Q. What can I do so that I can place concrete in the winter the same as I do in July?
A. The simple answer is, you can't! "Think of it this way: If you are cold and need a jacket, the concrete is also reacting to cold conditions and may need cover," says Dan Ellery, Consumers Concrete Co., Kalamazoo, Mich.
Concrete is very sensitive to temperature variations and can be unforgiving when the mercury drops below freezing. Contractors often question how much time is needed to properly cure concrete in cold weather, and they often ask for advice on the best mix to use when temperatures drop. In general, the finer the cement used, the higher the reactivity of the cement and therefore the higher the heat of hydration, which is good in cold weather. However, it's not that simple. In suspended slabs, you may have to heat the floor below to warm the overhead steel deck upon which concrete is placed. Again Ellery might advise: "If you are warm inside your jacket but your ankles and feet feel cold, chances are the slab may react negatively to the cold steel."
Curing concrete sufficiently is critical to the durability of the finished product. Without proper curing, concrete can be damaged by frost. If concrete freezes during the curing period, permanent damage usually occurs. Ice can form in concrete whenever the temperature drops to below about 29°F.
ACI defines cold weather concreting as any period when, for more than three consecutive days, the following conditions exist:
- Average daily air temperature is less than 40° F. The average daily air temperature is the average of the highest and the lowest temperatures occurring from midnight to midnight.
- Air temperature is not greater than 50° F for more than one-half of any 24-hour period.
Low temperatures reduce the rate at which concrete gains strength. If freezing occurs before the concrete can reach a compressive strength gain of about 500 psi, ice formation in the voids in the concrete will make the concrete less stable even to the point of being totally unusable. While contractors know this, delays caused by extended curing time and protection methods lead to increased project costs and schedule delays.
What is the coldest temperature at which to place concrete? ACI 306 provides the guidelines shown in the table at right, depending on the thickness of the section you are placing. From this you can see that it is unwise to place thin sections or slabs less than 12 inches thick during weather colder than 55°F. If you do have to place concrete in cold weather, you may consider the following:
- Use air-entrained concrete.
- Make sure that neither the sub-base nor any surfaces that may come in contact with the concrete are frozen.
- Use high-early-strength concrete.
- Cure concrete, providing adequate moisture with water or live steam for heating, taking care to vent all engine or heater exhaust.
- Place blankets over the concrete and make sure they are weighted down so they don't get blown off during the night.
- Leave forms in place as long as possible -- wood forms serve as a pretty good blanket.
- Use accelerating admixtures.
- Calcium chloride containing admixtures may not be acceptable with reinforced concrete.
- Minimize the temperature difference between the concrete and the ambient (external) conditions to prevent thermal shock when forms are removed; ACI 306 Table 5.5 provides guidance.