It’s that time of year again. As we begin to hit the heat of summer, it is important to think about proper concrete placement practices to combat the heat. But first let’s look at why hot weather is a problem.
Concrete sets as the cement hydrates. Hydration is an exothermic reaction, meaning it generates heat, and that reaction goes faster when the concrete is hot. So the main concern with the concrete's strength and set time isn't really the air temperature but the concrete temperature. When cement hydrates it sucks up water and grows crystals around the aggregate particles. When it's hot and that reaction is rapid, the crystals grow quickly but don't have time to grow strong. Early strength will be higher but 28-day strength suffers. If the concrete is about 18° hotter than normal (for example, 88° instead of 70°), the ultimate compressive strength will be about 10% lower.
In hot weather, as the cement sets up, slump decreases rapidly and more mixing water is needed. This can also contribute to lower strengths (as much as another 10% lower), and in integrally colored concrete, can lead to variations in water content which can result in significant differences in concrete color between adjacent pours.
Another potential problem in hot weather is surface drying—although this one comes with a caveat. If the concrete is warm and the sun is shining and there is a hot dry wind across the concrete, yes, you are likely to get more drying and surface shrinkage. But if it is cooler than the air by about 18°F or more, the chances are that water will be condensing on the surface rather than the surface drying out. The real problem with drying is when the air is cooler than the concrete. So, if we can get cool concrete, we should be OK. This is NOT to say that hot weather can't cause increased cracking and plastic shrinkage cracking, because it can if the concrete temperature is high and the humidity is low.
To determine if evaporation will be a problem, use the nomograph from ACI 305 (see below).
Knowing air temperature, relative humidity, concrete temperature, and wind velocity, you arrive at a rate of evaporation. If the rate is greater than 0.1 pounds per square foot per hour, shrinkage cracking is possible.
Hot dry subgrades 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 you are likely to get cracks. This tends to be a problem in mass concrete (members thicker than about 18 inches).
Finally, maintaining air content can be a problem in warm concrete. Mixing is more likely to drive air out of the concrete making the level difficult to control.