A Gamma Scattering Technique for Inspecting Concrete Structures

The present paper deals with a feasibility study of Compton scattering technique for inspection of concrete structures for the detection of local defects and the discrimination between materials of different density and composition, such as concrete, void and steel. The experimental setup consists of a Cs radioactive source and a HPGe planar detector providing high resolution energy dispersive analysis of the scattered spectrum. Reinforced concrete was represented by the insertion of two mild steel rods, 125 mm in length and diameter 9 mm into concrete slab of dimension 170 mm х 150 mm х 55 mm. An air cavity was created by inserting two cylindrical plastic voids each of size18.5 mm 3 into concrete slab. Scanning of the object was achieved by lateral movement of the concrete slab across the source and detector collimators in steps of 5 mm. The results show that the scattering method is highly sensitive to changes in electronic and physical densities of the volume element under study. The cylindrical voids and mild steel rods have been detected in the present study with a statistical accuracy of better than 0.1%. The density contrast is estimated to be 0.5 % and spatial resolution for detecting steel and void is estimated to be roughly 5 and 10 mm, respectively. The presence of steel and void can be clearly distinguished. As the steel bar intersects the sensitive volume, there is an increase in the total electron density of the material comprising of the sensitive volume, hence an increase in detector response. As the sensitive volume intersects the void cavity, there is a reduction in the total electron density and, therefore, a decrease in the detector response