## 3 Test Phenomenon  
The failure process and form of each specimen were as follows:  
First, small cracks appeared at the lower column corners when the specimen column C1 (unstrengthened column) was loaded to 550 kN. Then, the cracks started to grow and widen when loaded to 650 kN. Finally, when loaded to 760 kN, the bottom concrete began to fall off in a large area until the longitudinal bar was exposed. At that time, the C1 column was completely destroyed, showing more obvious brittle damage characteristics, as shown in Figure 5a. For C2 column, when it was loaded to 1040 kN, fine cracks began appearing. With the increase in the load, two small, nearly parallel cracks appeared at the joint interface of the new-to-old concrete when the load reached 1360 kN. While it was loaded to 1560 kN, two parallel cracks became larger in width and ran through the whole column (see Figure 5b). Moreover, the separation of the new-to-old concrete joints could be observed at close range. For C3 column, there were small cracks when it was loaded to 1080 kN. When it was loaded to 1512 kN, the crack penetrated the whole column. Afterward and up to  
Figure 5: Typical damage pattern of specimens. (**a**) Specim C1; (**b**) Specim C2; (**c**) Specim C3; (**d**) Specim C4; (**e**) Bending deformation of external longitudinal bars of specimen C5; (**f**) Specim C5; (**g**) Post-installed rebar of the specimen were pulled out; (**h**) Specim C6. the peak load (1641.3 kN), no significant phenomena was observed. In the process of unloading, while the load was unloaded to 1604 kN, a large piece of concrete came off at the lower surface of the column and exposed the reinforcement part of the stirrups (see Figure 5c). The damage pattern was lantern-shaped. No large penetration cracks were seen on the combined surface of the new-to-old concrete. Compared with specimen C2, the cracks and the penetration cracks in specimen C3 appeared later. The damage process was more abrupt. For C4 column, first, fine cracks were observed when it was loaded to 1160 kN. Then, when the loading reached 1680 kN, small pieces of concrete broke off at the corner of the upper part of the column (see Figure 5d). Meanwhile, two cracks appeared and joined together at the upper part of the concrete column at the end. Finally, the concrete in the column's upper part fell into large pieces when the load was unloaded to 1660 kN, and the surface cracks penetrated the interior. The bending and deformation of the reinforcement longitudinal bars were visible in the chiseled interior, as shown in Figure 5e. For C5 column, first, its cracks appeared at 1050 kN. Secondly, two parallel cracks were formed at the position of the joint surface at both ends of the column corner when it was loaded to 1450 kN. The peak load was 1641.8 kN. Lastly, when unloaded to 1610 kN, two cracks penetrated through the subsequent large bulge of concrete in the middle of the column, as shown in Figure 5f. The new-to-old concrete's separation and the post-installed rebar's extraction could be observed at close range, as shown in Figure 5g. For C6 column, first, fine cracks appeared at 1270 kN. Then, the cracks widened and formed arc-shaped cracks when it was loaded to 1730 kN. Small concrete fragments were dislodged from the top of the column. The peak load was 1811.8 kN. Finally, when it was unloaded to 1780 kN, the arc crack penetrated the whole column, and the concrete bulged in the middle of the column, as shown in Figure 5h. The test process of each specimen could be divided into three stages: (i) The stage in which no cracks appeared in the specimen belonged to the linear elastic stage. (ii) Specimens with large cracks and part of the concrete falling off belonged to the elastic-plastic stage. (iii) Specimens with penetration cracks, large pieces of concrete falling off and crushing, and exposed reinforcement belonged to the failure stage. After being strengthened, the C2-C6 specimens demonstrated a slow process from the appearance of penetration cracks to complete damage.