## 7 Conclusions  
Hybrid fiber reinforcement in UHPC leads to notable improvements in both fresh and hardened properties, with clear trends emerging for workability, mechanical strength, and dynamic performance. 1. The fluidity of UHPC continuously decreases as the total fiber content increases. In particular, adding more steel fibers (especially longer or hooked-end fibers) significantly reduces flowability. For example, the flow time of a UHPC mix can increase by over 70% (indicating a loss of slump flow) when shorter steel fibers are used, compared to mixes with longer fibers. Among the different types of fiber, basalt fibers impair workability more than steel fibers because of their rough surface and tendency to clump. Likewise, a higher proportion of polymer micro-fibers (e.g., polypropylene) in a steel-polymer hybrid mix causes a pronounced drop in fluidity as these small-diameter fibers absorb water and coil together. These findings underscore the need to balance the fiber content with the mix design to maintain self-compacting properties. 2. An optimal total fiber content of around 2% by volume was identified to maximize strength gains. Exceeding \(\sim\)2% fiber dose showed diminishing returns or even slight reductions in compressive strength due to fiber clustering and loss of workability. For example, steel + glass fiber hybrids at 2% total fiber achieved up to 15% higher compressive strength than plain UHPC, while increasing to 2.5% fiber caused a minor strength decline. In general, hybrid fibers moderately improved compressive strength (often of the order of 10-15%). The fiber length was crucial: mixtures with fibers \(\sim\)20-30 mm long achieved the highest compressive strengths, while the use of fibers shorter than 15 mm led to lower strength. This is attributed to the fact that longer fibers provide better crack bridging, whereas very short fibers may not anchor as effectively. Additionally, the base matrix quality (e.g., a low water-to-binder ratio and proper curing) remains important, as fiber effects on compressive strength can be overshadowed by a weak matrix. Among polymer fibers, the type of fiber influenced compressive results: polyolefin, polyoxmethylene (POM) and polyester fibers yielded the highest compressive strengths, whereas the use of polyvinyl alcohol (PVA) fibers led to comparatively lower strength. This suggests that stiffer polymer fibers contribute more to load-bearing capacity than more ductile or hydrophilic ones in compression. In general, the highest compressive strength gains observed with hybrid fibers were in the order of 10 to 15% in most studies, confirming that fibers primarily enhance tensile-related properties while providing a moderate boost in compressive strength. 3. Hybrid fiber reinforcement had a much more pronounced effect on the tensile capacity (including splitting tensile and flexural strength) of the UHPC. Using longer steel fibers (\(\geq\)30 mm) or fibers with improved anchorage (e.g., hooked ends) was particularly beneficial for tensile strength, as these fibers engage more fully across cracks. In contrast, increasing the proportion of shorter or more flexible fibers (such as certain polymers) tended to decrease the direct tensile strength if those fibers lack the high tensile capacity of steel. The best improvements in tensile performance were achieved by synergistic fiber combinations that utilize fibers of different types and sizes to bridge cracks at multiple scales. For example, a hybrid mix with \(1.5\%\,\text{steel}+0.5\%\) basalt fibers (both relatively short fibers) produced the highest tensile splitting strength in its category. Another effective mix was 2.0% steel + 0.5% basalt (using short steel and long basalt fibers), which maximized the combined benefits of both fibers. In steel-polymer hybrids, extending the steel fiber length while adding a small fraction of the micro-polymer fibers improved the overall tensile performance, as long steel fibers carry loads and polymer fibers help control microcracks.