## 7 Conclusions
In all the studies surveyed, increases in tensile strength of +23% to +50% with hybrid fibers (depending on fiber types and proportions) were recorded compared to an unreinforced UHPC. In particular, the combination of steel with carbon fibers (total 2%) yielded a roughly 35% increase in tensile strength, while a mixture of carbon + glass fiber showed up to 23% tensile improvement. The most dramatic tensile gains were seen with certain polymer-fiber mixes: for example, adding 0.5% basalt + 1.5% polypropylene fibers (short fibers) was identified as an optimal blend, boosting the tensile strength by more than 50%. The flexural strength also improved substantially. Many hybrid combinations achieved double-digit percentage increases in flexural strength; in one case, a balanced hybrid of 1% steel + 1% polymer fibers achieved a 51.4% higher flexural strength than the plain UHPC, outperforming even a mix with 2% steel fibers alone. In extreme cases with very high fiber content, the flexural strength more than doubled, e.g., a hybrid mix attained flexural strength of 32.8 MPa, which was 154% higher than the fiber-free reference. These results highlight that hybrid fiber reinforcement is most impactful in tension-driven properties, with carefully chosen fiber combinations (such as steel + synthetic fibers or steel + basalt) providing complementary benefits: stiff fibers (steel, carbon, basalt) carry load and bridge larger cracks, while ductile or finer fibers (polypropylene, POM, glass) fill micro-cracks and enhance post-crack ductility. 4. Under high strain-rate loading tests, impact or dynamic, UHPC with hybrid fibers demonstrated improved strength and energy absorption relative to static conditions. Dynamic compressive and tensile strengths increased with increasing strain rate for all fiber-reinforced mixes, reflecting sensitive behavior of the material strain-rate. Crucially, greater fiber content led to better performance under impact: substituting polymer fibers with more steel fibers consistently improved dynamic strength. For example, in one study, increasing the content of steel fibers from 0.5% to 1.5% (while decreasing polymer fibers) increased the axial tensile strength under impact from -29.4 MPa to 43.2 MPa, and the absorption of impact energy increased by 1.7 times. This shows a nearly 70% improvement in dynamic tensile capacity due to a higher fiber dose. Fiber size also played a role: mixtures with a higher proportion of short fibers exhibited better impact resistance, since a larger number of short fibers can intercept and blunt cracks more effectively under high strain rates. Short steel fibers, in particular, contributed significantly to energy dissipation, as they prevent crack coalescence and rupture sequentially, absorbing impact energy. Overall, UHPC with hybrid fibers showed improved toughness and crack resistance under dynamic loading, with performance gains most pronounced when steel fibers (especially short, well-distributed ones) were used to complement other fibers. These findings suggest that optimized hybrid fiber networks can make UHPC more resilient to impact, providing both higher dynamic strength and improved post-crack energy absorption compared to unreinforced matrices.