NDT methods (GPR and UPV) for steel rebar corrosion monitoring

Rebar corrosion is one of the main causes of deterioration of engineering reinforced structures and this degradation phenomena reduces their service life and durability. Non-destructive testing and evaluation of the rebar corrosion is a major issue for predicting the service life of reinforced concrete structures. The research work of the Geophysical Laboratory of University of Ferrara is committed to the study of RC corrosion. Several laboratory tests (Fornasari et al., 2022; Fornasari et al., 2023; Fornasari et al., 2024) were performed on a reinforced concrete beam where a corrosion was induced. In detail, this paper describes the results coming from Ground Penetrating Radar (GPR) and Ultrasonic Pulse Velocity (UPV) methods on induced corrosion experiment. A reinforced concrete sample of about 50 cm x 30 cm was used in this experiment. The concrete beam has a central ribbed steel rebar of 10 mm diameter and partially covered with epoxy resin, in order to focalize the corrosion only along the exposed part of the rebar (8cm). The same waterproof epoxy resin was applied on part of the concrete sample, in order to have a specific chlorides diffusion across a freeway zone of about 10cm x 8cm defined below the exposed rebar. The concrete sample was partially exposed (1cm) to a salty water with different NaCl concentrations. An initial NaCl concentration of 0.1 % was adopted for 7 days, then the concentration was increased to 1% and finally to 3.5% for further 7 days. The used instruments were GPR Proceq GP8000 with 2GhZ antenna and UPV Cronosonic MAE with Tx-Rx at 55KhZ. The NDT acquisitions were carried out along the same line across the central part of the buried rebar during the accelerated corrosion test. The GPR elaboration (Hilbert function) highlighted an increase of the envelope factor values with time corrosion of the rebar. In contrast, ultrasound data obtained from the rebar revealed a decrease in velocity as corrosion increased. Over the past few years, numerous experiments have been conducted using various NDT methods, each capable of illustrating signal variations during the corrosion phenomena. These results emphasize the sensitivity of NDT methods in detecting rebar corrosion. The use of multi-sensor tools serves as the starting point for integrated observation, facilitating the transition from qualitative assessments to monitoring the evolving corrosion phenomenon on reinforced steel rebars. This approach aims to establish a quantitative analysis of the observed phenomena.