Plastic shrinkage cracks are simply the result of moisture loss from the surface of the concrete while the concrete is still plastic and before it has hardened and attained strength. Most causal descriptions use the appropriate wording “rapid moisture loss”.
From a practical viewpoint, the moisture loss at the surface, caused by evaporation, is occurring at a greater rate than the rising bleed-water, from within the body of the concrete, can replace that evaporated surface moisture. This causes the surface of the concrete to shrink more than the “body” of the concrete. The PCA further describes this action by stating:
The National Association of Home Builders in their Residential Concrete publication adds the following: “Plastic shrinkage cracks are more likely to form in hot, dry, windy weather. However, they may occur in cool weather if evaporation is very rapid. Very warm concrete in cold surrounding air may cause evaporation and plastic shrinkage cracks.”
The American Concrete Institute in their document ACI 305, Hot Weather Concreting, provides a chart to estimate the rate of evaporation. It is based on wind speed, humidity and temperature. The chart indicates when special concreting precautions may be required.
Finally, if plastic shrinkage cracks begin to form, tamp the concrete on both sides of the crack with a float to close the crack together, then refloat. Remembering though, if rapid evaporation hasn’t been brought under control, this may be a futile effort.