A slab was cast on a dry subgrade without any vapor barrier and we are beginning to detect shrinkage cracks. How much more cracking can we expect than if a vapor barrier had been used?
Ultimately there should be no difference. The cracking depends on the amount of drying shrinkage. A slab without a vapor barrier will lose water to the subgrade by absorption, but water that is not absorbed by the subgrade will eventually evaporate as the slab dries out. This will take longer but the ultimate amount of drying shrinkage should be the same. For high-slump mixes (8 to 9 inches), however, investigators found that the incidence of cracking was higher when concrete was cast over a vapor barrier (see Richard H. Campbell, Wendell Harding, Edward Misenheimer, Leo P. Nicholson and Jack Sisk, "Job Conditions Affect Cracking and Strength of Concrete In-Place," Journal of the American Concrete Institute, January,1976, pages 10-13).