## 4 Hybrid Fiber Composition
### Steel--Polymer Hybrid Fiber
#### 4.3.2 Mechanical Properties
A positive effect of increasing the length of steel fibers from 13 to 18 mm and reducing the length of PE fibers from 19 to 13 mm was observed with a total fiber content of 2.5%. This suggests better use of longer steel fibers due to higher anchoring forces within the matrix and more effective crack-bridging by shorter polyethylene fibers. For the 3% fiber content, the strength increased with an increasing proportion of steel fibers. The most favorable combination was found to be 2.5% ST13 and 0.5% PE12. The lowest compressive strength was obtained with a mixture of short steel fibers of 13 mm at 2% and PE fibers of 19 mm at 1%, confirming the trends observed with a lower fiber content. Figure 6b presents the results of the compressive strength tests for concrete with the addition of steel and PVA fibers, which ranged from 95 to 160 MPa, depending on the type and amount of fibers, as well as the recipe for the mixture without fibers. For the 1% fiber content, a high concentration of results was observed at 118 MPa. Increasing fiber content to 1.8% and 2% resulted in a decrease in strength to 102 MPa, with a high concentration of results. This may be due to a significant reduction in the workability, leading to agglomeration of components and an impaired hydration process, creating areas of the heterogeneous matrix. The compressive strength of UHPC with the addition of steel and polypropylene fibers, with a total content ranging from 0.5% to 1.5%, was between 105 and 145 MPa (Figure 6c). The combination used hooked steel fibers of 50 mm in length and polypropylene fibers of 12 mm. Increasing the fiber content to 1% showed a positive effect on the compressive strength. Depending on the type of fiber, values between 110 and 145 MPa were achieved, and it was observed that the strength of the UHPC increased with the growing proportion of steel fibers. For a mix with 2% straight steel fibers of 13 mm and 0.5% PP fibers of 16 or 40 mm, a maximum value was obtained, with a slightly higher result observed for the longer PP fibers. Figure 6d shows the compressive strength results of mixtures with ST fibers and other types of polymer fibers (POM, PES, POL, P). Similar results were obtained for fiber contents ranging from 2% to 3.5%. A slight increase in strength was observed when the fiber content increased from 2% to 2.5%. Further increasing the fiber content to 3.5% did not provide significant benefits. Similar results for the flexural tensile strength of the mixtures were obtained with a fiber content of 0.5 to 1.6%, ranging from 9 to 10 MPa (Figure 7). These mixtures contained hooked steel fibers 50 mm long and polypropylene fibers 12 mm long. Increasing the total fiber content and changing the proportion between them did not lead to a rise in strength. The long hooked fibers, with their high tensile strength and ability to resist pulling forces from the matrix, were not fully used in load transfer. This was likely due to fiber clustering, which prevented them from effectively stopping crack formation. The addition of 2% fibers significantly increased the flexural tensile strength to 23-28.5 MPa. The highest results were achieved for UHPC with hooked steel fibers of 30 mm and plastic fibers of 30 mm, with an equal volume share of ST and P fibers. The highest UHPC strength was observed with 12 mm ST fibers and 19 mm PP fibers with an equal fiber content. Increasing the fiber amount to 2.5% and 3% resulted in a decrease in strength, linked to reduced workability and the creation of weakened areas.