The code does not say that the rebar in a slab is a grounding electrode. Only the rebar in a footing or foundation. See 250.52(A)(3). If the footing or foundation rebar is accessible, then you are required, by 250.50, to use it as part of the grounding electrode system. But since this sounds to be just a slab the rebar would not have to be grounded IMO. Hope this helps