Hot weather can have many negative effects on fresh and hardened concrete properties, including increased water demand, accelerated rate of cement hydration, increased moisture and slump loss, faster setting times, increased plastic- and drying-shrinkage cracking, and lower ultimate strengths. But construction doesn't have to stop when air temperatures rise above 90øF. By adding chemical admixtures to concrete mixes, you can improve concrete workability while reducing total water content. Other benefits included increased concrete strength and durability and lower total in-place costs.
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.