## 5 Static Load Failure Modes  
Figure 11 shows UHPC samples reinforced with various combinations of fibers compared to plain UHPC samples, damaged under compression. The failure of fiber reinforced cubes is more gradual and continuous. On the contrary, UHPC without fiber addition (Figure 11a) experiences sudden failure, with large concrete fragments detaching once the maximum compressive stress is exceeded. In the case of a large number of short polymer fibers, a chaotic network of cracks was observed (Figure 11b). The combination of short PP fibers and short B fibers (Figure 11c-e) reduced both the total number and width of cracks due to the higher strength of the basalt fibers. An effect was particularly evident as their proportion increased. Damage resulting from compressive forces for samples containing 1.5% nonmetallic fibers: long fibers (38 or 54 mm) and short fibers (12 mm) are shown in Figure 11f-m. The crack pattern was more regular and, in some cases, a network of fine cracks was formed (Figure 11f,f). Figure 11j illustrates the concrete failure model with 0.75% PP (38 mm) + 0.75% PVA (12 mm), which shows a different pattern compared to other cases. Due to the highest aspect ratio of the PVA fibers, they were the most numerous in the mixture. The complex network formed by these fibers effectively counteracted the formation of microcracks. In addition to a wide crack and vertical spalling in the corner of the cube, no clear vertical crack was observed. Failure patterns for the polymer and basalt fiber mixture show less complex crack networks (Figure 11g,k,m), attributed to the higher strength of basalt fibers, as particularly visible in Figure 11g. The spalling of concrete at the corner of the sample (Figure 11k) and the detachment of the surface fragment (Figure 11m) suggest a lack of fiber presence at these places, where the matrix strength was exceeded. Characteristic damage patterns are shown in UHPC images with long hooked-end steel fibers (Figure 11n-p). The cube in Figure 11n, which contains 1.5% ST fibers (50 mm) and 0.05% PP fibers (12 mm), failed due to wide horizontal cracks held together by long hooked steel fibers. The absence of microcracks indicates an insufficient amount of polypropylene microfbiers, which could not withstand the development stresses. The failure pattern of the mixture with 1.5% ST fibers (50 mm) and 1.5% B fibers (12 mm) (Figure 11o) shows two main horizontal cracks in the upper and lower parts of the sample. We can see a similarity to Figure 11n. However, the higher content and strength of the basalt microfibers in this case resulted in narrower microcracks, and the concrete did not spall. The UHPC  
Figure 10: Strength test results of UHPC reinforced with other types of fibers: **(a)** compressive strength, **(b)** flexural strength. containing three types of fiber--0.25% each of ST hooked end (50 mm), PP (40 mm), and G (18 mm), after compression stress, is presented in Figure 11p. A horizontal crack can be related to the presence of long ST and PP fibers, along with a complex crack network facilitated by glass fibers that effectively bridged the expanding cracks. Images of UHPC damage with hybrid fiber reinforcement confirm the beneficial effect of fiber reinforcement on the crack resistance of concrete and its ability to transfer residual stresses, reducing the risk of sudden failure. In the samples analyzed, the increase in compressive load (up to the point of ultimate strength) was more controlled, and material disintegration occurred gradually. Fibers altered the direction of microcrack development, resulting in increased energy absorption before failure. The width of the crack was limited, as the fibers maintained structural continuity and absorbed energy.