## 2 Direct Shear Test of Concrete-Granite Interfaces
### Results and Discussion
#### 2.2.2 Bush-Hammered Interface  
To improve the formation of stronger concrete-granite bonds, smooth surfaces of granite were bush-hammered to include the micro-roughness. The surfaces obtained through this process are still flat, but rougher compared to the smooth surfaces (Figure 1).  
Two types of failure were observed. The first type of failure occurs at low normal stress (0.5 MPa). Post-mortem analysis reveals that there is no damage neither in the granite nor in the concrete. Failure occurs almost exclusively at the interface (Figure 7a). The shear stress increases to reach the peak value, then drops suddenly and thereafter decreases gradually until reaching a residual value (Figure 8). The second type of failure occurs at high normal stress (1 MPa and 1.5 MPa). Post mortem analysis showed that the failure occurs mainly at the interface but there is a failure of the tips of concrete, which remains attached to granite (Figure 7b). The shear stress increases to reach the peak shear strength then drops gradually to reach the residual shear strength (Figure 9). Overall, it seems that in the first type, all the bonds break almost instantaneously, while in the second type there appears to be a progressive failure of both the bonds and the tips of concrete.  
Similar behavior was also reported in other works of ([PERSON] et al., 2019; [PERSON] et al., 2020; [PERSON] et al., 2020).  
Figure 10 is here presented as a mere illustration of the evolution of the shear strength with the normal stress, and thus there is no differentiation of the two types of failure identified above. From this figure, it can be assessed that the peak shear strength and the residual shear strength increase with the normal stress and have a linear correlation coefficient of 95.2%. For the peak shear strength, the interface has a friction angle of 37.8\({}^{\circ}\) and a cohesion of 831.7 kPa. For the residual shear strength, the linear correlation coefficient is 95.4% and the interface has a friction angle of 36.1\({}^{\circ}\) and cohesion of 86.7 kPa.  
Figure 7: Failure behaviors—bush-hammered interfaces. (**a**) Confinement stress 0.5 MPa: left-Concrete and right Granite; (**b**) Confinement stress 1.5 MPa: left-Concrete and right Granite.  
In comparison with the results obtained in the tests with smooth surfaces, in terms of the peak shear strength, changing the type of roughness from smooth interfaces to bush-hammered interfaces causes an increase of approximately 700 kPa in the cohesion and 10\({}^{\circ}\) in the friction angle. In terms of the residual shear strength, this change causes an increase of about 30 kPa and 10\({}^{\circ}\) in the cohesion and the friction angle, respectively.  
Figures 8 and 9 show that the peak shear strength obtained in the tests with bush-hammered surfaces is much higher than the residual shear strength, which is a clear indicator of the formation of strong bonds between concrete and granite.  
Figure 8: Shear evolution—Type I (bush-hammered interfaces).  
Figure 10: Peak and residual shear strength in terms of normal stress (bush-hammered interfaces).  
Figure 9: Shear evolution—Type II (bush-hammered interfaces).  
The formation of these strong bonds is directly related to the micro-roughness generated by bush-hammering the smooth surfaces.  
The results of the six tests performed with bush-hammered surfaces are summarized in Table 3.  
To further investigate the influence of roughness on the shear behavior of bonded concrete-rock interfaces, the natural granite interface was tested.