## 1 Introduction  
Steel bars are widely used as reinforcements for concrete structures in civil engineering [1; 2]. Although the use of non-metal bars in concrete is receiving more and more attention [3; 4], steel bars are still the most commonly used in concrete structures. During their service life, steel bars are prone to corrosion in the natural environment [5]. The consequences of the corrosion of steel rebars may result in non-uniform distributed points or pits on the steel surface, and the volumetric expansion of the reactant leads to the increase of radial stress on the reinforcement concrete, which reduces the constraints of the reinforced concrete, destroys the original bonding, and ultimately results in cracks in the structure [6; 7]. The corrosion of steel bars often occurs in coastal structures, hydraulic structures, and bridges, as well as roads and buildings, among others. The corrosion of steel bars poses a threat to the safety and health of these structures, and may cause catastrophic consequences if undetected. Monitoring corrosion is one of the best options to offer early warning about corrosion, and the high cost of later repairing can be avoided. Structural health monitoring (SHM) [8; 9] is a relatively new area that is currently being explored in corrosion monitoring in order to achieve the objectives of rapid and real-time detection of the corrosion of steel reinforcement, and to offer an early warning for timely maintenance [10]. In the current literature, physical and electrochemical methods are the two main methods used to detect the corrosion of steel rebars [11]. Physical methods are used to evaluate the corrosion of steel rebars by detecting changes in the physical characteristics, including electrical resistance, electromagnetics, heat conduction, and sound propagation. Such methods include time domain reflectometry (TDR) [12], X-ray diffraction and atomic absorption probe technique, infrared (IR) thermography [13], and fiber optic corrosion detection [14]. A disadvantage is that the external sensor wire is not sufficiently sensitive to detect the degree of corrosion of steel rebars in the TDR measurement. The X-ray diffraction and atomic absorption probe technique are not suitable for on-site monitoring, owing to their radiation hazard [15]. The fiber optic sensors can also be used to monitor the corrosion of steel bars, but the detection is restricted to the small area where the sensor probe is installed. In addition, the high cost of protecting the fiber and interrogator also poses challenges to implementation [16]. The electrochemical method is an approach that detects the electrochemical properties in order to evaluate the corrosive degree of a reinforced concrete corrosion system. It includes the half-cell potential method [17], galvanostatic pulse technique [18], and electrochemical impedance spectroscopy method [19; 20]. Although these electrochemical methods can be used for monitoring civil structures, their disadvantages include a long preparation time, low accuracy, saturation, unstable readings, insulation aging, poor electromagnetic interference resistance [21], and the loss of signal owing to a long electrode lead wire. The ultrasonic guided wave (UGW) technique is a type of physical method. With the advantages of in situ, real time, and more precision in corrosion monitoring, it has become increasingly popular in the corrosion monitoring of steel bars [22]. [PERSON] et al. [23] proposed a corrosion monitoring method of steel bars in an RC structure based on the time of arrival changes. [PERSON] et al. [24] used frequency sweeps of guided mechanical waves for the detection and estimation of corrosion damage in steel-reinforced mortar samples with seeded defects, and in samples undergoing accelerated corrosion owing to the impressed current. [PERSON] et al. [25] presented an approach for monitoring the corrosion evolutionary path in post-tensioned systems, based on the fractal analysis of UGW.