## 4 Hybrid Fiber Composition
### Steel--Polymer Hybrid Fiber
#### 4.3.2 Mechanical Properties
The axial tensile strength of concrete was found to increase for each hybrid mixture by 57-86% compared to the control sample. The influence of combining PE fibers (length/diameter = 6, 12, and 18/0.024 mm) with straight steel fibers (13/0.2 mm) on UHPC axial tensile strength of was presented in [147]. For each mixture, there was a 2.5-24% decrease in strength compared to the control sample with 2% ST, and this increased with a higher PE fiber content. The lowest strength results were observed for the longest PE fibers, suggesting that the lower amount of these fibers led to an uneven distribution across the sample cross section. All hybrid concretes with a total fiber addition of 1.8%, including 1.3% steel fibers (length/diameter = 13/0.2 mm) and 0.5% polymer fibers, showed an increase in axial tensile strength compared to the reference mixture without fibers [75]. The polymer fibers used included polyolefin (40/0.6 mm), PVA (12/0.04 mm), PVA (12/0.2 mm), polyester (30/0.75 mm) and polyester (30/0.9 mm). The axial tensile strength of hybrid mixtures with the addition of 1.5% PE fibers (length/diameter = 13/0.02 mm) and 0.5-1.5% steel fibers (18/0.16 mm) increased with the proportion of ST, ranging from 7.3% to 12.4%, due to the high tensile strength of the steel fibers [149]. Concretes with the addition of straight steel fibers (length/diameter = 16.5/0.2 mm at 0.33% and 19.5/0.3 mm at 0.67%) and polymer fibers at 0.5% achieved higher axial tensile strength compared to a mixture with 1.5% steel fibers [143]. With 0.5% polyethylene fibers (18/0.012 mm), the increase was 65.3%, and with PVA fibers (12/0.012 mm) it was 20.4%. A positive hybrid effect was observed when 0.5% of steel fibers were replaced with polymer fibers, especially PE fibers. The axial tensile strength was determined for mixtures with a total fiber content of 1% [140]. These included combinations of steel fibers (length/diameter = 19.5/0.2 mm) and polymer fibers, such as PVA (6/0.012 mm), PVA (12/0.012 mm), or PE (12/0.02 mm). Each mixture showed a linear decrease in tensile strength with a reduction in ST fiber content. The flowability results were significantly lower than those for steel fibers, due to the high shape factor of the polymer fibers because of further hindered proper fiber dispersion. The addition of shorter polymer fibers resulted in greater reductions in strength. The incorporation of microfibers has been observed to improve tensile strength, as they are more active during the cracking process, causing macrorack formation to delay and improve ductility [196; 197]. A loud sound heard after the first crack indicated fiber bridging. In UHPC without fiber addition, a sudden failure occurred, as once the concrete matrix reached its maximum strength, there was no component in the mixture capable of carrying the load [76]. Microcracks form in weakened areas, such as pores or interfaces. These cracks connect, but when microfibers are present, their expansion is hindered [198]. The effectiveness of microfibers is related to the smaller spacing between them as a result of their higher density per unit volume. The effect of hooked-end steel fibers (length/diameter = 50/1.0 mm) at 0.5 or 1% and polypropylene fibers (12/0.025 mm) at 0.06% on tensile splitting strength was reported in [151]. The UHPC mixture achieved a tensile strength increase of 104-120% with an increase in the steel fiber content from 0.5 to 1%. The addition of the aforementioned fibers in a total amount of 1.05-1.6% was presented in [153]. Significant strength gains were observed in the range of 64.5 to 67.7% with increasing total fiber content. The results indicate that for the mixture with 1.06% fibers, the packing density was already high enough that increasing the fiber content further did not result in a clear increase in strength. [PERSON] [152] observed that the splitting tensile strength, after adding the same fibers in a total amount of 1%, increased 36.8 to 51.7%.