## 3 Experimental Procedures
### Failure Process and Modes  
1. Unreinforced specimens: As shown in Figure 7, the failure processes for P1 and P2 were similar. For specimen P1, the first crack in the concrete appeared when the load was close to 800 kN, which expanded with the loading. When the load was close to 1100 kN, some of the cracks were connected together to form a long and oblique crack. As a result, the bearing capacity of P1 reached the peak value. Then, the load dropped rapidly and specimen P1 reached failure. For specimen P2, the first crack in the concrete appeared when the load was close to 500 kN, which turned into a penetrating crack when the load was close to 800 kN. As a result, the bearing capacity of specimen P2 reached the peak.
2. Ordinary strengthened specimen: Figure 8 shows the failure of specimen S1-A. Under the load of 1760 kN, the first concrete crack appeared at the interface between the filled concrete and the column. Then, with the increase in the load, the concrete crack gradually developed towards the segment sleeve and crisp sound could be heard, which was judged to be the fracture of the welded steel wire mesh. When the load was close to 2147 kN, several segments fractured and the bearing capacity of S1-A reached its peak, then the bearing capacity dropped until the failure of S1-A. The cracks were concentrated in the concrete cover of the longitudinal bolt, as well as  
Figure 5: Unreinforced specimens. (**a**) Specimens pouring; (**b**) Completed specimen.  
Figure 6: The production process for the strengthened specimens. (**a**) Preparing columns and LCSS; (**b**) Laying welded wire fabric; (**c**) Pouring SSAWC.  
the joint of the LCSS and the interface between the filled concrete and the column. It should be noted that all bolts remained unbroken.
3. Self-stressed strengthened specimens: The failure process and modes of the three strengthened specimens were similar, as shown in Figure 9. When the load was close to 2000 kN, the first crack appeared in specimen S1-S, then the crack developed with the increases in load, making a cracking sound in the process. When the load reached 2600 kN, several segments broke and the bearing capacity of specimen S1-S reached the peak and stabilized for a period of time. Then, the bearing capacity dropped and the cracks appeared at the interface between the filled concrete and the column when the bearing capacity dropped to 2250 kN. Finally, as the load decreased continuously, multiple cracks penetrated and the specimen failed. When specimen S1-SS was loaded to 1730 kN, cracks of concrete appeared in the segment concrete. When the load was close to 1850 kN, interface cracks appeared and developed toward the segments. When the load was close to 2150 kN, the segments at the top of the specimen almost failed and the bearing capacity reached the peak, then bearing capacity continued to decrease with long penetrating cracks appearing in many places until the specimen failed. When specimen S2-S was loaded to 1500 kN, concrete cracks appeared in the segment. When it was loaded to 1920 kN, fractures occurred in the segment due to the penetration of the cracks and the bearing capacity reaching the peak. Then, the bearing capacity decreased with the appearance of long penetrating cracks and the specimen finally failed. In this group, the cracks were concentrated on the concrete cover of the longitudinal bolt and the segment joints and none of the bolts were broken. Through testing and calculation, the self-stress values of the reinforced specimens could be obtained, as shown in Table 6.  
Figure 8: Failure of ordinary strengthened specimen. (**a**) Interface cracks; (**b**) LCSS fractures.  
Figure 7: Failure of the unreinforced specimens. (**a**) P1 axial compression failure; (**b**) P2 axial compression failure.