Q.: Does polyethylene vapor barrier under slabs decrease or promote surface cracking?

A.: Tests reported by Richard H. Campbell and others in the article "Job Conditions Affect Cracking and Strength of Concrete In-Place," Journal of the American Concrete Institute, January 1976, page 10, seemed to show that concrete placed over polyethylene is more likely to undergo cracking than concrete over a 3-inch layer of dampened sand on native soil or over a noncompacted 3-inch thickness of a dampened cement-treated sand mixture. The investigators suggested that when polyethylene is used a 3-inch pervious bed such as sand be spread on top of the membrane to reduce cracking. 1. Melvin S. Abrams, "Compressive Strength of Concrete at Temperatures to 1600 F,- Temperature and Concrete, ACI Publication SP-25. (This publication also contains other articles on the effects of temperature on concrete.)