# Study on Corrosion Monitoring of Reinforced Concrete Based on Longitudinal Guided Ultrasonic Waves
## 4 Testing of Corroded Reinforcement
### Test Results for RC
#### 4.3.3 GUW Signals and DI-SAD of RC  
Figure 15 shows the GUW signal waveforms of bare reinforcement and RC without corrosion. It can be seen from Figure 15 that the influence of the concrete cover on the GUW characteristics is mainly as follows:  
1. The amplitude of the GUW signal decreases as the covered thickness increases, and the more signal energy is leaked. The amplitudes in the reinforcement bars with the 75, 110, and 160 mm covered diameters are 1/19, 1/28, and 1/70 of those in the bare reinforcement, respectively. 2. With the increase in the covered thickness, the arrival time increases and the wave velocity decreases. The arrival time of the signal in the bare reinforcement is 292 us, whereas the corresponding arrival times are 405, 462, and 491 us for the reinforcements with 75, 110, and 160 mm covered diameters, respectively. 3. The mode components in the signals are increased and the signals are complex. [PERSON] et al. [28] pointed out that on the one hand, due to the small signal amplitude, the signal-to-noise ratio is lower and more interfered with by noise; on the other hand, the modal components of the GUW signals are more complex in the composite reinforcement-concrete waveguide medium. Figure 14: Diagram of specimens with different covered thicknesses before and after corrosion. Figure 15: GUW signals for reinforcement and RC without corrosion. The above phenomenon is consistent with the results discussed in Section 2.2. The effectiveness of DI-SAD has been discussed previously, and DI-SAD is still used here to identify the corrosion state of the reinforcement within the concrete. The relationship between the measured DI-SAD and corrosion time for the three RC specimens with the same excitation frequency at different corrosion times is shown in Figure (a)a. During the corrosion process, the corrosion inside the concrete was measured intermittently using GUWs. In the RC specimen with a 75 mm diameter, the measured GUW signals at different corrosion times are shown in Figure (b)b. It can be seen from Figure (b)b that the change in the GUW signal DI-SAD propagating in the reinforcement with the corrosion time (based on the minimum and maximum values of the normalized DI-SAD) can be divided into the following three stages in the case of covered concrete. 1. The first stage is the initial stage of reinforcement corrosion, which is characterized by a decrease in the amplitude of the GUWs. At this stage, the corrosion products are slowly filled in the pores of the concrete around the reinforcement, which does not cause the internal cracking of the concrete. Due to the filling of the corrosion products, the contact between the reinforcement and concrete is closer, and more GUW energy is leaked from the reinforcement to the concrete, making the amplitude decrease continuously. 2. The second stage is the development stage of the internal cracks in the concrete. After the corrosion products fill the pores, their volume continues to increase, and the resulting rust expansion pressure causes cracks to begin to develop within the concrete when the concrete stresses reach the tensile strength. The bond failure between the reinforcement and concrete leads to the gradual decrease in the GUW energy leaked into the concrete, so the amplitude of the GUWs propagating along the reinforcement tends to increase. 3. The third stage is the development stage of the concrete surface cracks. As the cracks within the concrete continue to extend and expand, cracks appear on the concrete surface. Corrosive products on the reinforcement surface seep out along the cracks, while the corrosion solution flows to the reinforcement surface along the cracks. Since the acoustic impedance of the liquid is much larger than that of the metal material, the amplitude of the GUWs propagating in the reinforcement decreases rapidly. However, the corrosion time of the concrete at each stage is related to the diameter of the concrete.