You will need to shoot at right angles to get depth and resolution will depend upon thickness of section examined, and size of rebar relevent to any aggregate in the mix. For very thick sections you are going to need real penetrating power using Colbalt 60 or a LINAC. There are other methods you might want to investigate such as electromagnetics mentioned in another repsonse or a portable ground penetrating radar device.
You talk about parallax radiograph procedure (double exposure) ? we already use this procedure. Yes we use Radar technique to detect rebar and it depth but is not sensitive enough to detect the corrosion of reinforcement.