## 4 Concluding Remarks  
An experimental program was presented to study the effects of different fiber reinforcements on substrate-repair debonding in Mode-I. Volume fractions of 0%, 0.5%, and 1% of 8 mm long PVA fibers, 12 mm long PVA fibers, and 13 mm long steel fibers were evaluated. The role of different fibers in improving interfacial bond and crack growth resistance was investigated. The role of crack deviation on the Mode-I strength of monolithic specimens was investigated. Moreover, the impact of interfacial roughness was studied. Using 3D surface scans and optical microscope, the relationship between repair material/fibers intersected at failure and the Mode-I bond strength was investigated. The main conclusions can be summarized as follows:  
1. Significant enhancement of the Mode-I crack growth resistance was observed in the repair materials and in the substrate-repair bond after introducing fiber reinforcement in the repair.
2. In the repair materials, a significant enhancement of the overall crack growth resistance curves, peak Splitting Load (SL) and critical stress intensity factors were observed for increasing fiber contents, as anticipated. For instance, peak SL increments of 50%-75% and 87%-250% were achieved at 0.5% and 1% V\({}_{\text{f}}\), respectively, with steel fibers providing the highest increases. Consistent correlations were found between the Mode-I crack growth resistance of the repair materials and quantified crack deviation, confirming the ability of fibers to promote crack tortuosity as one of their different contributing mechanisms to crack growth resistance.
3. For the concrete-concrete interfaces, 8 mm PVA fibers were the fibers exhibiting the best performance at V\({}_{\text{f}}\) = 1%, while, in case of V\({}_{\text{f}}\) = 0.5%, 13 mm steel fibers reached the highest fracture values. This has to do with a number of contributing factors, including the stiffness and size of the fibers with respect to the roughness of the interface (as far as failure mechanisms are concerned), the bonding of the fiber to the matrix, as well as the effect of fibers on compaction near and at the Interfacial Transition Zone, the different ability of different fiber types and contents to mitigate or prevent shrinkage/thermal cracking and bleeding.
4. Substrate roughness and interfacial tortuosity had a significant impact on Mode-I crack growth resistance. Minor changes in quantified substrate roughness parameters affected fracture parameters (such as the peak splitting load) and, for PVA fibers, also affected the amount of fibers intersected by the failure plane during splitting.
5. For PVA fibers, a correlation was also found between the fracture parameters and number of fibers intersected by the failure plane. This was not the case for steel fibers, due to their different stiffness and diameter. Regardless, the beneficial effect of steel fibers on concrete-concrete interfacial crack growth in Mode-I confirms that other bond enhancing mechanisms are in place.
6. The aim of this study was to provide a better understanding of complex mechanisms involved with the interfacial cracking in concrete-concrete composite systems. This information can be used to help the design of repairs and retrofits with fiber reinforced concrete with the scope to optimize stress transfer and durability along the substrate-repair bond, a parameter that is essential to the effectiveness and durability of the retrofitted structure. Finally, the investigation of the mechanisms set a database of information for the modeling of concrete-FRC debonding in Mode-I.  
Two authors contribute equal to the paper.  
This study was funded by the Natural Science and Engineering Research Council of Canada, NSERC (grant number RGPIN-2016-03863).  
The authors declare no conflict of interest.