Chlorides and moisture assessment in concrete by GPR full waveform inversion

Corrosion of rebar within reinforced concrete is a major problem for countries where salt is applied to roads for de-icing. Concrete structures are periodically inspected in order to monitor possible damage caused by chloride-induced corrosion of the reinforcement. However, the available drilling and visual inspections do not supply sufficient spatial information or can only be assessed in advanced stages of corrosion, respectively. Consequently, the condition of bridge decks can only be assessed with low certainty. Therefore, a spatially continuous and non-destructive method detecting chloride in concrete structures is desirable. This paper describes a novel method to estimate material properties using the full-waveform inversion of bistatic off-ground ground penetrating radar data. In this way, all information present in the ground-penetrating radar (GPR) traces is used, which enables the estimation of quantitative electromagnetic properties. A critical step for full-waveform inversion is a proper characterization of our horn antenna GPR system by estimating the phase centre and the effective wavelet using measurements over a stainless steel plate. The inversion of GPR data measured over nine concrete specimens having different moisture and chloride contents returned a relative dielectric permittivity and a conductivity that included a frequency-dependent component. As expected, the inversion results for almost all specimens showed for increasing chloride and humidity content specimens increasing conductivity and permittivity values, respectively. In contrast to traditional ray-based techniques we were able to distinguish between moisture and chloride effects and to obtain quantitative values for the permittivity and conductivity. For increasing chloride content increasing frequency-dependent conductivity values were obtained.