Concrete Slab Curling Over Vapor Retarders

摘要

Q. We have always been told that concrete slabs placed directly on a vapor retarder curl more than slabs cast directly on a prepared base. However, we recently measured a slab that we know had a vapor barrier only in limited areas. The portions cast over the vapor retarder had less curling than those cast directly on the base. What is the current industry understanding of this issue? A. Concrete slab curling (also termed "warping") is the upward deformation of the slab at cracks or joints due to the development of a temperature and/or moisture gradient through the depth of the concrete after slab placement. Shortly after the placement, heat is generated in the slab interior by hydration of the cement. If the surface of the slab is exposed to cooler air, a temperature gradient can develop, causing the slab surface to contract relative to the slab interior and bottom and resulting in curling deformation. This generally occurs shortly after slab placement (within the first day), which supports the need to saw-cut contraction joints so that cracking resulting from the curling stresses occurs beneath the saw-cuts and not randomly.