## 3 Results and Discussion  
### Mode-I crack growth resistance of the repair materials  
As anticipated, plain mortar specimens exhibited a brittle, abrupt failure soon after initial crack nucleation, while more controlled, steady crack propagation was observed in the fiber reinforced mortars overall at both pre-peak crack nucleation and post-peak crack propagation. Failure occurred along the groove at the mid section as desired. Experimental Splitting Load (SL) compared to crack mouth opening displacement (CMOD) curves obtained with 8 mm PVA fibers are shown in Figure 5a, where the curve of the plain repair material is also plotted for comparison. Only one type of fiber is shown here for the sake of conciseness, but all the different SL-CMOD curves obtained are available in the Appendix A. The allowable maximum crack width for durability of FRC structures ranges from 150 to 300 \(\upmu\)m, with the most stringent requirements specified by the American Concrete Institute (ACI) 224R [42]. According to ACI 224R the maximum crack width at the tensile face of reinforced concrete structures is specified as 150 \(\upmu\)m for exposure conditions of seawater, seawater spray, wetting, and drying, and 180 \(\upmu\)m for deicing chemical exposure. The FRCs investigated here achieve those crack opening values within the stable stage of subcritical crack growth prior to achieving the peak of the response curve and the unstable crack propagation in most cases, or otherwise exhibit significant residual stresses (Figure 5a, Figure A1a-c). Average values of the peak splitting loads of the different repair materials (representing the onset of unstable crack propagation, point B of Figure 4) are plotted as a function of the fiber type and content (V\({}_{\text{f}}\)) in Figure 6a. As expected, a consistent increase of the peak SL is observed for increasing fiber contents, with steel fibers reaching the highest values. With V\({}_{\text{f}}\) = 1% of the steel fiber, SL is increased from 0.8 kN to 2.8 kN (around 250%). The same contents of 8 mm and 12 mm long PVA fibers provide 175% and 87.5% SL increments, respectively. With a lower V\({}_{\text{f}}\), the SL increment is 75%, 62.5%, and 50% with steel, 8 mm PVA, and 12 mm PVA fibers. Therefore, the incremental benefit from 0.5% to 1% V\({}_{\text{f}}\) is much higher than the one from 0% to 0.5% V\({}_{\text{f}}\). Crack growth resistance curves in terms of stress intensity factor (K\({}_{\text{I}}\)) and effective crack length (a\({}_{\text{eff}}\)) were calculated following the procedure described in 2.2.3. Additionally in this case, only one representative fiber type is shown in Figure 5b and compared to the plain repair mortar, while the full curves for the other fiber types are available in the Appendix A. Compared to the plain repair material, fiber reinforced concretes exhibited a considerably greater subcritical crack growth branch (BOP-B, Figure 4) before the onset of the unstable propagation. The critical stress intensity factors (K\({}_{\text{IC}}\)), which  
Figure 5: (a) Splitting load (SL) versus crack mouth opening displacement (CMOD) curves of 8 mm PVA fiber reinforced concrete (FRC) and (b) crack growth resistance curves (in terms of SIFs, KI, compared to effective crack length, aeff) of 8 mm PVA FRC. represent the initiation of unstable crack propagation, are plotted in Figure 6b for different fiber types and contents. Consistently with the trends observed for the peak SL, KIC increased for increasing fiber contents and the highest increment were obtained with 1% Vf steel fibers. Microcracks encounter a higher toughness and crack growth resistance in the presence of fibers. Some fibers develop a chemical bond with the cementitious matrix that causes stress redistribution around the crack tip, promotes crack blunting, and bridges the cracks. This favors the tortuosity of the crack and causes directional deviations. It is expected that larger crack deviation leads to greater crack growth resistance.