# Study on Corrosion Monitoring of Reinforced Concrete Based on Longitudinal Guided Ultrasonic Waves
## 1 Introduction  
Reinforced concrete (RC) and prestressed concrete both make full use of the mechanical properties of reinforcement in tension and concrete in compression and are widely used in various types of structures such as bridges, houses, and nuclear power plants. During their long service life, these structures display long-term durability problems in bad operating environments, such as concrete carbonation [1] and reinforcement corrosion [2, 3] of which the latter is the most serious durability problem. Monitoring the corrosion status of reinforcement in concrete has been a topical research in civil engineering. The current detection methods are still based on indirect measurements [4], such as chloride content, concrete resistivity, and half-cell potential, which are used to indirectly determine the corrosion status of the reinforcement by measuring the corrosion environment parameters of the reinforcement inside the concrete [5]. GUW testing is widely used as a non-destructive testing technique for the damage monitoring of structures [6]. When GUWs propagate along a corroded reinforcement, the presence of a large number of surface pits due to corrosion will affect the propagation characteristics of the GUWs in the reinforcement, which is a more direct method for monitoring reinforcement corrosion. [PERSON] et al. [7] investigated the propagation characteristics ofGUWs in reinforcement and solved their displacement based on the frequency dispersion curves of the longitudinal wave modes, which showed that the strain energy of the L(0, 1) mode is mainly concentrated on the surface of the reinforcement, and is more suitable for corrosion studies of reinforcement damage. Many scholars [8; 9; 10; 11; 12] have simulated the local corrosion of reinforcement by grinding or notching and have found that the amplitude of the GUWs showed a linearly decreasing variation with the depth increase in grinding or notching. [PERSON] et al. [13] proposed a damage identification index based on the amplitude difference in the corrosion signal by carrying out GUW finite element simulations of corroded reinforcement and effectively validated the index through GUW tests of corroded reinforcement. [PERSON] et al. [14] carried out a test study concerning the corrosion process of GUWs on steel strands and proposed a method to identify the residual corrosion diameter of steel strands utilizing the variation in the group velocity. More signal processing techniques are being applied to GUW detection with the rapid development of computing software. [PERSON] et al. [15] used the time-of-flight method to obtain the travel time of wave packets as a monitoring indicator based on the variation in the wave velocity with the reinforcement diameter and found that the signal travel time obtained by the cross-correlation signal processing technique was more accurate for the identification of the remaining diameter of the corroded reinforcement. [PERSON] et al. [16] proposed a method to predict the corrosion status based on the \(K\)-means algorithm according to the dispersion and multimodal properties of GUWs. [PERSON] et al. [17] proposed a feature-level data fusion strategy method for monitoring reinforcements with different corrosion degrees using a combination of direct and coda waves. Extensive research on the GUWs' propagation characteristics of multilayer waveguide conductors reports on the propagation characteristics of GUWs in RC. There have been many studies [18; 19; 20; 21] to identify the corrosion stage of reinforcement in concrete by monitoring the signal changes under different corrosion states. [PERSON] et al. [18] analyzed the signal amplitude changes during the corrosion process and proved that it is feasible to identify the corrosion state by using the amplitude changes. Through a GUW test of corroded RC, [PERSON] et al. [21] found a corresponding relationship between the amplitude variation curve of the corrosion signal and the RC corrosion stage. [PERSON] et al. [22] and [PERSON] et al.