Don,
The type of PVC that is used for rigid conduit, is an un-plasticised version of Poly-Vinyl-Chloride.
The type used for wires and cables has a plasticiser added to it, to make it more flexible.
It is this plasticiser, that the acidic content of the concrete reacts with.
There are also forms of plasticiser that react with sunlight too and some overhead lines here have been known to lose thier insulation.
