## 5 Static Load Failure Modes
The network formed by these fibers is complex enough to effectively counteract the formation of microcracks. The resulting cracks are wide because of the full use of the properties of the PP fibers, some of which were partially broken. However, the PVA fibers, due to their shorter length and lower bond strength, were unable to prevent the further development of microcracks, leading to weakened areas through which the cracks propagated. The sample containing 1.5% PP fibers (40 mm) and 1.5% B fibers (12 mm) effectively transferred tensile stresses without significant cross-sectional damage. The large number of long polymer fibers bridged the cracks, aided by the short basalt fibers, which have higher tensile strength. The failure modes of the samples with hooked-end ST fibers (50 mm) combined with PP fibers (40 mm) are shown in Figure 12o,p. In the mixture with 1.5% ST and 1.5% PP, after exceeding the tensile stresses, three main vertical cracks appeared near the center, indicating the effective use of long fibers, where a greater part of the cross section was engaged in  
load transfer. The beneficial effect of adding 2.5% ST fibers at the hook end and 0.5% PP fibers is confirmed by the failure pattern in Figure 12p, which shows only one narrow main crack with a small crushed concrete zone under the upper loading fiberboard. The addition of fibers to the UHPC transformed the sudden and brittle failure observed in the UHPC without fibers into a controlled damage process in all hybrid fiber combinations, allowing the transfer of residual stresses. Fibers visible in the crack zone, some anchored in the concrete matrix, and others broken after exceeding the tensile strength, highlight their active role in limiting the crack width. In the case of fibers with high tensile strength, they are observed to gradually pull out of the matrix, creating additional resistance and improving crack propagation resistance. On the other hand, fibers with lower tensile strength tend to rupture. Fibers reduce crack openings, improving structural integrity after the UHPC matrix exceeds its tensile strength, slowing the failure process. In  
Figure 12: Tensile splitting fracture modes of UHPC with different hybrid fiber content (**a**) 0% fiber, (**b**) 0.75% POL 12 mm + 0.75% PP 12 mm, (**c**) 0.75% PP 12 mm + 0.25% B 12 mm, (**d**) 1% PP 12 mm + 1% B 12 mm, (**e**) 1.5% PP 12 mm + 0.5% B 12 mm, (**f**) 0.75% POL 38 mm + 0.75% PP 12 mm, (**g**) 0.75% PP 38 mm + 0.75% POL 12 mm, (**h**) 0.75% PP 38 mm + 0.75% PP 12 mm, (**i**) 0.75% PP 38 mm + 0.75% PVA 12 mm, (**j**) 0.75% POL 38 mm + 0.75% B 12 mm, (**k**) 0.75% PP 38 mm + 0.75% B 12 mm, (**l**) 0.75% POL 54 mm + 0.75% PP 12 mm, (**m**) 0.75% POL 54 mm + 0.75% B 12 mm, (**n**) 1.5% PP 40 mm + 1.5% B 12 mm, (**o**) 1.5% ST 50 mm + 1.5% PP 40 mm, (**p**) 2% ST 50 mm + 0.5% PP 12 mm. all failure modes presented, various combinations of fibers improved the plastic behavior of the material, which is crucial for fatigue resistance, long-term operational safety, and resistance to dynamic loads. Figure 13 presents photographs of UHPC samples reinforced with various combinations of fibers and a plain sample in the bending test. The UHPC beam without fibers (Figure 13a) suddenly broke mid-span after reaching ultimate stress, completely losing its load-bearing capacity. The remaining images show the damage patterns of the UHPC samples with hybrid fiber reinforcement. In Figure 13b-l, notched samples with hybrid fiber additives are illustrated. The main crack propagated from the tip of the notch toward the upper edge of the sample, undergoing multiple deflections. In the beam with 0.25% ST fibers (50 mm) and 0.75% PP fibers (12 mm) (Figure 13b), hooked steel fibers and straight PP fibers are visible within the fractured cross section, maintaining the concrete structure and preventing complete separation. The absence of microcracks is attributed to the high concentration of short PP fibers, which, because of their low density and small diameter, are more numerous than steel fibers.