Q.: Some concrete specifications contain a maximum temperature for the freshly mixed concrete as delivered. Typical values are between 80° and 95° F as measured by ASTM C 1064-86. Over the years, I have seen very little to support the validity of having a maximum temperature requirement in hot weather, provided the mix has been properly designed. ACI 305R section 3.2.1 states that “concrete can be produced in hot weather without maximum limits on placing temperature and will perform satisfactorily if proper precautions are observed in proportioning, production, delivery, placing, and curing. As part of these precautions, an effort should be made to keep the concrete temperature as low as practical.” Is there a need for maximum temperature limitations on freshly mixed concrete in hot weather?
A.: The simple truth of the matter is that concrete placed and cured at a moderate temperature (60° to 80° F) will outperform +90° F concrete in strength and durability. If you are looking for superior concrete, control the temperature.
The other problem with warmer concrete is cracking. These maximum specified temperatures are necessary to help control early cracking in concrete. Concrete is usually poured during the day when it is warm. Early cracking in slabs and other large concrete structures is often caused by a steep temperature gradient through the concrete caused by the cooling of the surface by the night air when the concrete has very low strength. After hydration starts, concrete will gain in temperature reaching a maximum, which will depend on member thickness, type, and quantity of cement, and so forth. As soon as the concrete begins to cool, the gradient, which depends strongly upon external temperature, will determine the cracking risk of that concrete. During this phase (from 24 to 72 hours after pouring), if the concrete is able to generate tensile stresses higher than the tensile strength, a crack will appear. Any measure to reduce temperature differences, such as using special low hydration cements, lower cement content, cooling down the compounds before mixing, cooling down concrete during the first hours, or avoiding extreme temperature drops after 1 day, will help. One way is to pour during the night, first to reduce the maximum temperature because, during the night, external temperature helps cool the concrete, and second, when the day starts, the temperature increase helps decrease temperature differences and gives the concrete more time to generate tensile strength.