## 4 Analysis of Test Results
### Strain Analysis of Longitudinal Bar  
Figure 9 shows the load-strain relationship between the strengthening longitudinal bar and the existing longitudinal bar in C1-C6 specimens.  
As can be seen from Figure 9:  
In the linear elastic stage, the compressive strains of the existing longitudinal bar and strengthening longitudinal bar were basically the same and the growth rate was basically the same. This indicated that the existing and strengthening longitudinal bars were jointly and synergistically stressed during the loading period. When the specimen was loaded to the elastic-plastic stage, the strain curve of the strengthening longitudinal bars increased slower than the existing longitudinal bars. In the stage of failure, the strain in the existing longitudinal bar would increase rapidly. In contrast, the strengthening longitudinal bar strain would increase at a lower rate until the specimen was damaged, which was still basically in the elastic deformation stage.  
Figure 8: Strain–load variation in post-installed rebar in specimen C5 and specimen C6.  
Figure 9: Load–strain curves of strengthening longitudinal bars and existing longitudinal bars in specimens C1–C6. (**a**) Specim C1; (**b**) Specim C2; (**c**) Specim C3; (**d**) Specim C4; (**e**) Specim C5; (**f**) Specim C6.  
The longitudinal bar load-strain curves correlated well with the damage characteristics of the specimens. Using the C3 specimen as an example, the strain curve separation between strengthening longitudinal bars and existing longitudinal bars occurred at 1050 kN. At that time, fine cracks appeared on the joint surface of the specimen. The strengthening longitudinal bar and the existing longitudinal bar curve were separated at 1400 kN when large cracks appeared at the joint interface of the specimen. At the same time, the C4 curve separation of the specimen with epoxy resin interface agent on the concrete interface was slower than that for the C3 specimen, and the load for which cracks appeared was higher than that for the C3 specimen. Specim C6 showed the maximum compressive strain in the strengthening longitudinal bars near the ultimate load. This indicated that it exerted the maximum effect. Its ultimate load capacity was also the maximum. The strain in the existing longitudinal bar was larger than that in the strengthened longitudinal bar when it was closed to the damage. The strain growth rate increased gradually, while the strain growth rate of the strengthening longitudinal bar was basically constant. When the specimen reached the ultimate load capacity, the strain in the existing longitudinal bar would be much larger than the strengthening longitudinal bar. Then, the strength of the strengthening longitudinal bar was not fully developed.  
In summary, it was clear that the unsynchronized compressive strain between the strengthening longitudinal bar and the existing longitudinal bar of the specimen was caused by the separation of the new-to-old concrete interfaces. The higher the compressive load at the time strain asynchrony occurs, the higher the ultimate bearing capacity of the short column. The variation in the integrated longitudinal bar strain values with load growth could be inferred. The damage to the strengthened column specimens began in the existing concrete column section. Then, the existing column deformed substantially in the interior, and bulging and cracking occurred in the outer reinforcement immediately afterward. Finally, the whole specimen lost its bearing capacity.