Radiographic Evaluation of Concrete
Radiographic evaluation of concrete involves the use of radiographic test on building elements to examine the internal condition of concrete. This test is capable of examining concrete members without causing any damages. It determines the location of reinforcing steels, voids in concrete, and the position of stressing wires and ducts.
The radiographic test is based on the ability of X-rays or gamma rays to pass through materials under examination. When radiation travels through concrete element, different parts of reinforced concrete element such as steel bars, concrete, and voids would absorb different quantity of x-rays.
Radiographic film which is sensitive to radiation is used to catch images of defects and steel bars. This enables the observer to detect and specify the location of steel bars, voids, and cracks, accurately.
Radiographic Test Equipment
- X-ray equipment or Gamma ray sources to direct gamma or x-ray into concrete element.
- Radiographic film to catch image of defects and steel bars.
Principles of Radiographic Test
Radiographic is based on the penetration of x-ray or gamma ray into concrete and employing a film to take image of defects in concrete elements. The concrete member to be tested is placed between the radiation source and detector. 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.
Differential absorption of the travelling radiation through reinforced concrete member generate distinct differences when recorded on radiographic film. The resolution of produced images rely on the orientation of the defects in concrete member with regard to the source of x-ray or gamma ray and radiographic film.
Reinforcing bars absorb more energy than the surrounding concrete and show up as light areas on the exposed film. Cracks and voids 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.
Finally, extreme care shall be practiced to avoid risks of radiation during the test. So, necessary measures and modifications need to be considered prior to the test.
Limitations of Radiographic Test
Radiographic test encounter certain restriction when it is used to test thick concrete members. This because heavy equipment is need to direct x-ray. Such equipment would be difficult and time consuming to set up in the field, and the cost of the test could increase.
Moreover, a system of shielding is required when thick concrete element is tested due to the presence of scattered radiation. The system of shielding may include collimation, masking, filters, back shielding and special intensifying screens.
The interpretation of concrete radiographs can also be difficult since there is no standardized terminology for imperfections and no standardized acceptance criteria.
Applications of Radiographic Test
- It can be utilized to locate the position of reinforcement bar in reinforced concrete
- Radiographic test result can be used to evaluate bar diameter and depth below the surface.
- It can reveal the presence of voids, cracks and foreign materials, the presence or absence of grouting in post-tensioned construction and variations in the density of the concrete.
- Pipeline welded joint inspection
- Pressure vessel fabrication quality control
- Welder qualification testing
- Structural steel fabrication
- Detect Discontinuities in weld
- Determine the extent of corrosion (internal and external)