## 1 Introduction  
In order to extend the service life of deteriorating reinforced concrete structures, and to grant safety in the case of increased loading demand, interventions of repair and rehabilitation have become of frequent practice worldwide. Nevertheless, poor compatibility and debonding of the repair material can jeopardize these interventions. In a repaired system, the interface between the concrete substrate and the new repair layer is typically weaker than the materials on either side. Due to such weakness, combined to stress concentrations (emphasized in case of poor substrate-repair compatibility), the interface is much more vulnerable to cracking and failure. As a result, the performance of repaired systems and, thus, their safety and durability, are highly dependent on the properties of the interface [1; 2; 3; 4; 5]. Fiber reinforced concrete (FRC) is recognized as a promising option for improving concrete durability [6; 7; 8; 9]. These benefits become even more relevant in repaired structures, where fibers can help improve compatibility to the substrate or, at least, reduce the extent of damage arising from poor compatibility [8; 10]. It has been proven that fiber reinforcement can also improve the bond of a repair concrete layer to an existing substrate, though this is highly dependent on several factors including substrate preparation and roughness, fiber bonding, size, aspect ratio, and stiffness, type of stress applied at the interface, etc. [PERSON] and [PERSON] [11] investigated the bond strength, failure mode, and cracking behavior of Engineered Cementitious Composites (ECC) and introduced the concept of interface crack trapping. Their test results indicate that the ECC repair system has higher strength, more ductile behaviour, and better crack width control [12]. [PERSON] et al. [13] studied the interface between strain hardening cementitious repair layers and concrete substrate. The researchers concluded that bond strength was highly affected by surface roughness and added the qualitative observation that the beneficial effect of fiber reinforcement was impacted by the amount of repair material that remained attached to the substrate after failure (in other words, by the deviation between failure and bond plane). [PERSON] et al. [14] characterized the behaviour of strain-hardening cement-based composite systems aimed at enhancing the overall durability of structural retrofits, by addressing both interfacial behaviour and drying shrinkage. [PERSON] et al. [15; 16] studied the interface behaviour under both tensile and shear loading. The results show the effectiveness of fibers in enhancing the quality of the interface both in tension and in shear. Further studies investigated the effect of fiber properties and surface preparation on the shear bond strength of substrate-repair interfaces in mixed modes of fracture. It was found that surface preparation has a strong effect on the crack deviation from the interface followed by fiber activation and multiple cracking before failure. A study by [PERSON] et al. [17] explored the suitability of fiber-reinforced cementitious composites with high-volume fractions of microfibers for thin repair applications. Results of tensile tests indicate a significant enhancement of the repair bond strength as a result of introducing microfibers to repair layer. Although various studies indicated the potential effectiveness of fiber reinforcement in improving interface performance, the exact governing mechanisms are neither well understood nor quantified for the different concurring contributing factors, so that accounting for these effects in the design and analysis of a repair application can be challenging.