We haven’t quite developed Superman-grade X-ray vision, but we’re getting close. Some would argue too close, given the invasive capabilities of equipment used for airport body scans; but that’s fodder for other blogs.
Radiography (erroneously referred to as X-ray) examination used to be the only reliable way of determining the presence and depth of reinforcing steel or conduits in concrete. That technology employs a radioactive source capable of penetrating concrete sufficient to project an image onto a photo/radiographic film or screen placed on the side opposite the source. X-rays project a high-energy beam of electrons with similar effect and the energy requirements make those systems essentially non-portable. Radioactive materials are inherently hazardous and their possession and use are tightly controlled by the Nuclear Regulatory Agency.
A safer and more accessible way to see inside concrete is the use of ground-penetrating-radar (GPR). GPR directs high-frequency, high-energy radio waves into the material to be examined (concrete, asphalt, soil, etc.) and the materials located therein will reflect the waves back to the source. Because the degree and nature of the reflections vary with density and other factors, an image of the scanned structure is created.
GPR has been around for awhile, but resolution issues limited its use in building diagnostics. Improved resolution and such innovations as 3-D imaging have elevated GPR’s use to preeminence in non-destructive testing of concrete and other building elements.