## 5 Static Load Failure Modes
These PP fibers initially bear developing stresses until they reach their load capacity, after which the steel fibers take on the additional load. Increasing the steel fiber content to 0.5% (without changing the total fiber volume) reduced the width of the main crack, although additional microcracks appeared due to the lower fiber content of PP (Figure 13c). Figure 13d shows the damage pattern of a beam with 1.5% B fibers (12 mm) and 1.5% PP fibers (40 mm), in which, due to the large number of basalt fibers of high tensile strength, crack bridging is possible. Longer and wavy PP fibers have lower tensile strength but better adhesion, effectively providing crack inhibition. The failure pattern of the sample containing 1% ST fibers (50 mm) and 2% G fibers (18 mm) is shown in Figure 13e, in which a major crack is visible that extends from the notch upward through the cross section. Long ST fibers, visible in the center of the crack, prevent it from widening due to their high tensile strength and hooked ends, which anchor them within the concrete matrix. Increasing the ST fiber content to 1.5% reduced the width of the crack, while 1.5% short G fibers prevented the development of microcracks, leading to more controlled propagation of the main crack (Figure 13f). The damage pattern of the sample with 1.5% ST fibers (50 mm) and 1.5% B fibers (12 mm) is shown in Figure 13g. The crack originating at the tip of the notch and extending into a fracture does not propagate vertically to the top, but it shifts horizontally. The high tensile strength of the fibers and the forces that anchor them in the matrix prevent further vertical cracking. The resulting stresses followed the weakened zone of the beam, indicated by the horizontal crack. The presence of microcracks highlights the lower effectiveness of basalt fibers in preventing their formation compared to polymer fibers, which have a greater elongation capacity and therefore do not fracture suddenly. The impact of three types of fibers (ST 50 mm + PP 40 mm + G 18 mm) with different volume contents during the bending test is illustrated in Figure 13h-l. ST fibers remain embedded in the matrix due to their length and hooked ends, preventing crack widening, but we see surface spalling of concrete fragments as a result of straightening the hooks, as presented in Figure 13i,j. Long polypropylene fibers prevent microcrack formation, which can be observed in failure modes in Figure 13h-k. Increasing their quantity from 0.25% to 0.83% resulted in a reduction in the number of microcracks until they did not occur in the case of Figure 13k. Reducing the content of PP fiber to 0.42% (see Figure 13l) resulted in the formation of additional cracks in the upper part of the cross section. Glass fibers, as a high tensile strength material, help to restrict crack widening. Additionally, because of their low density and small diameter, they are much more numerous in the matrix compared to ST fibers. These fibers, along with the polypropylene fibers, effectively create a complex structural network that prevents the formation of microcracks. However, because of the low ultimate strain of the glass fibers, there are certain limitations. Figure 13m-o show images  
of the damage patterns of the beams without notches, tested for flexural tensile strength. The UHPC with the addition of PP fibers (12 mm) and B fibers (12 mm) fractured after reaching the maximum load, with a visible crack spreading upward. The absence of visible microcracks is attributed to the presence of polypropylene fibers, while basalt fibers mainly contribute to slowing down crack propagation because of their high tensile strength. The influence of 1% ST fibers (50 mm) combined with 0.05% PP fibers (12 mm) on the flexural strength of the beam is evident in Figure 13(a). The ST fibers hold the cross section together, preventing a complete separation of the beam into two parts, even at an advanced stage of loading.