## 2 Direct Shear Test of Concrete-Granite Interfaces
### Results and Discussion  
#### 2.2.1 Smooth Interface  
Figure 5 shows the evolution of the shear stress to the shear displacement during the test, for the three levels of normal loading. In the beginning, there is an increase in the shear stress until a peak is reached, then there is a slight decrease in the shear stress and afterward, the shear stress follows a plateau.  
Figure 4: BCR3D. (**1**: Sample; **2**: internal metallic boxes; **3**: external metallic boxes).  
Figure 5: Shear stress evolution for tests with a smooth surface.  
The results of the eight tests performed with smooth interfaces are summarized in Table 2.  
As expected, the shear strength varies with the normal stress, the relationship between the peak shear strength and the normal stress has a linear correlation coefficient of 95.4% (Figure 6), and the interface has a cohesion of 96.2 kPa and a friction angle of 29.7\({}^{\circ}\). For the residual stress, the correlation coefficient is 96.8%, and the cohesion and the friction angle are, respectively, 53.8 MPa and 27.9\({}^{\circ}\).  
Regardless of the normal stress, the shear behavior generates a peak shear strength not very different from the residual shear strength (Figure 5). This type of behavior is very similar to the one of the direct shear tests of unbonded smooth surfaces, which could indicate that the formation of strong bonds in the smooth concrete-granite interface is negligible.  
The unsuccessful formation of strong concrete-granite bonds could be related to the chemistry and the roughness of the granite surface [32] and the shrinkage during the curing of concrete which may damage the already low local cohesion.  
In addition, it is worth mentioning that the oscillations in Figure 5 and other experimental results are related to the configuration of the shear machine and the phenomenon of stick and slip [50]. In the BCR3D, the test was conducted by applying displacement in the two halves shear boxes at a certain rate. This rate was maintained during the test using a loop that computes the necessary force to produce the same increment of displacement in time. The problem is that the variable force in this loop depends on the resistance of the interface and this later is not constant. There are instants of shear displacement with low resistance and instants of shear displacement with high resistance. The change between these two types of instants depends on the interface being tested and can occur in a very short interval of time, such that the correction of force to produce the same increment of shear displacement needs updating constantly, and this does not give enough time to the all system to adjust, generating oscillations in force. These oscillations have an insignificant  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Test**} & \multirow{2}{*}{**Normal Stress (MPa)**} & \multirow{2}{*}{**Peak Shear Strength (MPa)**} & \multicolumn{2}{c}{**Residual Shear Strength (MPa)**} \\ \hline
1 & 0.5 & 0.47 & 0.35 \\
2 & 0.5 & 0.38 & 0.33 \\
3 & 0.5 & 0.37 & 0.30 \\
4 & 1.0 & 0.63 & 0.55 \\
5 & 1.0 & 0.62 & 0.60 \\
6 & 1.0 & 0.60 & 0.55 \\
7 & 1.5 & 0.98 & 0.92 \\
8 & 1.5 & 1.00 & 0.80 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Summary of the direct shear tests of samples with smooth surfaces.  
Figure 6: Peak and residual shear strengths in terms of normal stress (smooth surface).  
influence on the shear evolution and are left \"as it is\", without filtering, intentionally. Similar oscillations can also be observed in the results of direct shear tests presented in [PERSON] et al. (2019).