## 4 Hybrid Fiber Composition
### Steel--Polymer Hybrid Fiber
#### 4.3.3. Dynamic Properties
Fiber failure due to exceeding its tensile strength is unfavorable, as it prevents full use of the mechanical properties of fibers [201,202]. Energy is suddenly released, which can cause a sharp decrease in the strength of the material. A mixture with 1.5% PE fibers (length/diameter = 13/0.02 mm) was enriched with 0.5%, 1%, or 1.5% ST fibers (18/0.16 mm) [149]. For each concrete, an increase in strain rate led to higher axial tensile strength and impact energy. As the ST content increased, the dynamic behavior of the UHPC improved. The axial tensile strength of the mixture with 0.5% ST fibers and a strain rate of 49 s\({}^{-1}\) was 29.4 MPa, gradually increasing with the ST fiber content to a maximum of 43.2 MPa at a strain rate of 45 s\({}^{-1}\) with 1.5% ST. The impact energy increased 1.7 times with increasing steel fiber content from 0.5% to 1.5%. This indicates a significant improvement in energy dissipation efficiency by steel fibers. Additionally, they activate the potential of PE fibers for pull-out under dynamic impact Due to their low deformability, steel fibers bear higher external loads from the beginning of crack development. The increase in the dynamic strength of the matrix occurs mainly before the first crack forms, and after its formation, multiple cracks are possible due to fiber bridging [203]. Increased short-fiber density contributes significantly to the energy dissipation capacity of the UHPC under dynamic loading. The higher number of fibers per unit volume leads to more frequent fiber-matrix interactions, which in turn enhances frictional pull-out resistance and crack bridging. The described relationships were confirmed based on the influence of steel microfibers (length/diameter = 6/0.12 mm) in amounts ranging from 1 to 2.5% in concrete mixtures [204]. Three-dimensional tomography-based reconstructions illustrated the spatial distribution of pores and fibers, allowing a precise visualization of fiber alignment and clustering, providing insight into crack formation and propagation pathways within the concrete. For plain concrete, the cracks were concentrated and exhibited pronounced brittle behavior, while fiber-reinforced concrete showed evenly distributed cracks. In regions with high fiber density (reinforcement zones), macroracks were absent, and only minor microcracks were observed. Fiber inclusion altered the original crack propagation directions, allowing stress redistribution throughout the matrix. This change mitigates concrete brittleness, improving ductility and impact resistance. Peak stress, maximum strain, and ultimate stress were highest for 2% fiber content. Increasing the fiber content to 2.5% resulted in a reduction, indicating an uneven distribution of fibers within the matrix. For a mixture with ST fibers (12/0.175-0.35 mm) and PE fibers (12/0.039 mm) totaling 2%, high-rate dynamic compression tests (9.5-11.5 m/s) showed some integrity of the sample [205]. Samples with 2% ST fibers exhibited brittle failure with concrete fragmentation, while 2% PE fibers resulted in wide cracks with fewer microcracks. This difference arises due to the significantly higher number of PE fibers compared to ST fibers at the same volumetric fraction (due to the lower density and diameter). Several studies using high-speed imaging and post-fracture SEM analysis have shown that short fibers are activated earlier during impact, forming a dense bridging network that resists crack propagation. The energy absorbed during debonding and pulling out is largely governed by the extent of interfacial bonding and the frictional resistance during fiber extraction. These mechanisms are particularly efficient in short fibers as a result of their higher probability of activation and shorter embedding lengths. The dynamic properties of UHPC reinforced with steel and polymer fiber are summarized and analyzed in Table 11.  
The dynamic compressive strength of UHPC with the addition of steel and polymer fibers, depending on the strain rates, is presented in Figure 8a.