## 1 Introduction
In addition, acoustic emission (AE) can also be used to monitor and classify early corrosion damage in RC piles exposed to saltwater [26]. The time reversal (TR) method has been increasingly popular in the fields of signal processing and imaging in recent years. TR is a reverse operation for the received signals in the time domain. Two properties, namely spatial focusing and temporal focusing, enable the TR operated signal to have a high signal to noise ratio (SNR) [27]. [PERSON] et al. [28] proposed a TR method using stress wave-based active sensing in order to detect the state of looseness of a cuplock connection. [PERSON] et al. [29] used a method to detect corrosion damage in reinforced concrete beams, which is based on the time reversal of the guided waves. [PERSON] et al. [27] proposed a method to monitor the grouting quality using a piezoceramic transducer enabled time-reversal method. [PERSON] et al. [30] proposed an impact image method, which is time-reversal-focusing-based, to estimate the position of the impact for complex composite structures. [PERSON] et al. [31] proposed an active sensing method to monitor the degree of looseness of a pipeline tapered thread connection, which used piezoceramic transducers based on time reversal, and demonstrated it to be more robust in rejecting noise in SHM applications. [PERSON] et al. [32] proposed a TR method to identify the depths of artificial pitting corrosion on the galvanized steel wires. [PERSON] et al. [33] proposed a technique regarding time reversal to improve the performance of detecting a cylindrical object bottoming at the seafloor in shallow water. Piezoceramic is one kind of functional material that can realize the conversion of electrical energy and mechanical energy [34]. In addition, piezoceramics have many advantages, including a high sensitivity, low cost [35], quick response [36], wide frequency response range [37; 38], suitability for high-frequency excitation [39], and small size, which have almost no effect on the structural properties [40]. Piezoceramics have been widely used in medical imaging, acoustics, ultrasound,and energy harvesting [41; 42; 43; 44; 45; 46; 47], among others. Piezoceramics can be conveniently developed into sensors or actuators of different shapes [48; 49], because of their positive piezoelectric effect and inverse piezoelectric effect [50]. Piezoceramic devices are commonly used to generate and detect ultrasonic waves for structural health monitoring [35; 51; 52]. Piezoceramics can be embedded into structures or pasted onto the surface of structures in order to monitor the health of structures [53; 54; 55; 56]. Lead zirconate titanate (PZT) is the most commonly used piezoceramic material, with a strong piezoelectric effect. [PERSON] et al. [38] used PZT to detect bolt looseness. [PERSON] et al. proposed an active sensing method to detect the debonding between the reinforcing bar and the concrete [57]. To monitor the corrosion of steel bars, this paper proposes a new approach that is simple to implement and offers real time monitoring. First of all, the corrosion resistance of the piezoceramic transducers with epoxy resin protection were investigated. A rapid corrosion test environment was created using a 20% solution of sodium chloride (NaCl) with an amplified current. The experimental results demonstrated that the NaCl solution had no influences on the epoxy resin proofed piezoceramic transducers. Then, two PZT transducers were surface-bonded on the two ends of a steel rod, respectively. One PZT worked as an actuator, which emits a Gauss signal, and the other one was a receiver. By the TR operation, the amplitude of the focused signal works as the criteria to evaluate the corrosion severity of the steel bars. The accelerated corrosion experiments of steel bars were conducted in a water solution containing 5% NaCl. The ratio of mass loss was applied to evaluate the corrosion severity of the steel bar. The corrosion result shows that the corrosion on the steel bar significantly effects the propagation of the stress wave.