## 5 Static Load Failure Modes
Failure patterns depend  
Figure 11: Compressive 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**) 1.5% PP 12 mm + 0.5% B 12 mm, (**e**) 1% PP 12 mm, (**f**) 0.75% POL 38 mm + 0.75% PP 12 mm, (**g**) 0.75% POL 38 mm + 0.75% B 12 mm, (**h**) 0.75% PP 38 mm + 0.75% POL 12 mm, (**i**) 0.75% PP 38 mm + 0.75% PP 12 mm, (**j**) 0.75% PP 38 mm + 0.75% PVA 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% ST 50 mm + 0.05% PP 12 mm, (**o**) 1.5% ST 50 mm + 1.5% B 12 mm, (**p**) 0.25% ST 50 mm + 0.25% PP 40 mm + 0.25% G 18 mm. on the type of fiber, its aspect ratios, the total fiber content, and the percentage contribution of each fiber type within the concrete matrix. Figure 12 presents various failure models of UHPC samples after splitting tests. The UHPC cube without fiber addition (Figure 12a) fractured along the center of the cross section after reaching the ultimate tensile stress, with a clear separation into two main parts, completely losing its load-bearing capacity. In the case of the sample containing two types of polymer fibers at 0.75% each (POL 12 mm and PP 12 mm) (see Figure 12b), a single main crack was observed, indicating the loss of load bearing capacity by the matrix and the partial transfer of residual stresses by the fibers, which, due to their short length, were pulled out. A network of vertical microcracks was also visible, showing that some of the energy from the fracture was absorbed. The fibers prevented the complete separation of the sample by bridging the cracks, delaying the propagation of the crack, and reducing the width of the crack. The addition of a mixture of PP and B fibers (Figure 12c-e) demonstrates the higher strength of basalt fibers, as the main crack was narrower and only a few vertical microcracks appeared. This indicates that basalt fibers absorbed significantly more fracture energy compared to PP fibers. Damage patterns for samples with 1.5% non-metallic fibers: longer (38 or 54 mm) and shorter (12 mm) are shown in Figure 12f-l. The longer fibers were partially broken, while the shorter ones were pulled out of the separated concrete parts, but both types together were effective in stopping sudden cracking. The network of microcracks and macro-cracks suggests energy dissipation by the fibers and local stress redistribution, providing the UHPC with a higher capacity to absorb energy. The increased length of the POL fibers (54 mm compared to 38 mm) in combination with the same B fibers (12 mm) reduced the width of the macro-cracks and generated more micro-cracks, indicating a more efficient use of the fibers (Figure 12m,j). The effect of fiber length was also observed in Figure 12l,f, where in the mixture with 0.75% POL and 0.75% PP (12 mm), increasing the length of the POL fibers from 38 to 54 mm resulted in three vertical cracks that run through the entire cross section and a reduction in the width of the crack. This reflects the participation of an additional part of the matrix and a portion of the fibers in the load-carrying with the improved energy absorption capacity. The failure pattern for the mixture with 0.75% PP fibers (38 mm) and 0.75% PVA fibers (12 mm) (Figure 12i) provides insight into the role of PVA fibers in the transfer of tensile stresses. 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 locations through which the cracks propagated. The failure pattern for the mixture with 0.75% PP fibers (38 mm) and 0.75% PVA fibers (12 mm) (Figure 12i) provides information on the behavior of PVA fibers in transferring tensile stresses.