Question: My belief is that increasing the cement content in mixes designed for placement in conditions below 32° F is counterproductive to initial strength gain due to the additional hydration required to generate heat. We use a Type III cement and generally deliver concrete at 60° F at time of placement. Since hydration slows in colder temperatures, would a mix design with less cement and more water-reducing admixture produce higher one-day strength than a mix with higher cement content?

Answer: Even if customers order concrete on days with below-freezing temperatures, ACI 306R, Cold Weather Concreting, recommends that they maintain concrete at a minimum temperature for one or more days. For structures with minimum dimensions less than 12 inches, concrete must be placed and maintained at 55° F. The required duration of protection from early freezing depends on the type and amount of cement, whether an accelerating admixture is used and the service category of the structure.With standard mixes placed and maintained at 55° F, contractors will certainly get enough cement hydration to generate heat. But to reduce the duration of the protection period, ACI 306R suggests that contractors request mixes that increase the rate at which the concrete gains strength by using a Type III cement, an accelerating admixture or an extra 100 pounds of cement per cubic yard. The extra cement reduces water-cement ratio, producing a higher strength at any age, and also supplies more heat of hydration at early ages.Using less cement and more water-reducing admixtures, as you suggest, would be counterproductive because, even though you might get a lower water-cement ratio, the concrete wouldn't develop as much heat of hydration. A Type A water reducer might also cause undesirable set retardation in cold weather. However, some Type E water-reducing, accelerating admixtures meeting the requirements of ASTM C 494 substantially improve the 24-hour strength of concrete maintained at 50° F. According to ACI 306R, the strength of concrete containing these admixtures approaches the strength obtained by using 2% calcium chloride and will be appreciably greater than for some, but not all, concretes made with Type III cements. The data also indicate that water-reducing accelerating admixtures may reduce setting times and produce substantial strength increases at all later ages. However, ACI 306R makes no mention of reducing cement content when these admixtures are used.