## 3 Results and Discussion  
### Failure Forms and Ultimate Flexural Load Affected by Steel Corrosion  
In the actual RC bridge structures, the longitudinal reinforcement corrosion-induced cracks are located on the bottom and top surfaces of the bridge superstructure. Thus, in order to better mimic the actual RC bridge structures, the two most seriously cracked two opposing side surfaces of the RC specimens were set as the loading surface and the support surface. The failure forms of the five RC specimens after the bending test are shown in Figure 7.  
From Figure 7a, it can be seen that the concrete was crushed at the load point during the loading process, and there was one shear crack that appeared on the side surface, indicating that the failure form of the uncorroded beam was over-reinforced.  
From Figure 7b, it can be seen that the concrete at the load point was crushed while the concrete on the underside was cracked in the 10% corroded beam. Furthermore, after loading, one flexural crack and two shear cracks can be observed on the side of the specimen. These phenomena indicate that the failure form of the 10% corroded beam was balanced-reinforced.  
From Figure 7c, it can be seen that the concrete at the load point was not crushed, and the concrete underside was cracked in the 20% corroded beam. Furthermore, there was only one shear crack on the side of the specimen after the bending test. These phenomena indicate that the failure form of the 20% corroded beam was rare-reinforced.  
From Figure 7d, it can be seen that the concrete at the load point was not crushed, and the concrete underside was cracked severely in the 30% corroded beam. There was one extended irregularly bending crack on the side of the specimen. Furthermore, after loading, the concrete was spalling at the lower right corner of the specimen, from which it can be inferred that steel corrosion causes severe damage inside the concrete. The failure form of the 30% corroded beam was rare-reinforced.  
Figure 5: The arrangement of acoustic emission (AE) sensors (unit: cm).  
Figure 6: Experimental setups for the bending test and AE test.  
From Figure 7e, it can be seen that there was one bending crack with a large crack width that existed in the 40% corroded RC specimen. Furthermore, the side of the specimen was almost spalling after the loading process. In addition, the steel bars embedded in the concrete were corroded severely, suggesting the tiny bonding force between concrete and steel bars. Based on these phenomena, it can be judged that the specimen was almost at the point of failure before the bending test due to steel corrosion.  
The ultimate flexural load of these five RC specimens is shown in Figure 8.  
The ultimate flexural loads of 10%-40% corroded RC beams decreased significantly with the increase of the steel corrosion ratio; the ultimate flexural load of the uncorroded beam was close to that of the 20% corroded beam. The reason for this is that the uncorroded beam is an over-reinforced beam; during the loading process, the concrete on the top of the RC beam will be initially damaged, resulting in low flexural strength. Therefore, the balanced-reinforced RC structures should be designed to meet the requirements of compression and flexural strength. Also, as can be seen from the failure forms of RC beams, the corroded steel bars crack the surface of concrete, decrease the bonding force between steel bars and concrete, as well as the mechanical properties of concrete material, then reduce the flexural strength of the RC beams.  
Figure 7: Failure forms of corroded reinforced concrete (RC) beams: (**a**) Uncorroded RC beam; (**b**) 10% corroded RC beam; (**c**) 20% corroded RC beam; (**d**) 30% corroded RC beam; (**e**) 40% corroded RC beam.