What is the most critical action to take when concrete is placed in hot weather conditions to prevent plastic shrinkage cracking?

The most critical action to take when concrete is placed in hot weather conditions to prevent plastic shrinkage cracking is to minimize the rate of evaporation from the concrete surface. Plastic shrinkage cracking occurs when the rate of evaporation exceeds the rate at which bleed water rises to the surface to replace the evaporated moisture. This leads to shrinkage of the surface concrete while the underlying concrete is still plastic, resulting in tensile stresses that exceed the concrete's early tensile strength. Therefore, reducing evaporation is paramount. This can be achieved through several methods, including: erecting windbreaks to reduce wind speed over the concrete surface; shading the concrete from direct sunlight to lower the surface temperature; using fog sprays to increase the humidity near the surface; applying an evaporation retardant to the surface; and starting curing as soon as possible after finishing. While all these methods contribute to preventing plastic shrinkage cracking, minimizing evaporation is the overarching goal. For example, on a hot, windy day, using a combination of windbreaks, fog sprays, and an evaporation retardant is crucial to keep the concrete surface moist and prevent cracking.