# Experimental Study of Interfacial Bond Properties between CGM and Existing Concrete
## 3 Results and Discussion
### Interfacial Bond-Slip Curve  
The bond-slip relationship and bond stiffness of the interface are important indicators that reflect the bond performance of the interface. The test results of this study showed that the interfacial bond-slip curves of all the specimens were basically similar. They all exhibited a slow upward trend at the initial stage of specimen loading. After reaching the peak, the load suddenly and sharply decreased, and the displacement increased rapidly, resulting in a relatively insufficient data acquisition in the descending phase. Therefore, considering the reliability of the regression analysis data, this study preferred to safely ignore the bearing capacity of the descending phase, and only the test data of the ascending stage (before failure) of the interface bond-slip curve were used for regression analysis.  
Figures 8 and 9 show the ascent test curves of interfacial shear bond stress, \(\tau\), versus the interfacial slip, \(s\), of each group of specimens. Most of the rising sections of the test specimens were close to a straight line. Therefore, the linear method was used to conduct a regression analysis of the test curves. The regression equation was assumed to be  
\[\tau=\tau_{0}+Ks\ \ (0\leq s\leq s_{u}), \tag{3}\]  
where \(\tau\) is the interface bond stress, \(\tau_{0}\) is the critical bond stress before the interface slipped, \(K\) is the interface bond-slip curve stiffness, \(s\) is the relative slip of the interface, and \(s_{u}\) is the slip when linear segment bond strength, \(\tau_{u}\), is reached. The specific values and correlation coefficients of each group of specimens are listed in Table 6. For the specimens of the Z-C35-T and Z-C35-S groups, to conduct a thorough regression analysis, the ascending section was divided into two stages (elastic and damage stages), and the regression analysis was carried out. Hence, each parameter of the groups in Table 6 had two values.  
Figure 7: Influence of shear key on interfacial bond strength.  
Figure 8: Bond stress–slip curves of direct shear specimens: (**a**) Z-C35-I; (**b**) Z-C35-T; (**c**) Z-C35-S; (**d**) Z-C45-S; (**e**) Z-C55-S.  
Figure 9: Bond stress–slip curves of slant shear specimens: (**a**) X-C35-30-W; (**b**) X-C35-45-W; (**c**) X-C35-30-Y; (**d**) X-C35-45-Y; (**e**) X-C45-45-Y; (**f**) X-C55-45-Y.  
Table 6 shows that most of the correlation coefficients of the linear regression equations of each group of specimens were close to 1, indicating a good correlation with the experimental results. The ratio of the bond stress at the end of the linear growth (the linear segment bond strength) obtained by linear regression to the ultimate bond strength was generally above 90%. The critical bond stress of each group of specimens before slippage occurred at the interface did not exceed 11.02% of the linear segment bond strength. In addition, with the increase in the existing concrete's strength or shear angle, the bond strength and slip of the specimen increased, and the interfacial bond strength and slip of the specimen with shear keys further increased.