Should you be pouring concrete in cold weather?
Much of the country is an ice box and that has put a stop to most remodeling and new construction projects in the northern part of the U.S. However, those of you who live in the South have experienced some pretty cold weather too. You may be wondering if you should delay your remodeling or go forward with the concrete work. Much depends on local conditions, your concrete suppliers, and whether your concrete contractor has a track record with cold weather concreting.
You can pour concrete footings, walls, walks or driveways. Just remember that as a general rule, concrete does not like the cold. So if you want to work with it in cold weather, you and your concrete contractor have to follow some rules. The term used when you are pouring structural concrete in cold weather is called Cold Weather Concreting.
The American Concrete Institute in ACI 360 defines cold weather as a period of more than three consecutive days in which the following conditions exist:
1) The average daily air temperature is less than 40°F and
2) The air temperature is not greater than 50°F for more than one-half of any 24-hour period.
That's pretty complicated. A simpler rule of thumb is "the air temperature has fallen to, or is expected to fall below, 40°F." Some people will tell you that if the temperature is 20°F, then don't bother to pour it. When it doubt consult your building department in your locality.
Why is temperature critical for Concrete?
Temperature is really critical, because when concrete is poured it requires time to set or cure to achieve it's full strength. This chemical reaction takes place in the concrete and is controlled by temperature. When the temperature of the concrete is above 50°F, the hydration reaction proceeds normally and within 24-48 hours the concrete will reach full strength. As the temperature drops below 50°F the reaction slows down and below 40°F it stops completely. At these cooler temperatures, the concrete doesn't gain strength and can't support the load of the structure. In fact the water in the concrete can freeze and it that happens the concrete freezes and can actually break.
Cold Weather Concreting
If you are going to pour concrete during cold weather you have to take steps to prepare the ground and heat it, modify the concrete mixture, and protect the concrete from freezing once it is poured. The overall goal is to get the temperature of the concrete greater than 40°F and to keep it as warm as possible so that it cures and reaches it's full strength. See this excellent video on Cold Weather Concreting from the Concrete Network.
The contractor can protect and warm the ground with thermal blankets and and heaters prior to pouring the concrete.
Usually the concrete supplier will provide air entrainment, heat the concrete mix with hot water, and add chemical additives and extra Portland cement to the mix. All of the above are designed to promote curing so the concrete reaches full strength. Once the concrete does reach full strength it will be able to handle the cold. Non chloride additives that will not corrode steel are desirable if you are using rebar or wire mesh.
When concrete is poured in cold weather a contractor will have to make sure that the ground is free of snow and ice and not frozen. The contractor can cover the ground with thermal blankets too. If the ground is frozen, Do Not Pour the Concrete under any circumstances. If the concrete is still poured, it may break as the ground thaws out. Special care must be taken to make sure that rebar and wooden forms are not cold since this will prevent curing in those areas.
Use blankets and heaters to cover the concrete for several days to keep it warm. In more severe cases, the concrete contractor may even enclose the area in plastic and heat it.
The Concrete Network has an excellent video on cold weather concreting and full series on the subject. They are also an excellent source to find a concrete contractor in your area. Here are several links to get your started: