First, let's define cold weather with respect to concrete. Any time you have 3 consecutive days where the average daily temperature is less than 40°F or if the temperature is lower than 50°F for less than half of any of the 3 days, concrete thinks it's cold. Cold weather can have a detrimental affect on concrete curing for several reasons.
Concrete transforms from a liquid to a solid material through a chemical reaction. The speed of the reaction depends upon the temperature of the concrete. When the weather is warm, the reaction proceeds quickly. When it's cold and the ground hasn't been thawed, the reaction slows down. That's the problem: the concrete needs to harden as rapidly as possible to resist pressures caused by water freezing within the concrete.
If the temperature is too cold, the concrete may not have reached a minimum strength of 500 psi soon enough to resist the effects of freezing temperatures. If your concrete isn't protected with concrete curing blankets after it's poured, it may cool too rapidly, slowing the chemical reaction.
Poor finishing techniques can also doom your slabs. Freshly poured concrete often bleeds. The water in the mix floats to the top, since it's the lightest ingredient. Floating or troweling this water into the concrete weakens the top layer. Troweling the concrete too early can seal this bleed water just below the surface as well. If your slab is then exposed to freezing temperatures several days later, this water can freeze and fracture the top layer. Using a concrete curing blanket can eliminate the potential of freezing.
Concrete can be successfully poured in cold weather.
Several precautions need to be taken:
- Never pour concrete on frozen ground, snow, or ice. Use powerblanket™ concrete curing blankets for ground thawing ahead of time.
- Be sure to order air-entrained concrete. Request a heated mix or order 100 lbs of extra cement for each cubic yard of concrete. This extra cement helps develop early strength.
- Be sure the concrete is ordered with a low slump (drier mix). This minimizes bleed water.
- After the final finish is completed, cover the concrete with a powerblanket™ concrete curing blanket. The powerblanket™ concrete curing blankets will prevent freezing and keep the concrete at optimal curing temperature.
- After about 3 days, remove powerblanket™ curing blankets to allow the concrete to air dry.
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