Accelerating chemicals reduce the initial set time of concrete and are recommended in cold weather. They do not act as an antifreeze; rather, they speed up the strength gain and make the concrete stronger to resist damage from freezing. Each drop of ten degrees in temperature below 70 degrees F ( the temperature for which all concrete is designed) substantially delays setting and the rate of strength development. A drop to 50 degrees will reduce the one-day strength by almost 40%, while a drop to 40 degrees will reduce the strength more than 60%. Tests show that it takes 2 ½ days to develop the same strength at 40 degrees that can be developed in only one day at 70degrees. Accelerators are also used in fast track construction. Accelerators are covered by ASTM C494 Types C and E.

There are two common accelerators:

•  Calcium Chloride is the most common and least expensive accelerator for non-reinforced concrete and is specified at not more than 2% by the weight of the cement. The addition of 2% calcium chloride will more than offset the losses in strength that normally result from temperatures as low as 40 or 50 degrees.

•  Non-chloride accelerators are specified when corrosion is an issue. In addition, certain types of non-chloride accelerators have the ability to place concrete at an ambient temperature as low as 20 degrees F, without freezing in its plastic state.