Radiographic Evaluation of Concrete
Radiography is similar to taking X-ray or gamma radiation pictures in the medical field. Radiography can determine the internal condition of a structural member and can locate embedded steel. As the radiation passes through the member its intensity is reduced according to the thickness, density and absorption characteristics of the materials within the member. The quantity of radiation passing through the member is recorded on X-ray film. Reinforcing bars absorb more energy than the surrounding concrete and show up as light areas on the exposed film. Cracks and voids, on the other hand, absorb less radiation and show up as dark zones on the film. Crack planes parallel to the radiation direction are detected more readily than cracks perpendicular to the radiation direction.
The penetrating ability of portable X-ray units is limited and is used for members less than 300 mm thick. Gamma rays result from the radioactive decay of unstable isotopes and a gamma ray source cannot be turned off. Extensive shielding is needed to contain the rays when not used for inspection. Gamma ray sources are available that can penetrate up to 450 mm of concrete. For thicker structural elements, a hole may be drilled and the source placed inside the member.