[lucas]

I suppose it might depend on the nature of the reinforcement too. If you are looking for the traditional steel rebar that is embedded, you might consider the ACS system (MIRA) https://acs-international.com/
It uses an array of dry-point contact shear wave transducers (I think they operate on the principle of bimorphs). Because it is ultrasonic it should be unaffected by the surrounding EM.
I am not sure how ground penetrating radar would work when surrounded by an active EM field but perhaps the manufacturers can advise you on this (or demonstrate the equipment for you).