Failure to manage temperature while concrete cures can be disastrous
I’m sure the delayed onset of cold weather this year was much appreciated by those who spend their days in the open air of construction sites; even more by those whose projects are in the somewhat closed-in, but not yet heated phase. Cold weather presents challenges that builders in warmer climates don’t face. One of them is protecting fresh concrete from freezing so that the finished product will perform as intended.
Failure to properly plan for cold weather concreting can be disastrous. Concrete that is allowed to freeze (which occurs at 25 degrees F for plastic concrete) will not cure properly and can reduce its final strength by more than 50%.
The American Concrete Institute (ACI) defines cold weather concreting as occurring when, for more than three consecutive days, the average daily air temperature is less than 40 degrees F, and the air temperature does not exceed 50 degrees F for more than one-half of a given 24-hour period. During winter in the Front Range, it is pretty normal for these conditions to exist. Typically, newly placed and properly protected concrete will reach 500 psi strength during the second day, after which protection is generally no longer required, unless there is a need to reach high early strengths. Without supplying heat, it may be necessary to delay removing forms for longer periods to allow adequate strength to develop. While liquid chloride may be used in unreinforced slab-on-grade construction to speed curing, it should not be used for structural concrete containing reinforcing steel. Non-chloride admixtures may be used in structural concrete applications to achieve similar results.
Concrete should never be placed on frozen ground. After excavating a foundation in cold weather, the soil should be covered with insulating blankets until concrete is placed. During extremely cold weather, it may be necessary to provide supplemental heat below the blankets to keep the soil from freezing. The reinforcing and other embedded components should also be at an appropriate temperature prior to concrete placement. If embedded steel components are cold enough to cause freezing of the concrete, the damaged concrete will not properly bond to the reinforcing steel. Providing heat after concrete placement may be necessary, too, but this should be done carefully to avoid introducing carbon monoxide or dioxide into the concrete during curing, which can cause carbonation. It is also important to provide a relatively even temperature distribution across concrete elements. Most often in our climate, simply keeping the concrete covered with insulating blankets provides adequate protection from the heat that is produced by the chemical reaction of concrete hydrating and curing. However, there are a few weeks when temperatures drop well below freezing, or even sub-zero, for several days at a time. If you must place concrete under these conditions, it would be wise to provide heating in addition to blankets.
If there are concerns that a client, inspector, city official, etc. might not be satisfied with the efforts made to protect your concrete, using maturity meters will provide a code-approved approach to document that the concrete was maintained at appropriate temperatures until adequately cured. This method uses temperature probes cast into the concrete to provide a continuous record of the temperature of the concrete. This method will also allow the user to determine when forms can be removed safely.
While cold weather concreting presents unique challenges, the biggest hurdle is lack of knowledge. There are many resources available in the Colorado construction market to help address temperature and weather-related concrete placement issues. When you do your research and plan appropriately, you will have a successful pour.