In general, Concrete sets as the cement hydrates, and is an exothermic reaction, means it generates heat reaction goes quickly when the concrete is hot condition. The main reason with the concrete's strength and setting time is not really the air temperature but the concrete temperature. During the time of cement hydration it sucks up water and forms crystals around the aggregate particles as shown in fig 1. The crystals grow faster but do not have time to grow strong when it is hot and is

more »

... . The 28day strength suffers even though early strength is higher. If the concrete is about 16° hotter than normal (for example, 86° instead of 70°), the compressive strength will be about 10% lower. The slump decreases rapidly as the cement set up In hot weather, and the mixing water is more needed. It can also contribute to lower strengths ( another 10% lower), can lead to variations in water content which can result differences in concrete color between adjacent pours. Next problem in hot weather is drying of surface, comes with a caveat. If the concrete is warm, sun is shining and hot dry wind across the concrete, it is likely to get more drying and surface shrinkage. By the help of nomograph from ACI 305 (Fig 8) the relative humidity, concrete temperature, air temperature and wind velocity, it arrive at rate of evaporation. The shrinkage cracking is possible when the rate is greater than 0.1 pounds per square foot per hour. Hot dry sub grades and formwork can also result in cracks by absorbing water from the mix. Another problem with heat and concrete is thermal differentials. All that means is that one part of the concrete mass is warmer than another part. If the differential is greater than about 20°F then the concrete likely to get cracks. This tends to be a particular problem in mass concrete (members thicker than about 18 inches).