# Study on Corrosion Monitoring of Reinforced Concrete Based on Longitudinal Guided Ultrasonic Waves
## 4 Testing of Corroded Reinforcement
### Test Results for Bare Reinforcement
#### 4.2.2. GUW Signals and DI-SAD of Reinforcement  
As shown in Figure 11, ultrasound waves propagate within the reinforcement and form GUWs under 100 kHz sinusoidal pulse excitation, and evidently reflected waves can be seen. The direct waveforms of the reinforcement at different corrosion degrees are not visibly different after amplification. When the reflected waves are formed, the waveforms of the GUWs with different corrosion degrees are different. From the direct waveforms, it can be seen that the time corresponding to the peak of the direct wave is almost the same (about 292 us) for reinforcements with different corrosion degrees. From the reflected waveforms, it can be seen that with the increase in the corrosion degrees, the reflected waveforms of reinforcement show a forward shift and its peak value gradually decreases, that is, the time corresponding to the peak value gradually decreases (from 800.27 us to 798.58 us). In conclusion, the greater the corrosion degree, the lower the waveform amplitude and the lower the arrival time.  
It has also been noted that the amplitude of the reflected wave is significantly decreased with respect to the direct wave, and [PERSON] et al. [41] pointed out that this decreasing trend conforms to an exponential function. This is due to the refraction and scattering of the reflected wave after passing through the interface between the two media. Part of the energy is reflected back while the other part propagates through the medium during the GUW propagation. This results in the decrease in the amplitude of the reflected wave compared with the direct wave.  
It can be seen in the frequency dispersion curve shown in Figure 2a that the group velocity of the reinforcement is 4510 m/s at a frequency of 100 kHz. The propagation velocities of the specimens in the finite element and test studies were calculated from the arrival times of the absolute peak amplitudes: The propagation velocity of the GUW in the reinforcement in finite elements is 4449 m/s according to Figure 7b,c; the propagation velocity of the GUW in the reinforcement in the test is 4329 m/s according to Figure 11. It can be seen in Figure 12 that the deviations in the propagation velocities obtained by these three are small, all within the 4% deviation range. This further validates the correctness of the finite element method and test. For the reinforcement embedded in concrete, various longitudinal GUW modes are observed at a single frequency, which makes it complicated to separate the wave modes and calculate the group velocities. Considering this complexity, the group velocity dispersion is only validated for bare reinforcement.  
Figure 11. GUW signals of reinforcement at different corrosion degrees.  
The same excitation frequency as the finite element simulation is used, and the test and simulated DI-SAD obtained after amplitude normalization are compared, as shown in Figure 13. The results show that the changing trend between the DI-SAD and corrosion degree of the corroded reinforcement is consistent. The sensitivity of the index is positively correlated with the frequency of the GUWs. Overall, both the test and simulated results indicate that it is feasible that the DI-SAD can be used as an index for determining the degree of corrosion of reinforcements.