Cold weather concrete has superior properties to concrete placed in hot weather. At low temperatures, though, concrete sets and gains strength more slowly because the cement doesn't hydrate as fast. Setting time is increased about one-third for each 10 degree decrease in concrete temperature down to 40 degrees Fahrenheit. Accelerating admixtures can help to offset these effects of low temperatures on setting and strength gain. They should meet the requirements of ASTM C 494, Standard Specifications for Chemical Admixtures for Concrete. Accelerators can be divided into four classes:
- Calcium chloride is the oldest and most widely used accelerator. Regardless of the way in which it's added to concrete, the weight of calcium chloride shouldn't exceed 2 percent of the weight of cement.
- Accelerating admixtures containing calcium chloride are water reducers with enough calcium chloride added to accelerate setting.
- Nonchloride accelerating admixtures are used as accelerators in situations where calcium chloride admixtures are prohibited.
- Nonchloride accelerating admixtures that also provide freeze protection permit concrete to be placed at subfreezing ambient temperatures and may reduce or eliminate the need for other protective measures such as insulation blankets and heated enclosures.