One of the most contentious topics in meetings of ACI Committee 302, Construction of Concrete Floors, has long been vapor barriers. Is a vapor barrier needed at all? Does it increase curling? Does it increase the problem of dominant joints? Should there be a gravel layer on top of the vapor barrier? Is the vapor barrier a nuisance or a necessity, as Peter Craig famously asked in a 2004 CC article? (His answer was, yes.)
The evidence is in on curling and it seems placing a floor slab directly on a vapor barrier actually does not increase curling with low-shrinkage concrete. The issue of dominant joints, however, is not so clear. It actually makes sense—if the vapor barrier reduces the friction (restraint) beneath a slab, then some joints don’t activate and others widen as the slab shrinks. But can this be controlled?
This is what I would like to explore during the Quality in Concrete Slabs Luncheon and Forum at World of Concrete in 2016. I hope you will join us.