# Study on Corrosion Monitoring of Reinforced Concrete Based on Longitudinal Guided Ultrasonic Waves
## 5 Conclusions  
In this paper, firstly, the GUW dispersion characteristics of the reinforcement and RC are analyzed. Secondly, the finite element simulation of a corroded reinforcement is carried out using random corrosion to determine the frequency of the excitation signal and the damage index. Finally, the identification of the corrosion stages of a reinforcement in concrete is carried out using GUW tests on reinforcement and RC specimens. The main conclusions are as follows:  
1. The five proposed Dis all show a linear increasing trend with the increase in the reinforcement corrosion degree in the finite element analysis, and DI-SAD is the most sensitive to the corrosion state of the reinforcement compared with other DIIs, as the excitation frequency increases in the range of 50-100 kHz. The results of the reinforcement corrosion test and the finite element are in good agreement, and their data correlation is 0.998.
2. The direct waveforms of the reinforcement in the finite element simulation and the test almost coincide, while the reflected waveforms tend to shift forward with the increase in the corrosion degree of the reinforcement, and their amplitudes are significantly decreased compared with the direct waveforms. The deviation between the GUW propagation velocity for the specimens with a different corrosion degree from the finite element and test and the numerical solution of the frequency dispersion curve (4510 m/s) is within 4%, which is in good agreement.
3. As the corrosion time of the RC specimens increases, the DI-SAD values of the GUW show three-stage changes. The larger the covered thickness of the RC specimens, the longer the corrosion time required for each stage. This is the result of the combined effect of the corrosion expansion pressure generated by corrosion and the ultimate tensile strength of the concrete material.
4. Increasing the covered thickness is beneficial to delay the appearance time of corrosion cracks in the RC specimens. When the covered thickness is small, the test value of the cracking time of the RC specimens is basically consistent with the theoretical results, and the error is within 12.5%, while the test values are significantly larger than the theoretical values when the covered thickness is larger, which is due to the assumed conditions of the theoretical model.  
Conceptualization, J.Q.; methodology, J.Q.; software: P.Z.; validation, J.Y.; formal analysis, Y.W.; investigation, Y.W.; data curation, R.J.; writing--original draft, J.Q.; visualization, P.Z.; writing--review and editing, J.Q.; project administration, J.Y.; funding acquisition: J.Q. All authors have read and agreed to the published version of the manuscript.  
This work was supported by State Key Laboratory of Bridge Engineering Structural Dynamics, grant number: ZSJK-KF01; the National Natural Science Foundation of China (Grant No. 52378283); China Postdoctoral Science Foundation (Grant No. 2021M702782); Natural Science Foundation of Chongqing, China (Grant No. CSTB2023 NSCQ-MSX0633); Chongqing Education Commission Science and Technology Research Youth Project (Project Number KJQN202300745).  
Not applicable.  
The data presented in this study are available in the main article.  
The authors would like to appreciate [PERSON], who discussed the finite element simulation and suggested useful comments.  
The authors declare no conflicts of interest.