## 6 Microstructural Analysis and SEM Observations  
The behavior of long hooked-end steel fibers (65 mm) indicated crack formation at the fiber-matrix interface [172]. This was due to internal pressures generated by the fiber's hooked ends attempting to straighten during pull-out. This phenomenon was not observed with straight steel fibers (even up to 97.5 mm in length), due to their smooth surface that facilitates pulling out of the matrix without causing significant damage. An increase in tensile strength was also observed with increasing straight fiber length (from 65 to 97.5 mm). The higher aspect ratio increased the effective bonding area. Basalt fibers with lengths of 6 and 12 mm and a diameter of 0.017 mm at concentrations up to 3% improved tensile strength by transferring loads after matrix cracking [113]. At 1% fiber content, a uniform distribution allowed effective load transfer from various directions. Due to their small diameter, these fibers were particularly effective in preventing microcrack formation. High tensile strength prevented their rupture and strong anchoring in the matrix reinforced their effectiveness. However, increasing the fiber content from 2.5% to 3% caused fiber agglomeration and weakened fiber-matrix bonding at these sites, potentially reducing mechanical properties. SEM images of concrete mixtures containing 0.35% steel fibers and basalt fibers of lengths of 12, 25, 37, and 50 mm (up to 0.3%) confirm a strong bond between the fibers and the matrix [62]. The basalt fibers are visibly attached to fragments of the concrete matrix on fractured surfaces, indicating excellent adhesion. However, longer basalt fibers tended to agglomerate, potentially creating weak zones. Macroracks propagated through steel fibers, confirming their role in bridging cracks under high-load conditions. The localized agglomeration observed with longer fibers highlights the importance of optimal fiber length selection to achieve uniform dispersion and prevent crack initiation sites. Microstructural images of UHPC mixtures with basalt (18/0.013 mm), glass (12/0.01 mm), or polypropylene fibers (10/0.023 mm), added in amounts ranging from 0.5 to 2.5%, are presented in [40]. Basalt fibers showed attached matrix fragments, confirming their strong matrix adhesion. Higher fiber content resulted in fiber agglomeration zones, potentially creating areas of weakness. The glass fibers demonstrated strong anchorage, indicated by fractures propagating along the fibers rather than pulling them out. Polypropylene fibers bridged microcracks effectively, but ruptured once their limited tensile strength was exceeded, effectively limiting crack propagation. Quantitative SEM analysis indicated typical microcrack widths ranging from 5 to 20 \(\upmu\)m, with fiber pull-out lengths observed in the range of 50-200 \(\upmu\)m, clearly demonstrating effective crack bridging and stress dissipation. The strong bond of basalt fibers to the matrix was also confirmed in [83], as evidenced by the partial coverage of the fiber surfaces with cement paste. The additions of glass fibers (24/0.01-0.017 mm) exhibited agglomeration, with cracks propagating along the fiber-matrix interfaces, indicating weak areas. To support the described mechanisms of crack inhibition and fiber-matrix interaction in UHPC, selected scanning electron microscopy (SEM) images from the previous research of the authors are presented in Figure 14.  
These micrographs illustrate key microstructural phenomena observed in hybrid fiber-reinforced UHPCs, including fiber anchorage, pull-out behavior, crack bridging, and fiber rupture. In Figure 14a, a hooked-end steel fiber (50 mm in length, 1 mm in diameter) is shown embedded within the concrete matrix that contains 0.5% steel and 0.06% polypropylene fibers (12 mm). The image clearly reveals a strong mechanical anchorage, suggesting effective stress transfer and interfacial bonding.