weather pouring so that they can deliver the concrete at a temperature predetermined for optimal cold weather curing.
The dimensions of the finished concrete construction also affect the risk of freezing during curing. Thinner forms are more likely to freeze. Edges and corners are especially susceptible to rapid temperature variations. A high risk of cracking exists when the difference between interior and exterior concrete temperatures is 1.67 degrees Celsius or more.