GPR full-waveform inversion of chloride gradients in concrete

The use of salt for deicing roads is a major problem for reinforced concrete structures. With time chlorides migrate within concrete pores and causes reinforcement to corrode, which leads to the reduction of the structure´s load carrying capacity. Due to alternating wet and dry climates, chlorides are distributed in gradients within concrete. Here, an experiment was carried out where concrete specimens were exposed to controlled wetting-drying cycles using different saline solutions for different exposure times to generate chloride gradients. During this experiment repeated off-ground GPR measurements were made. In this paper, two GPR processing methodologies, enabling chloride content determination, are developed to provide an assessment tool for structural engineers. A ray-based forward model provides average permittivity and conductivity values and a novel full-waveform method enables the reconstruction of conductivity gradients within concrete slabs. The destructive determination of chloride gradients distribution confirmed the obtained results. Finally, the obtained results indicate that the chloride solution concentration is dominating the chloride gradient distribution, whereas the exposure time has a minor influence.