Detection of weak scatterers in reinforced concrete structures

GPR surveys on concrete structures like bridges, decks and buildings are successfully applied to detect and localize construction elements mostly made of metal. Defects like air voids behind rebars in incompletely compacted concrete still represent major challenges due to their weak reflection amplitudes. In a systematic study at a concrete test specimen with vertical rebars and simulated air voids of different size and depth, the effects of different GPR data acquisition parameters like antenna frequencies (1.2 up to 2.6 GHz) and antenna orientation on the quality of imaging of these air voids were investigated. Very dense measurements with high position accuracy were carried out using an automated scanning system. Despite the strong shadowing and disturbing interference effects of the reinforcement and the associated strong reflections, the weak scatterers can still be identified to a certain depth using appropriate antenna configurations. Measurements with the antenna orientation perpendicular to the rebars of 1.2 to 2.0 GHz give the best results. Preand postprocessing steps like filtering, gain, 3D SAFT (Synthetic Aperture Focusing Technique) reconstruction and some adapted visualization options were properly applied to improve the imaging results of these weak scatterers.