## 3 Results and Discussion
### Substrate-Repair Interface Cracking: Comparison and Discussion  
Peak values of the bond splitting load and the critical stress intensity factor, corresponding to the stress intensity factor at the crack tip prior to the start of unstable cracking, are plotted in Figure 10 as a function of fiber type and content. A steady improvement was observed as the fiber content (V\({}_{\text{f}}\)) was increased. With V\({}_{\text{f}}\) = 1%, the peak splitting load and critical stress intensity factor are twice as high as those of the plain concrete. However, while steel fibers outperformed PVA fibers in the material fracture tests (Figure 6, Section 3.1), quantitative differences between steel and 8 mm PVA fibers are negligible when it comes to concrete-FRC interfaces (Figure 10). This obviously has to do with the different mechanisms involved with the failure of the material and the failure of the interface. In the case of concrete-FRC bond, the beneficial effects of fibers can be classified into two main categories as follows:  
1. Fibers reduce interfacial damage, help compatibility, reduce shrinkage and bleeding [8; 10; 16];
2. Fibers can provide additional toughening mechanisms by the interface. This is highly dependent on the mode of fracture, fiber bonding, size relative to the interface roughness, stiffness, and orientation with respect to the bond plane, and overall deviation of the failure plane from the bond plane [11; 12; 18]. With respect to point 2, it may be useful to investigate how many fibers were intersected by the failure plane during crack propagation. An optical microscope was used for the measurement of the number of fibers along the interface. This information is correlated to the peak Splitting Load in Figure 11 for the different fibers considered. One can observe that there is a consistent trend between the number of PVA fibers that remained attached to the substrate and were crossed by the failure plane (mixed adhesive-cohesive interfacial failure). No significant correlation was found for the number of steel fibers. This has to do with the different fiber sizes, stiffness, and bonding. Nevertheless, steel fibers had a favorable contribution to bond, comparable or superior to that of PVA fibers, thus confirming that, for different fibers, different mechanisms are involved. For PVA fibers, instead (due to their smaller size and different bonding), the number of fibers crossed at failure can be interpreted as an indication of the amount of cohesive rather than purely adhesive splitting occurring by the interfacial area. Figure 10: (**a**) Peak splitting load (SL) and (**b**) critical stress intensity factor (K\({}_{\text{I}}\)) of substrate-repair interfaces for various fiber types and volume fractions. In light of the good correlation found in Figure 11 for PVA fibers, the analysis was extended to investigating possible correlations between the number of fibers crossed by the failure plane and the roughness of the interface. Although the sandblasting was performed in the same manner for all the specimens, differences in the roughness parameters obtained (especially when calculated on a relatively small interface area) are inevitable. Based on the 3D surface scanning procedure and analysis described in Section 2.2.2, roughness quantification parameters were obtained. To the best of the authors' knowledge, there is no other work at the state of the art that investigates the correlation between quantitative roughness parameters and concrete-concrete bond parameters under Mode-I fracture tests. In this work, the authors investigated the correlation between quantitative roughness parameters and Mode-I crack growth resistance parameters. It was found that the average roughness (Sa) exhibited the best correlation with the splitting load (Figure 12a). However, this finding cannot be generalized and other bond strength/cracking parameters (e.g., in Mode-II) may be best correlated to other roughness parameters.