## 2 Direct Shear Test of Concrete-Granite Interfaces
### Results and Discussion
#### 2.2.3 Natural Granite Interface  
In this set, fifteen tests were carried out with natural granite surfaces. These tests are crucial to improve the understanding of the mechanisms that drive the shear behavior of natural rough concrete-rock interfaces. Indeed, as pointed out by [PERSON] et al. [51], the heuristic rough interface, like the \"sawtooth\", does not always capture very well the mechanisms involved in the shear test of natural rough interfaces. The results of the tests performed are summarized in Table 4.  
The two types of failure observed earlier in the tests with bush-hammered interfaces were also observed here in the tests with natural granite interfaces (see Figures 11 and 12). Under low normal loading (0.5 MPa), failure occurs mainly at the interface, while at higher normal loading (1.0 and 1.5 MPa), failure occurs at the interface but also the tips of concrete are sheared off and remained attached to the granite surface (Figure 13). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Test** & **Normal Stress (MPa)** & **Peak Shear Strength (MPa)** &
\begin{tabular}{c} **Residual Shear Strength** \\ (MPa) \\ \end{tabular} \\ \hline
1 & 0.5 & 0.97 & 0.62 \\
2 & 0.5 & 1.18 & 0.80 \\
3 & 0.5 & 1.24 & 0.47 \\
4 & 0.5 & 1.33 & 0.62 \\
5 & 0.5 & 1.57 & 0.43 \\
6 & 0.5 & 1.66 & 0.49 \\
7 & 1.0 & 2.05 & 0.95 \\
8 & 1.0 & 2.20 & 1.12 \\
9 & 1.0 & 2.25 & 0.88 \\
10 & 1.0 & 2.54 & 0.90 \\
11 & 1.0 & 2.77 & 1.45 \\
12 & 1.5 & 2.70 & 1.50 \\
13 & 1.5 & 2.96 & 1.74 \\
14 & 1.5 & 3.06 & 1.90 \\
15 & 1.5 & 3.13 & 1.86 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of the direct shear tests on natural granite interfaces. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Test** & **Normal Stress (MPa)** & **Peak Shear Strength (MPa)** &
\begin{tabular}{c} **Residual Shear Strength** \\ (MPa) \\ \end{tabular} \\ \hline
1 & 0.5 & 1.24 & 0.40 \\
2 & 0.5 & 1.25 & 0.55 \\
3 & 1.0 & 1.50 & 0.74 \\
4 & 1.0 & 1.61 & 0.80 \\
5 & 1.5 & 1.93 & 1.26 \\
6 & 1.5 & 2.11 & 1.15 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of the results of direct shear tests of bush-hammered interfaces. Figure 14 shows the evolution of the shear stress with the normal stress without differentiation of the two types of failure identified above. The peak shear strength and  
Figure 11: Shear evolution—Type I (natural granite interfaces). Figure 12: Shear evolution—Type II (natural granite interfaces). Figure 13: Failures behavior—natural granite interface. (**a**) Confinement stress 0.5 MPa: left-Concrete and right Granite; (**b**) Confinement stress 1.5 MPa: left-Concrete and right Granite. the residual shear strength increase with the normal stress and have a linear correlation coefficient of 88.0%. For the peak shear strength, the interface has a friction angle of 59.1\({}^{\circ}\) and a cohesion of 551.1 kPa. For the residual shear strength, the interface has a friction angle of 48.9\({}^{\circ}\). Figure 14 and Table 4 also show that for each confinement level the values of the peak shear strength and residual shear strength obtained vary significantly from one sample to another. The reasons for this variability are: the difference of roughness between the samples and the complex relationships between, on one hand, the roughness and the shear stress and on the other hand, the roughness and the formation of strong concrete-rock bonds. In comparison with the results of tests with the bush-hammered interface, three observations can be presented. The tests with natural granite interfaces yield a lower cohesion (about 300 kPa), which could be related to the difference of roughness between the bush-hammered interface and the natural granite interface. The roughness in the bush-hammered granite surface is freshly generated roughness that is topographically uniform, unweathered, and rough, while the roughness in the natural granite surface is created by cracking and sliding within a rock mass which could lead to some local smoothing, furthermore, the natural granite surface can also be affected by weathering.