## 3 Experimental Setup
### Detection of Corrosion in Steel Bars  
In a concrete structure, natural corrosion is a time-consuming process. The corrosion electric current has significant effect on the corrosion speed of the steel bar. Thus, in this study, the impressed current was used to shorten the corrosion time and to obtain the corroded specimens quickly. The experimental instruments included a real-time data acquisition system (NI-USB 6363), PC, specimens, piezoceramic disk transducers, and support foam. With the consideration of the properties' difference of the steel bars, four Q235 steel specimens with a different date of production were used to verify the consistency of the experimental results. The diameter and length of the steel bars were 20 mm and 400 mm, respectively. The piezoceramic disk transducers were bonded to the ends of the steel bar, one of which was used as a signal receiver, and the other one was used as a signal generator. Then, both of the ends of the steel bar and the wires were wrapped with epoxy resin. In addition, the impressed current technique was used for accelerated corrosion under a constant current of 2 A. The experimental apparatus is shown in Figure 7.  
Figure 6: Relationship diagram of time and voltage. Figure 7: Experimental setup consisting of piezoceramic transducers to detect the corrosion of steel bar. The steel bar and a copper rod were immersed in a tank filled with a water solution containing 5% NaCl. The steel bar was connected to the positive electrode of a DC power supply source as the anode, and the copper rod was connected to the negative electrode, and it acted as the cathode, which formed the accelerated corrosion system. Figure 8 shows the accelerated corrosion setup. An NaCl solution has been chosen in this study, which is based on the large amount of NaCl in seawater, and because the long-span bridge structure is vulnerable to seawater erosion. Piezoceramic transducers were bonded on the surface of the two ends of the steel bar, and a multimeter was used to detect whether the piezoceramic transducers were properly insulated. First, the Labview data acquisition system was used to carry out a frequency sweep so as to acquire the resonance frequencies of the steel bars and to record the waveform data. The obtained data showed that the amplitude of the received signal had a better response at frequencies ranging between 25 KHz to 100 KHz. The Gaussian pulse signals, whose frequencies were set as 70 KHz, 60 KHz, and 50 KHz, respectively, and with an amplitude of 5 V for all of the frequencies, were generated using a Labview program hosted by a PC. The pulse signals were recorded every 5 h using a Labview program, and the quality of the steel bar during the accelerated corrosion process was measured using the current control technique. Figures 9 and 10 show the steel bar without corrosion and the same bar with corrosion after 35 h, respectively. Figure 8: Corrosion experimental setup. Figure 10: Corrosion of steel bar after 35 h.  
Figure 9: Steel bar without corrosion. After 50 h, the corresponding signals for the different degrees of corrosion were obtained. To analyze the influences of the corrosion degrees on the amplitudes of the focused signal received by the piezoceramic transducers, the peak values of the focused signals for the four specimens are listed in Tables 1-4, respectively. Each table includes three cases for the frequency of the emitted Gaussian pulse signal 50 KHz, 60 KHz, and 70 KHz. It can be seen from the tables that with the decrease of the magnitude of the focused signal, the mass loss ratio of the steel bar is increased, which means that the steel bar was corroded more seriously.