Concrete in Cold Weather
The building code says concrete must comply with ACI 318. This standard contains requirements for placing concrete, including cold weather concreting. Here is a summary of ACI cold weather requirements.
Concrete must be protected from freezing until it has reached a strength of at least 500 lbs./sq. in. That normally takes about 24 hours after the pour. However, in cold weather it can take much longer. Concrete cure slows as the weather gets colder and practically stops when the concrete is below 40 degrees. It is essential to protect it from becoming colder than this as it sets. The ACI standard requires the air temperature to be at least 40 degrees when it is poured and be protected when freezing is predicted to occur within 24 hours. Concrete that freezes during this time will be permanently weakened and can lose half its strength.
To maintain temperature concrete is prohibited from being poured on frozen ground. The water and aggregate in the mixture can be heated to help it maintain temperature. Windbreaks, heated enclosures, blankets and adjusting the admixtures or increasing the cement content may be needed. Admixtures such as calcium chloride can help maintain the temperature, but calcium chloride can corrode embedded steel and stain the concrete surface. When heat is used be careful to not dry out the concrete surface because concrete needs the moisture to property cure. Concrete should be gradually brought to the ambient temperature to prevent cracking. Adding extra cement to the mixture or using Type III high early strength cement will speed the hydration process. If heating is necessary during the curing time it is recommended direct fired heaters not be used or the concrete surface may become carbonized.
Plan ahead. The finishing crew will likely need to stick around longer because the concrete will cure slowly in cold weather. Wait unit the weather improves if you can. If you cannot, work with the concrete supplier to have the material delivered at the right temperature and with the best mixtures.
You will not find these requirements in the code book, but the code reference to ACI 318 which contains them makes them a required part of the code, so they must be followed.