## 4 Hybrid Fiber Composition
### Steel--Polymer Hybrid Fiber
#### 4.3.2 Mechanical Properties
The highest value was obtained by UHPC with a maximum steel fiber volume fraction of 0.75%, indicating optimal utilization of the parameters of the hooked-end fiber, which contributed to the bridge macro cracks. The positive effect of adding hooked-end ST and PP fibers was also confirmed in [154]. The splitting tensile strength was observed to increase by 98-120% and 61.5-64.6% for water/binder ratios of 0.3 and 0.25, respectively. The increase in UHPC strength with 1.05% fiber addition and w/b ratio = 0.3 was higher than that of 1.6% fiber addition at a lower w/b = 0.25. This can be explained by the uneven distribution of fibers at a higher fiber content, linked to reduced workability. [PERSON] and [PERSON] [76] determined the elastic modulus of concrete with hybrid fiber mixtures, including 2% steel fibers (length/diameter = 13/0.2 mm) and 0.5% polymer fibers. The following types of polymer fibers were used: PP (16/0.15 mm), PE (40/0.6 mm), PVA (12/0.04 mm), polyester (30/0.75 mm), and polyester (30/0.9 mm). For each mixture, the elastic modulus increased by 12.3 to 39.5% compared to the control concrete without fibers. Compared to the mixture with 2.5% steel fibers, only the concrete with steel and polyester fibers (30/0.75 mm) achieved an increase in elastic modulus, suggesting a positive hybridization effect for this combination. The elastic modulus of UHPC with a total fiber content of 3% was tested in [42]. Steel fibers (length/diameter = 30/0.2 mm) and POM fibers (12/0.2 mm) were used. The highest elastic modulus was obtained by mixing with 3% steel fibers, while increasing the content of POM fiber to 1% and 2% caused a slight linear decrease in the elastic modulus. Mixtures with a total addition of hybrid fibers of 1.8%, including 1.3% steel fibers (length/diameter = 13/0.2 mm) and 0.5% polymer fibers, achieved an increase in elastic modulus [75]. Among the polymer fibers used were POL (40/0.6 mm), PVA (12/0.04 mm), PVA (8/0.2 mm), PVA (12/0.2 mm), polyester (30/0.75 mm), and polyester (30/0.9 mm). The highest elastic modulus was recorded for the ST and PVA (12 mm) mixture, which was 51% higher compared to the reference mixture. The elastic modulus of UHPC with steel and polyolefin fibers was observed to decrease by 9% compared to concrete with 1.8% ST. This was related to the lower elastic modulus of polyolefin fiber compared to that of ST fiber. The elastic modulus of concrete with the addition of 0.5 or 1% hooked-end ST fibers (length/diameter = 50/1 mm) and 0.06% PP fibers (12/0.025 mm) increased from 0.5 to 1.8% as the ST fiber content increased [151]. The addition of the fibers mentioned above in amounts of 1.05% and 1.6% was presented in [153]. The UHPC mixtures achieved elastic modulus gains of 1.8% and 2.1%, respectively, with an increasing total fiber content. The impact of hybrid mixtures of identical fibers with a total content of 1% was assessed in [152]. A decrease in elastic modulus of 8.9% and 10.7% was observed for two combinations: 0.25% ST/0.75% PP and 0.75% ST/0.25% PP, but no effect was observed for the 0.5% ST/0.5% PP mixture. The addition of the aforementioned fiber types in total amounts of 0.525% and 1.6% resulted in a decrease in elastic modulus by 4.1% and 16.3%, or a slight increase of 0.8% and 1.8% [154]. In the case of concrete with a total fiber content of 1.05%, reducing the w/b ratio resulted in a decrease in elastic modulus, which can be attributed to insufficient water content for proper mixing and hydration. These results suggest that hooked-end ST has a minimal effect on the elastic modulus value. Figure 6a presents the results of the compressive strength tests of UHPC with the addition of steel and polyethylene fibers. An increase in compressive strength was observed  
from 115 to 225 MPa with the growing fiber content. The lowest results were obtained for mixtures with a total fiber content of 1%. For a 2% fiber addition, small variations in strength results were recorded, ranging from 135 to 142 MPa.