## 3 Accelerated Corrosion Test of Steel Bars in Different Corrosion Degree
### Electromagnetic Wave Reflected Voltage of Steel Bars with Different Degrees of Corrosion  
The electromagnetic wave reflected voltage of steel bars in concrete with different thicknesses of the protective layer (4 cm, 6 cm, 7 cm, and 9 cm) was analyzed at different durations of accelerated corrosion (0-408 h):  
1. Analysis of the reflected voltage for the 4 cm protective layer  
The results of the ground penetrating radar showed that in the three different acceleration periods of 72 h, 192 h, and 288 h the reflected voltage with different degrees of corrosion had different changes, as shown in Figure 13. From the results, the degree of corrosion increases with the increasing accelerated duration.  
The changes of the reflected voltage of the ground penetrating radar became apparent at 72 h, which demonstrated that the steel bars began to corrode after 72 h of electrification. When the duration of the accelerated corrosion was 288 h, the reflected voltage only had small changes.
2. Analysis of the reflected voltage for the 6 cm protective layer  
From the detection results of the ground penetrating radar, the reflected voltages had different changes with the degree of corrosion at the three different accelerated corrosion durations of 72 h, 192 h, and 312 h, as shown in Figure 14. At 72 h, the reflected voltage of the electromagnetic wave propagated by the ground penetrating radar for the 6 cm protective layer had significant changes. The steel bars with 4 cm and 6 cm protective layers began to corrode at 72 h. At 312 h, the change of the reflected voltage slowly increased.
3. Analysis of the reflected voltage for the 7 cm protective layer  
From the detection results of the ground penetrating radar at the three different accelerated corrosion durations of 48 h, 168 h, and 288 h, the reflected voltage had different changes with different degrees of corrosion, as shown in Figure 15.  
From the figure, the reflected voltage of the electromagnetic waves propagated by the ground penetrating radar for the 7 cm protective layer changed significantly at 48 h, which was 24 h earlier than that for the 4 cm and 6 cm protective layers. After the current passing through the steel bars for 48 h, they began to corrode. When the duration of the accelerated corrosion was 288 h, the variations of the reflected voltage slowly increased. For the moderate and serious corrosion, there is a 24 h gap between the corrosion potential  
Figure 15: Reflected Voltage of 7 cm Protective Layer.  
and the reflected voltage of the ground penetrating radar. The reason for this phenomenon is that the protective layer of the steel bars has a soaking level of 7 cm, which makes the corrosion faster than the other protective layers. Thus, the corrosion potential cannot explain whether the steel bars have uniform corrosion. However, the reflected voltage of electromagnetic waves propagated by the ground penetrating radar can explain the uneven corrosion in the accelerated corrosion process.  
Analysis of the reflected voltage for the 9 cm protective layer  
From the detection results of the ground penetrating radar, there were three different accelerated corrosion durations, i.e., 72 h, 192 h, and 312 h, as shown in Figure 16. Furthermore, the changes of the reflected voltage of the electromagnetic wave propagated by the ground penetrating radar for the 4 cm and 6 cm protective layers became significant at 72 h, which indicates that the steel bars with 4 cm and 6 cm protective layers began to corrode after the current passing through for 72 h. At the accelerated corrosion duration of 312 h, the variations of the reflected voltage gradually decreased.  
Figures 13-16 are the reflected voltage measurement results of steel reinforcement corrosion at different protective layer depths, and the images tend to be similar, indicating that with the increase in accelerated corrosion time, the more serious the corrosion degree of the steel reinforcement.