## 4 Hybrid Fiber Composition
### Steel--Polymer Hybrid Fiber
#### 4.3.2 Mechanical Properties
In the case of mixes with two types of fibers: 2% straight steel fibers with a length/diameter ratio of 13/0.2 mm and 0.5% of one of five different types of non-metallic fibers (PP 16/0.15 mm or 40/0.6 mm, PVA 12/0.04 mm, and polyester 30/0.75 mm or 30/0.9 mm), compressive strength increases ranging from 17.6% to 27% were noted compared to the mix without fibers [76]. The mixture with PVA fibers showed the lowest strength gain compared to the control mixture and an 8.6% reduction compared to the mix with 2.5% steel fibers. The same volume content of non-metallic fibers contained many more fibers than steel fibers, resulting in uneven fiber dispersion, mixing difficulties, and fiber agglomeration. [PERSON] [147] investigated the effect of polyethylene fibers with lengths/diameter ratios of 6, 12, and 18/0.024 mm and straight steel fibers of 12/0.2 mm, on the compressive strength of UHPC with a total fiber content of 2%. As the percentage of non-metallic fibers increased, the compressive strength decreased, with no significant differences observed between the different lengths of the fiber. However, according to [PERSON] et al. [193], excessively short fibers can lead to complete slippage within the mixture. Even if the bond strength is sufficient at the initial crack, increasing the applied force could cause the short fibers to fail without providing adequate anchorage. The best combination was achieved with steel fibers and 12 mm polyethylene fibers, resulting in the smallest strength reductions, suggesting that the fibers were evenly distributed and effectively prevented microcracking without negatively affecting peak strength. A reduction in the compressive strength of mixtures with a total fiber content of 3% (steel and polyoxymethylene) was confirmed in [42]. A linear decrease in strength was observed, from 163 MPa for 3% steel fibers to 125 MPa for 3% POM fibers, with intermediate strength values for hybrid mixes. It was observed that the POM fibers were either pulled out or fractured, whereas the steel fibers were predominantly pulled out because of their high strength. Mixtures with a total addition of 1.8% hybrid fibers, including 1.3% straight steel fibers with a length/diameter of 13/0.2 mm and 0.5% polymer fibers, showed an increase in compressive strength compared to the control sample [75]. Among the polymer fibers used were polyolefin fibers with a length/diameter of 40/0.6 mm, PVA fibers of 12/0.04 mm, 8/0.2 mm or 12/0.2 mm, and polyester fibers of 30/0.75 mm or 30/0.9 mm. An increase in compressive strength was obtained compared to the 1.3% steel fiber mixture only for two mixtures: steel-polyolefin and steel-polyester (30/0.9 mm) fiber combinations. It can be seen that these fibers had the largest diameters, which effectively inhibited macrocracks. The lowest compressive strength was achieved with the addition of fibers with the smallest diameters, which was also confirmed in [76]. [PERSON] et al. [148] observed an increase in the compressive strength of a UHPC mixture with a total fiber content of 2% as the proportion of straight steel fibers with a length/diameter of 14/0.22 mm increased compared to PVA fibers with a length/diameter of 12/0.026 mm. Compressive strength increased from 35.7% to 49.3% as the proportion of steel fibers increased from 0.5% to 1.5%. [PERSON] et al. [149] determined the compressive strength of mixtures containing 1.5% polyethylene fibers with a length/diameter of 13/0.02 mm and steel fibers with a length/diameter of 18/0.16 mm in amounts of 0.5%, 1%, and 1.5%. The lowest increase in compressive strength was observed with a 0.5% addition of steel fibers (24.4% compared to the fiber-free mixture). Further increases in the steel fiber content to 1% and 1.5% resulted in increases in strength of 27.8% and 33.1%, respectively.