## 4 Hybrid Fiber Composition
### Steel--Polymer Hybrid Fiber  
4.3.1. Physical Characteristics  
Table 9 shows the influence of steel-polymer hybrid fibers on the properties of the fresh UHPC mix. \begin{table}
\begin{tabular}{p{28.5 pt} p{28.5 pt} p{28.5 pt} p{28.5 pt} p{28.5 pt} p{28.5 pt} p{28.5 pt} p{28.5 pt} p{28.5 pt} p{28.5 pt}} \hline \multicolumn{1}{c}{**Hybrid Fibers**} & & & & & & & & & **Properties (Effectiveness \%)** & **Ref.** \\ \hline \multicolumn{1}{c}{**Part A**} & & & & & **Part B** & & & & & \\ \hline
**Type** & \(\mathbf{l_{t}}\) (mm) & \(\mathbf{d_{t}}\) (mm) & \(\mathbf{V_{f}}\) (\%) & **Type** & \(\mathbf{l_{t}}\) (mm) & \(\mathbf{d_{t}}\) (mm) & \(\mathbf{V_{f}}\) (\%) & & & & \\ \hline straight steel & 13 & 0.22 & 1.0 & polyethylene & 19 & 0.023 & 1.0 & flowability = 174 mm (\(-\)37.8\%) & 146 \\  & & & 2.0 & 2.0 & & & 1.0 & flowability = 145 mm (\(-\)33.1\%) & 146 \\ \hline straight steel & 13 & 0.2 & 1.5 & polyvinyl-alcohol & 8 & 0.038 & 0.5 & flow time = 48 s (\(+\)65.5\%) & 139 \\  & & & 2.0 & polypropylene & 16 & 0.15 & 0.5 & flowability = 540 mm (\(-\)14.3\%) & 138 \\ straight steel & 13 & 0.2 & 2.0 & polyvinyl-alcohol & 40 & 0.6 & 0.5 & flowability = 543 mm (\(-\)13.8\%) & 157 \\  & & & 2.0 & alcohol & 30 & 0.75 & 0.5 & flowability = 542 mm (\(-\)14.0\%) & 147 \\  & & & 2.0 & polyester & 30 & 0.9 & 0.5 & flowability = 533 mm (\(-\)15.4\%) & 148 \\ \hline straight steel & 13 & 0.2 & 1.0 & polypropylene-thylene & 12 & 0.2 & 2.0 & flowability = 155 mm (\(-\)20.5\%) & 142 \\  & & & 2.0 & ethylene & 12 & 0.2 & 1.0 & flowability = 160 mm (\(-\)18.0\%) & 148 \\ \hline  & & & 0.25 & polyvinyl-alcohol & 6 & 0.012 & 0.75 & flowability = 140 mm (\(-\)36.4\%) & 148 \\  & & & 0.75 & alcohol & 6 & 0.012 & 0.5 & flowability = 140 mm (\(-\)36.4\%) & 148 \\ straight steel & 19.5 & 0.2 & 0.25 & polyvinyl-alcohol & 12 & 0.012 & 0.75 & flowability = 160 mm (\(-\)27.3\%) & 148 \\  & & & 0.75 & alcohol & 12 & 0.012 & 0.5 & flowability = 165 mm (\(-\)25.0\%) & 148 \\  & & & 0.25 & polyethylene & 12 & 0.02 & 0.75 & flowability = 160 mm (\(-\)27.3\%) & 148 \\  & & & 0.25 & polyethylene & 12 & 0.02 & 0.75 & flowability = 160 mm (\(-\)27.3\%) & 148 \\  & & & 0.25 & polyethylene & 12 & 0.02 & 0.75 & flowability = 160 mm (\(-\)27.3\%) & 148 \\  & & & 0.75 & polyethylene & 12 & 0.02 & 0.75 & flowability = 160 mm (\(-\)27.3\%) & 148 \\  & & & & 0.75 & & & 0.25 & flowability = 160 mm (\(-\)27.3\%) & 148 \\ \hline \end{tabular}
\end{table} 
Table 9: Influence of hybrid steel–polymer fibers on flow of UHPC mixes. Figure 5: Strength test results of UHPC reinforced with steel and basalt fibers: (**a**) compressive strength, (**b**) flexural strength. Hybrid mixes with a total fiber content of 1%, consisting of ST fibers with a length/diameter ratio of 19.5/0.2 mm and PVA fibers of 6/0.012 mm or PVA 12/0.012 mm, or PE fibers of 12/0.2 mm, exhibited a reduction in slump flow diameter compared to the control mix with 1% steel fibers [140]. The shorter PVA fibers, because of their lower aspect ratio, caused a greater reduction in workability compared to the longer PVA and PE fibers. A significant reduction in workability was observed with a fiber content of 0.5%. Increasing the content of straight steel fibers with a length/diameter of 13/0.22 mm, as well as polyethylene fibers with a length/diameter of 19/0.023 mm, negatively affected the flowability of the hybrid UHPC mix [146]. However, the steel fibers reduced the fluidity to a lesser extent. An increase in polyethylene fibers from 0.5% to 1% resulted in a greater decrease in flowability compared to the additional application of 1% steel fibers. This is because steel fibers are heavier, have a larger diameter, and are present in smaller quantities by volume, which makes the reinforcing network less complex. As a result, the components can move more easily because there are fewer bonds that hold the mixture in its initial shape.