In the UK the current standard for specifying concrete, BS 8500, states that the concrete temperature at point of discharge shall be less than 35°C. There is no stipulated requirement for a maximum air temperature but higher air temperatures will increase the temperature of the fresh concrete, typically resulting in the following:

• Loss of workability/consistence causing placing issues.
• Rapid evaporation of moisture from exposed surfaces which may cause plastic shrinkage cracking, crazing and dusting of the hardened concrete.
• Rapid hydration of the cement leading to accelerated setting.
• Higher early strength but reduced long-term strength.

Higher temperatures for the fresh concrete will lead to more rapid hydration and higher early age maximum temperatures. This will result in higher thermally induced stresses as the concrete cools to the ambient temperature, with a correspondingly higher risk of cracking. In addition, differential temperatures between the interior and the surface of the concrete will be increased unless controlled, again increasing the risk if cracking.

Design Standards give guidance on maximum differential temperatures; generally, a figure of maximum of 20°C is considered acceptable for gravel aggregate-based concrete.

In areas with extremely high ambient temperatures, such as in the Arabian Peninsula, it may be necessary to cool the concrete prior to placing it, for example by replacing part of the water in the mix with flaked ice. Other measures such as shading the aggregate, working at night etc. form part of the approach to these extreme environments.

A good summary for hot weather concreting is given in GCG8 Concrete Practice, Section 14. This refers to moisture loss, and loss of consistence.
CIRIA Report C766 is suggested for assessing early age thermal crack control.