The concrete contractor has little love for retarders because he knows that, when one is specified under his concrete slab on grade, that vapor retarder also prevents the slab from curing evenly. To elaborate, the free water is retained by the retarder under the slab, which keeps the bottom of the slab moist while the top dries out faster. This dynamic causes shrinking tensions that produce a curl in the concrete.
It’s been said, although I personally have not seen it done, that a cement contractor often will deliberately puncture the vapor retarder to allow the free water to pass through and thus allow the slab to cure — from top to bottom — at a relatively equal rate. This, according to the contractor’s rationale, will help keep the slab from curling.