What is a common cause of plastic shrinkage cracks in concrete?

Plastic shrinkage cracks in concrete primarily occur due to rapid evaporation of moisture from the surface, particularly during the initial setting phase when the concrete is still in its plastic state. When exposed to conditions such as high temperatures, low humidity, and wind, the surface moisture can evaporate quickly, leading to a differential in moisture levels within the concrete. This causes the surface to dry out faster than the underlying concrete layers, resulting in tension that the concrete cannot withstand, which ultimately leads to cracking.

The other options may have effects on concrete but do not directly cause plastic shrinkage cracks. Excessive water in the mix can lead to a variety of issues such as segregation and increased porosity, but it does not specifically promote the rapid moisture loss that triggers plastic shrinkage cracks. Low ambient temperatures during curing can slow down the setting and strength development of concrete, but it does not necessarily induce the rapid evaporation that results in plastic shrinkage. High air content in the concrete mix can affect the strength and density but doesn't create the conditions required for moisture loss that leads to shrinkage cracking.