Mixing and placing concrete at a high temperature may causeflashset in the mixer, during placing, or before finishing can be completed. Also, loss of strength can result from concreting in hot weather .

In practice, most concrete is cast at about 70 20F. Research on the effects of casting temperature shows highest strengths for concrete cast at 40F and significant but practically unimportant increasing loss of strength from 40 F to 90F. For higher temperatures, the loss of strength becomes important. So does increased shrinkage. The increased shrinkage is attributable not only to the high temperature, but also to the increased water content required for a desired slump as temperature increases.
For ordinary building applications, concrete suppliers control temperatures of concrete by cooling the aggregates and, when necessary, by supplying part of the mixing water as crushed ice. In very hot weather, these precautions plus sectional casting, to permit escape of the heat of hydration, may be required for massive foundation mats. Retarding admixtures are also used with good effect to reduce slump loss during placing and finishing.
(‘‘Hot Weather Concreting,’’ACI 305R; and ‘‘Standard Specifications for Structural
Concrete,’’ ACI 301.)

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References:

• BUILDING DESIGN AND CONSTRUCTION HANDBOOK for Frederick S. Merritt & Jonathan T. Ricketts