Rebar Local Corrosion Monitoring of RC Structures Based on Fractal Characteristics of Piezoelectric Guided Waves

In offshore reinforced concrete (RC) structures, the phenomenon of rebar corrosion is widespread, and how to monitor the early production and development of the rebar corrosion is of great significance. However, the rebar corrosion in RC structures is localized and develops along the interface, which brings great challenges to the evaluation of rebar corrosion level (especially in corrosion areas). In this paper, theoretical analysis, numerical calculation, and experiment validation are used to study the rebar local corrosion monitoring and evaluation of RC structures with piezoelectric ultrasonic guided waves (UGWs). For this purpose, a reasonable selection of piezoelectric guided wave excitation and reception method and corresponding setup is studied. Frequency dispersion curves of the selected guided waves under different corrosion conditions are obtained by analyzing the wave dispersion and multimodal characteristics. Based on echo signal energy values and their fractal dimension characteristic values for different corrosion levels, an evaluation index of rebar corrosion is proposed, and a corresponding evaluation equation is established. The effectiveness of the proposed algorithm is verified by a rebar corrosion monitoring test with the accelerated corrosion test and guided wave technology. A fitting relationship between the corrosion level (length and thickness) and the basic characteristics of the sensing signal is established. A corrosion evaluation method is established based on the corrosion index. The results show that the rebar corrosion has a certain effect on the energy and fractal characteristics of longitudinal guided waves. The larger corrosion length and the thicker corrosion layer result in the smaller energy value of echo signal, smaller fractal characteristic value, and the larger corrosion index value.