Hot enough?

Concrete Construction, August, 2004 by Terry Holland

I want to comment on the hot weather article in the June issue of CONCRETE CONSTRUCTION (p. 27).

First, I am completely uncertain what the author means when he states that the design engineer is responsible for "designating" the concrete mixture. Later, he writes that the design engineer "specifies hot weather concrete mixes." I think that he is implying that the design engineer should prepare a prescriptive specification for the concrete mixture, which is exactly opposite to the direction that the industry is trying to go with the current P2P movement. The design engineer should be establishing the requirements for the hardened concrete and leave the mixture proportioning to the producers. It is certainly within the designer's responsibility, however, to include appropriate hot weather language in the specification.

Second, the rate-of-evaporation predictions made using the nomograph from the ACI documents are very rough approximations. The 2001 version of ACI 308 discusses the limitations of this approach and defines exactly where the various measurements are to be taken. This is critical information that should be included in any discussion of the use of this device.

Third, the recommendation regarding using a retarding admixture is incomplete and technically incorrect. Adding a retarder to flatwork will extend the period during which the concrete is susceptible to drying and plastic shrinkage or crusting. In some cases, an accelerator may be more appropriate. A retarder may be useful in formed concrete if there are concerns that the concrete is setting too quickly to achieve bond between lifts.