## 3 Results and Discussion  
### Crack Patterns and Failure Modes  
Figure 7 illustrates the cracking patterns and typical failure modes of eight pier specimens observed after testing. The failure processes of the PP-ECC and RC piers were carefully monitored and recorded during the test; the details are described as follows. Six PP-ECC specimens (PP-ECC-1\(\sim\)PP-ECC-6) exhibit some similar failure processes and modes. For example, the scenario of PP-ECC-6 is described here. The first crack usually occurs at approximately 130 mm above the bottom of the pier, while the lateral loading is about 10 kN. As the test loading increases, many tiny transverse cracks and shear oblique cracks are formed in the PP-ECC region. The spacing between adjacent cracks is about 40-60 mm. Before the yielding of longitudinal steel bars, the cracks are able to completely close during unloading, and the residual lateral displacement is almost invisible. Numerous transverse cracks are formed in the PP-ECC region of the pier after the yielding of longitudinal reinforcement bars, while the lateral load is about 37 kN. The PP-fibers in the PP-ECC exhibit tensile failure with a \"missing\" sound. When the lateral displacement reaches 80 mm and the lateral load reaches 51.3 kN, a lot of transverse cracks display in the non-PP-ECC region of the pier, and the spacing between the adjacent cracks is about 150 mm. After reaching the peak load, the width of the existing cracks increases and few new cracks form. A large number of vertical cracks occur within the range of 150 mm from the bottom of the pier. In the range of 150-200 mm from the bottom of the pier, the local PP-ECC has bulged outwards under pressure, but no peel off of PP-ECC cover is observed over the test period. When the pier loaded to a lateral displacement of 100 mm, the bearing capacity has dropped to less than 85% of the peak loading, and the longitudinal reinforcement bars are buckled significantly. Because of the fiber bridging effect, it controls the crack development effectively in the PP-ECC pier. The pier remains in good integrity after failure. For the RC specimens (RC-7 and RC-8) also exhibit similar failure processes and fail in a flexural manner. The scenario of specimen RC-8 is discussed here as an example. It experiences four typical failure stages during the test, including initial crack in concrete, yield of steel bar, spalling of concrete  
Figure 7: Crack patterns and failure modes of specimens. cover, and buckling of longitudinal reinforcement bars. At the beginning of the test, there are no obvious cracks on the concrete surface. The first crack is observed in the plastic hinge region when the test loading is about 20 kN. As the lateral loading keeps increasing, the number and width of cracks both increase. The spacing between adjacent cracks is about 60 mm, and the cracks propagate to both sides. The longitudinal reinforcement bars yield when the lateral load is about 55 kN, and the corresponding lateral displacement is 20 mm. Crack width and length increase with the increasing lateral displacement level, more and more transverse cracks appear in the middle and lower portions of the pier, and the spacing between adjacent cracks is about 100 mm. As the lateral displacement level continues to increase, few new cracks form. However, the previously formed cracks become wider and longer gradually, and four macro cracks form in the plastic hinge region at the lateral displacement of 40 mm. When the displacement reaches 60 mm, the concrete cover is crushed and peeled off from the pier. At the same time, the longitudinal steel bars and stirrups are exposed, and a distinct plastic hinge is generated at the bottom of the pier. The peak load is achieved at the lateral displacement of 80 mm, and the majority of concrete cover at the bottom is crushed and peeled off. The longitudinal reinforcement bars are buckled due to high stress, and the stirrups are pulled outward. The active load then reduces to less than 85% of the peak load, indicating that the RC8 specimen is destroyed.