Winter is not the optimal time to be pouring concrete, but sometimes a job can’t wait for the weather to warm up. There are a number of problems that can arise when pouring concrete in cold weather. Two of the biggest problems that occur when pouring concrete in cold weather are that the concrete may freeze before it becomes strong enough, and that it sets much less quickly when it’s cold.
At temperatures below 50°F, the setting process is very slow, but below 40°F the concrete essentially stops strengthening because of the effects of the cold on the hydration reaction. Bear in mind that these temperatures are that of the actual concrete, not the air, so it’s most important to take great care in protecting the concrete before it can handle being exposed to the cold air temperatures.
A rule of thumb is that once the concrete has strengthened to about 500 psi, it’ll set just fine. Once the concrete has gained enough strength to measure 500 psi, the cement has absorbed enough of the water in the concrete mix through hydration that there isn’t enough water left in the pores to cause damage to the concrete if it were to begin to freeze. There are two things that contractors can do in cold weather to help their concrete reach 500 psi: change the mix so that it will set more quickly or protect the concrete from the cold. Usually both measures are required.
Some changes you could make to concrete in cold weather include:
Hot water: the producer of your ready mixed concrete will usually use hot water when it’s cold outside. When concrete leaves the plant during winter months, most producers try to have it measure at least 65°F, which is usually sufficient for work being done.
Accelerators: As we mentioned, cold weather slows the setting of the concrete, so when pouring concrete in cold weather, you can expect delayed set times. In order to keep your job on schedule, you can add calcium chloride to accelerate the hydration reaction. Determine the weight of your cement, and add 2% of that amount in calcium chloride. This method is not only very effective, but is also fairly inexpensive. HOWEVER, if you’re planning to embed steel into the concrete, like rebar, adding that much chloride could potentially lead to corrosion. It can also result in a mottled surface appearance in colored concrete.
Next time, we’ll continue looking at changes you can make to the mixtures when pouring concrete in cold weather, and also at how to protect the poured concrete while it sets!