## 3 Experiment Results and Discussion
### Failure Process and Modes
#### 3.1.2 Strengthened Specimens  
All strengthened specimens (RP-0 to RP-5) displayed flexural failure modes involving the crushing of concrete at the bottom of the core pier column and filling layer, as well as buckling and fracture of longitudinal rebars (Figure 11). All strengthened specimens experienced segment cracking, loosening of the circumferential connection of the segments, filled concrete crushing, concrete crushing and spalling at the core pier column root, longitudinal rebars pulling to fracture, and stirrups buckling. (a) In RP-1, the first transverse crack appeared at the pier column root when the lateral displacement reached 6 mm. Owing to the presence of LCSS, cracks on the surface of the core pier column could not be inspected during the test. The longitudinal rebar reached the yield strength when the displacement reached 8 mm, and the specimen reached yield strength, indicating that \(\Delta_{y}\)= 8 mm. During the \(\Delta_{y}\) to \(2\Delta_{y}\) loading cycles, several transverse cracks began to expand on the tensile side of the core pier column root. Subsequently, the initial transverse cracks widened and extended, gradually developing into diagonal cracks during the \(3\Delta_{y}\) to \(5\Delta_{y}\) loading cycles. Several new transverse cracks were also generated at the core pier column root. During the \(6\Delta_{y}\) to \(9\Delta_{y}\) loading cycles, the filled concrete and bottom concrete of the core pier column graduallyspalled due to the rapid development and penetration of cracks. Moreover, several vertical cracks appeared at the bottom of the LCSS, and part of the circumferential connection of the segments began to loosen and separate. Finally, during the 10\(\Delta_{y}\) to 11\(\Delta_{y}\) loading cycles, the concrete at the core pier column root was crushed, and vertical cracks at the LCSS developed rapidly. A \"bang\" was heard during this progress, which was considered to be due to a longitudinal rebar fracture. The test was terminated when the horizontal load decreased to 85% of the peak load. (b) The test failure processes of specimens RP-0, RP-2, and RP-3 were similar to those of RP-1. (c) The initial cracks for specimens RP-4 and RP-5 appeared at the core pier column root when the lateral displacement reached 12 mm, and \(\Delta_{y}\) was set to 14 and 18 mm, respectively. Similar to that of RP-1, during the \(\Delta_{y}\) to 8\(\Delta_{y}\) loading cycles for RP-4 and RP-5, cracks developed at the core pier column root and LCSS, the circumferential connection of the segments loosened, the bottom of the filled concrete was crushed, and crushing and spalling of the concrete occurred at the core pier column root.  
Figure 11g,h shows the internal damage conditions at the bottom of the strengthened specimens via demolishing the segment sleeves and filled concrete in sequence after the test. Both the segment sleeves and filled concrete remained relatively intact, indicating that the LCSS and SSAWC designs could satisfy the seismic strengthening demand.  
Figure 10: Typical failure phenomena and modes of unreinforced specimens. (**a**) UP-1; (**b**) UP-2; (**c**) UP-3.  
Figure 11: Typical failure phenomena and modes of strengthened specimens. (**a**) RP-0; (**b**) RP-1; (**c**) RP-2; (**d**) RP-3; (**e**) RP-4; (**f**) RP-5; (**g**) Demolition of the segment sleeves; (**h**) Demolition of the filled concrete. Note: W1 represents the average looseness width of LCSS, W2 represents the average crack width of concrete (unit: millimeters).