## 4 Hybrid Fiber Composition
### Steel--Polymer Hybrid Fiber
#### 4.3.2 Mechanical Properties
The load bearing capacity increased due to the steel fibers that feed into the crack interfaces. The tensile strength of the fibers and their behavior during removal from the matrix was used. The flexural tensile strength of UHPC mixtures with the addition of 1% or 2% straight steel fibers with a length/diameter ratio of 13/0.22 mm and 0.5% or 1% polyethylene fibers 19/0.023 mm was determined in [146]. In the case of the mixture with 1% steel fibers, the addition of polyethylene fibers in the amount of 0.5% or 1% resulted in increases in strength of 83.3% and 90%, respectively. Increasing the amount of steel fibers to 2% and adding polypropylene fibers in the amount of 0.5% or 1% led to strength increases of 90% and 60%, respectively, indicating that with a total fiber content of 3%, they were not evenly distributed and the hydration process may have been hindered. The results demonstrated the positive effect of polymer fibers, which contributed to the increase in flexural strength. These fibers prevented the formation of microcracks and allowed the steel fibers to carry the ultimate load. The mixture containing a total of 2% fibers, composed of hooked steel fibers with a length/diameter of 30/0.9 mm and polymer fibers of 30/0.9 mm, achieved an increase in the flexural tensile strength compared to the reference mixture [150]. The mixture with an equal volume content of steel and polymer fibers (1% each) reached the highest increase in strength, which was 51.4% compared to the control mixture. The concrete strength for this fiber combination was also higher than the strength of the mixture with a 2% addition of steel fibers, indicating the positive effect of fiber hybridization. The addition of hooked steel fibers with a length/diameter ratio of 50/1 mm in the amount of 0.5% or 1% and polypropylene fibers of 12/0.025 mm in the amount of 0.06% resulted in an increase in flexural tensile strength as the content of steel fibers increased [151]. The mixture achieved an increase in strength of 11% and 14.6%, respectively, for the increase in the content of steel fibers. The total addition of 1.05 to 1.6% of the fibers mentioned above was presented in [153]. The increase in strength was 17.6% for 1% steel fibers and 0.05% PP fibers compared to the control mixture without fibers. Increasing the share of ST fibers to 1.5% and PP to 0.1% resulted in a further increase in flexural strength of 5.4%. [PERSON] [152] examined the effect of 1% of the aforementioned ST and PP fiber mixtures on the properties of UHPC. For each fiber combination, an increase in flexural tensile strength was achieved from 16.7 to 61.4%, with the best results obtained from an equal distribution of fibers at 0.5% each. The positive effect of adding hooked steel and polypropylene fibers was confirmed in [154]. An increase in strength was observed from 8.5% to 23.1% as the fiber content increased. Furthermore, reducing the water/binder ratio from 0.3 to 0.25 resulted in improved flexural tensile strength. The axial tensile strength of UHPC mixtures with straight steel fibers (length/diameter = 13/0.2 mm) and polyethylene fibers (12/0.024 mm) with a total fiber content of 3% was determined by [PERSON] et al. [144]. The study considered the following fiber combinations in hybrid mixtures: 1/2%, 1.5/1.5%, and 2/1%. Strength increases were achieved compared to UHPC with 3% steel fibers, except for the 1.5/1.5% combination. Increasing the content of PP fibers to 2% had a positive effect, suggesting that with equal fiber shares, fiber clusters could have formed, creating spaces through which cracks spread. Hybrid concretes with a total fiber content of 2.5% were tested for axial tensile strength [76]. The mixture included 2% straight steel fibers (length/diameter = 13/0.2 mm) and 0.5% polymer fibers. The effects of several types of synthetic fibers were compared, including PP (16/0.15 mm), PP (40/0.6 mm), PVA (12/0.04 mm), polyester (30/0.75 mm), and polyester (30/0.9 mm).