Plastic shrinkage cracks in concrete during placement in hot weather occur because:

Plastic shrinkage cracks in concrete during placement in hot weather primarily occur due to the rapid evaporation of water from the surface of the fresh concrete before it has set. When temperatures rise, especially in hot and windy conditions, moisture can evaporate quickly, leading to a decrease in volume in the concrete as it begins to dry. This evaporation causes the surface layer of the concrete to shrink more quickly than the underlying layers, creating tensile stresses that often result in cracks.

It's important to note that while factors such as improper mixing ratios, inadequate curing, and ground instability can affect concrete quality, they do not directly cause the specific phenomenon of plastic shrinkage cracks related to heat and evaporation. Proper mixing and curing techniques can help mitigate the impact of environmental conditions, but if water is lost too quickly, the risk of cracking increases significantly. Understanding this relationship between temperature, evaporation, and the properties of concrete is crucial for landscape contractors to prevent such issues during the installation of concrete features.