## 3 Results and Discussion
### Reinforcement Bar Behavior  
Because there are many reinforcement strain measurement points in each bridge pier (16 points \(\times\) 8 piers), considering the length limitation, the testing results of PP-ECC-6 pier are only included here, as depicted in Figures 12 and 13. Other related plots for the lateral loading versus transverse reinforcement strain and the lateral loading versus longitudinal reinforcement strain can be found in [35], and are available from the authors by reasonable request. Based on the hysteresis curves of lateral loading versus reinforcement strain, it can be found that:  
Figure 12: Lateral load-longitudinal reinforcement strain curves of PP-ECC-6 pier. Figure 13: Lateral load-transverse reinforcement strain curves of PP-ECC-6 pier. The longitudinal reinforcement in the bridge pier exhibits an elastic-plastic stress process under the cyclic reversed lateral loading. The hysteresis curve of longitudinal reinforcement strain versus lateral loading hysteresis is quite long but narrow. When the loading level is relatively small at the beginning, the increasing trend of the longitudinal reinforcement strain is slow. However, after the cracks form in the pier, the longitudinal reinforcement strain increases rapidly. After unloading of each cycle, a certain residual strain still exists. As the lateral displacement increases, the residual deformation after unloading keeps increasing, which indicates that the plastic deformation of longitudinal reinforcement also increases gradually. Under the same loading displacement level, the longitudinal reinforcement strain is smaller when it locates further away from the pier bottom. The maximum longitudinal strain usually presents at the bottom of the pier. There is a very slight difference in longitudinal reinforcement strains between the left and right sets of strain gauges. During the test, the strain gauges near the bottom of the pier are very seriously damaged, resulting in a certain distortion of the strain value for some plots. This is mainly contributed to the reinforcement at the bottom of the pier is more stressed and complicated, so the strain gauge is more prone to damage. Due to the inhomogeneity of concrete materials and the complexity of stress in the plastic hinge region, the lateral loading-transverse reinforcement strain curves of piers present a certain irregularity. With the increase in loading lever and the increase in lateral displacement, the transverse strain of stirrup in the plastic hinge region increases, and the interlocking between stirrups and concrete is also obviously generated. The stirrups still exhibit some certain residual strains after unloading. The residual strains in stirrups are very small at lower loading levels but become greater when the lateral displacement level increases. Stirrups generally help to hold in place the longitudinal reinforcement bars. Transverse shear and vertical tension are caused by the combined loadings. Stirrups hold the cracked concrete surface together and prevent the pier from buckling. It is shown that stirrups can effectively constrain the lateral deformation of concrete, improving the lateral deformability of concrete. It thus improves the ductility performance and seismic resistance of bridge piers. When the loading mode changes from the loading control to displacement control, the maximum longitudinal reinforcement strain at the bottom of the pier has exceeded the yield strain of longitudinal reinforcement bars (i.e., about 2212 \(\mu\varepsilon\) for HRB400), which means that the longitudinal reinforcement is yielded. When reaching the ultimate load, the maximum longitudinal reinforcement strain at the bottom of the pier exceeds the ultimate strain of reinforcement bars (i.e., about 3084 \(\mu\varepsilon\) for HRB400). The stirrups are always under tension state during the cyclic reversed lateral loading, and the maximum stirrup strain is about 1500 \(\mu\varepsilon\). This means that the stirrups in the pier have not yielded during the test.