Before you take a job in the fall or winter, you need to examine how you'll deal with problems like frozen subgrades, extended concrete setting times, slower strength gain rates, and decreased worker comfort (and efficiency). To obtain high quality concrete under these conditions, it's important to take the five general precautions listed below.
The first is plan ahead. Make sure all the equipment you need is on-site before you begin to place concrete. To ease finishing operations in cold weather, you may need additional equipment. Insulating blankets, for example, may be needed to cover the subgrade before the pour.
The second is choose cold-weather concrete mixes. Because cement hydration is slowed in cold weather, the concrete cools quickly, extending setting time by about one-third for each 10 degree reduction in temperature. To expedite your finishing operations and speed form removal and reuse, ready mixed concrete producers usually heat the mixing water or aggregates. You may also want to consider ordering a mix with increased cement content, faster-setting cement, or an accelerating admixture.
The third is maintain adequate concrete temperatures. To prevent early-age freezing, protection must be provided immediately after concrete placement. In some cases, you may only need to cover the concrete with insulating materials and use the natural heat of hydration to maintain the temperature at the recommended levels. In more extreme cases, consider building enclosures and using heating units to maintain the desired temperatures.
The fourth is maintain proper curing conditions. Concrete exposed to cold weather is normally unlikely to dry at an undesirable rate. But this may not be true for concrete within heated protective enclosures. When concrete warmer than 60 degrees F is exposed to air at 50 degrees F or higher, it's important to prevent drying. Using an exhaust steam heater is probably the most effective method, since it provides both heat and moisture.
The fifth is limit rapid temperature changes. ACI provides the following maximum allowable drops in temperature during the first 24 hours after the end of the protection period: For concrete that's less than 12 inches thick, the maximum allowable drop is 50 degrees F; 12 to 36 inches thick, 40 degrees F; 36 to 72 inches thick, 30 degrees F; and greater then 72 inches thick, 20 degrees F.