## 3 Experimental Results and Discussion
### Hysteretic Behavior and Maximum Shear Resistance  
Figure 5 shows typical hysteresis loops for the tested interfaces. The behavior of the interfaces is characterized by the pinching effect and the limited area of the hysteresis loops, as well as by a pronounced reduction in the interface resistance during the second and the third loading cycles. Hysteresis loop envelopes for all tested specimens are presented in Figure 6. It is noted that the maximum interface resistance is not mobilized for the same shear slip value in the two loading directions. Thus, the \(\tau_{u,exp}\) in Table 2 is calculated as the average of two values, namely the absolute maximum recorded resistance (usually during the first loading direction) and the resistance mobilized for the same shear slip value, \(s_{u}\), in the other loading direction. Figure 4: Cracks on the specimens: (**a**) Specim R-31/B/20/0.1, crack along the interface; (**b**) specimen NR-25/E/6/0.1, oblique cracks; (**c**) specimen R-25/E/6/0.1, block “B”, parallel, multiple oblique cracks; (**d**) specimen Re-26/B/6/0.1, crack parallel to the interface, at a distance almost equal to the embedment length of the post-installed bars in block “A”. Figure 5: _Cont._Figure 5: Typical hysteresis loops of interface resistance (V) versus slip (s). The hysteresis loop envelopes, together with the test results listed in Table 2, clearly show the effect of the presence of an external normal stress, \(\sigma_{N,ext}\), of different magnitude, on the behavior of interfaces. Indeed, the interface resistance increases, together with the stiffness of interfaces (Figure 7). Thus, the ultimate shear resistance is mobilized for significantly smaller imposed shear slips than for interfaces without external compression, this being the case for both cast-in and post-installed interface reinforcement. It is also observed that interfaces subjected to external compression \(\sigma_{N,ext}\) exhibit a practically linear behavior up to their maximum resistance. It should be noted that this behavior does not seem to depend on the value of the imposed external compression. However, such a conclusion has to be considered as tentative, in view of the fact that the overall number of tested specimens is rather limited, while the inherent scatter of the experimental results must also be taken into account  
The ratio V/s for the first cycle, corresponding to the shear stiffness of the specimens, is shown in Figure 7 to clearly demonstrate the significantly higher stiffnesses of the interfaces under external compression compared to those without normal stress. Furthermore, it is also observed that while, in the absence of external compression, the stiffness seems to depend on the reinforcement parameters (ratio, diameter of bars, and embedment length), in the presence of \(\sigma_{N,ext}\), those parameters do not affect the interface stiffness. Figure 6: Hysteresis loop envelopes for the first cycle for specimens (**a**) without external compressive stress and (**b**) with external compressive stress; (**c**) close up of the graphs (**a,b**), s \(\leq\) 2.00 mm. The effect of the external normal stress on the maximum interface resistance \(\tau_{\text{u,exp}}\) is illustrated in Figure 8, where it is interesting to observe the interaction between the presence of \(\sigma_{N,ext}\) and the embedment length of the interface reinforcement. Figure 8a shows the well-known ([PERSON] et al. [29], [PERSON] et al. [30], [PERSON] [31]) dependence of the interface peak resistance on the embedment length for \(\sigma_{\text{N,ext}}\) equal to zero. Indeed, an increase in the normalized embedment length from 6.25 to 20.0 leads to an increase in the interface resistance. The increase in the interface resistance depends on the reinforcing ratio as well. On the contrary, as shown in Figure 8b, the presence of external compressive stress makes the interface resistance rather insensitive to the embedment length, as well as to the interface reinforcing ratio. This observation is in accordance with [PERSON] et al. [4].