## 3 Experimental Results and Discussion
### Crack Openings and Tensile Strains of the Bars Crossing the Interface  
In Figure 11, the opening, w, of the crack at the interface is plotted against the imposed shear slip, s. It is observed that the crack opening increases when the applied shear slip increases. However, the number of applied shear slip cycles does not seem to significantly affect the magnitude of crack opening. Similar observations can be made in relation to the residual crack opening, i.e., the value recorded on the unloading branch, when the shear slip value is equal to zero. At that point, there is always a residual crack opening, the value  
Figure 10: Energy absorbed during the first cycle, estimated by the area included in the hysteresis loops. (**a**) Specimens without external compressive stress. (**b**) Specimens with external compressive stress, \(\sigma_{N,cat}\). Figure 9: Strain of the reinforcement, measured for the maximum interface resistance, plotted against the applied external stress, \(\sigma_{N,cat}\). Empty symbols are used for specimens with cast-in bars, with full symbols for specimens with post-installed bars. of which depends on several parameters and mobilized mechanisms, such as (a) the cut-off of cement paste particles and aggregates forming the roughness of the interface as the new and the old concrete blocks slip relative to one another (where the roughness of the interface is gradually reduced, whereas the material produced by the cut-off of asperities remains along the interfaces, thereby preventing the crack from closing) and (b) the contribution of the interface reinforcement, which depends on its bonded length. Thus, when the interface reinforcement is short, due to the tension induced by the clamping mechanism, the bars exhibit significant and irreversible pullout displacements that contribute to the residual crack opening. If, on the contrary, the interface reinforcement is provided with anchorage length sufficient for its yield strength to be developed, there may be not fully reversible strains in the interface reinforcement post-yield. Those strains also contribute to the value of residual crack opening. Further observations can be made in Figures 12 and 13. In the absence of external compressive stress, larger embedment lengths allow larger shear slip values to be imposed (Figure 12a), associated with larger crack widths (Figure 13a) and, as a result, higher steel strains due to clamping effects. Thus, as shown in Figure 8a, the interface resistance increases for larger embedment lengths. The opposite is observed in cases where an external compressive stress, \(\sigma_{\text{N,ext}}\), is acting in the interface: in this case, both the shear slip \(\text{s}_{\text{u}}\) and the crack opening at the moment of the maximum interface resistance is activated (\(\text{w}_{\text{u}}\) at \(\text{s}_{\text{u}}\)), which decrease with increasing embedment length of the reinforcement. As an example, consider the case of the post-installed interface reinforcement embedded at \(6.25\,\text{d}_{\text{b}}\). In the absence of external compression, the (limited) interface resistance is mobilized at a relatively small imposed shear slip, causing concrete breakout failure (Figure 4d). In the presence of external compression, however, the interface can sustain larger shear slip  
Figure 11: Typical crack width (w) vs. shear slip(s) curves. values, associated with larger crack opening, and therefore exhibits higher resistance. As the embedment length of the reinforcement increases, the interface resistance increases significantly (Figure 8a). In this case, the positive effect of external compression is less pronounced (compare Figure 8a and 8b). With the increase in interface resistance being smaller under external compressive stress, the maximum resistance is mobilized for smaller \"\(\text{s}_{\text{u}}\)\" and \"\(\text{w}_{\text{u}}\)\" values (Figures 12b and 13b).