## 4 Hybrid Fiber Composition
### Steel--Steel Hybrid Fiber
#### 4.1.2 Mechanical Properties  
Fibers enhance UHPC by inhibiting microcrack propagation and bridge developing cracks. Short fibers are activated early, while longer fibers delay macrocrack formation and enhance ductility. [PERSON] et al. [140] noted that the highest compressive strength was achieved in a hybrid mixture that combined straight steel fibers with a length/diameter ratio of 19.5/0.2 mm at a dose of 0.5% and hooked fibers of 13/0.2 mm at a dose of 0.5%. Increasing the length of the hooked fibers to 25 mm resulted in a decrease in compressive strength, which was associated with reduced workability and difficulty evenly dispersing the fibers in this hybrid combination. The results were confirmed by [PERSON] et al. [160], who investigated the effect of straight fibers (13 mm) and hooked fibers (13 mm) with a total content of 2.5% in UHPC. The compressive strength decreased as the proportion of hook-end fibers increased to 1.25% and was the lowest when 2.5% of these fibers were used. In the case of UHPC mixes containing hybrid straight steel fibers with a total content of 1.5%, the highest compressive strength was achieved with the addition of 0.5% fibers (length/diameter 10/0.12 mm) and 1.0% fibers (13/0.12 mm) [48]. The lowest values were observed in the mixes containing the shortest fibers (6 and 10 mm). The results presented in [135] confirm the relationships with respect to the influence of steel fibers with a length/diameter ratio of 6 and 13/0.2 mm. The highest compressive strength was obtained for hybrid mixtures with a fiber content of 2% for the highest proportion of 13 mm fibers. The strength increased by 46.2% compared to the fiberless mix. \begin{table}
\begin{tabular}{l c c c c c c c c c c} \hline \hline  & & \multicolumn{3}{c}{**Hybrid Fibers/Steel—Steel**} & & & & **Properties (Effectiveness \%)** & **Ref.** \\ \hline \multicolumn{1}{c}{**Part A**} & \multicolumn{3}{c}{**Part B**} & & & & & & & \\ \hline
**Type** & \(\mathbf{l_{f}}\) (mm) & \(\mathbf{d_{f}}\) (mm) & \(\mathbf{V_{f}}\) (\%) & **Type** & \(\mathbf{l_{f}}\) (mm) & \(\mathbf{d_{f}}\) (mm) & \(\mathbf{V_{f}}\) (\%) & & \\ \hline straight & 6 & 0.2 & 1.0 & straight & 13 & 0.2 & 1.0 & flowability = 240 mm (\(-\)3.0\%) & [135] \\  & & & 1.5 & & & & 0.5 & flowability = 243 mm (\(-\)1.8\%) & \\ \hline straight & 6 & 0.16 & \(\begin{array}{c}0.5\ \\ 1.0\ \\ 1.5\ \\ \end{array}\) & straight & 13 & 0.2 & \(\begin{array}{c}1.5\ \\ 1.0\ \\ 1.5\ \\ \end{array}\) & & & & & & & & & \\ \hline straight & 19.5 & 0.2 & \(\begin{array}{c}0.5\\ 0.25\\ 0.25\\ 0.25\\ 0.25\\ \end{array}\) & hooked-end & 13 & 0.2 & \(\begin{array}{c}0.5\\ 0.75\\ 0.25\\ 0.25\\ \end{array}\) & & & & & & & & & \\ \hline straight & 13 & 0.2 & \(\begin{array}{c}1.5\\ 1.0\\ 0.5\end{array}\) & hooked-end & 30 & 0.5 & \(\begin{array}{c}0.5\\ 1.0\\ 1.5\end{array}\) & flow time = 42 s (\(+\)44.8\%) & & \\ \hline \hline \end{tabular}  
\({}^{1}\) comparison with a mixture with 1% straight steel fibers; \({}^{2}\) comparison with a mixture with 2% straight steel fibers 13 mm long; \(\mathbf{l_{f}}\) = length; \(\mathbf{d_{f}}\) = diameter; \(\mathbf{V_{f}}\) = fiber volume.  
\end{table} 
Table 4: Influence of hybrid steel–steel fibers on flow of UHPC mixes. The decrease in compressive strength of concrete mixes with an increasing proportion of short fibers is consistent with [161], where identical-length steel fibers were also used. Similar conclusions were presented in [136], where the compressive strength was tested for a mixture containing straight steel fibers with a length/diameter ratio of 6/0.16 and 13/0.2 mm, with a total content of 2%. The highest strength was obtained in the mixture with the highest proportion of 13 mm fibers and a total content of 1.5%. The results of hybrid mixes with a total steel fiber content of 2.5% indicate a greater influence on compressive strength for fibers with a length/diameter ratio of 6/0.2 mm compared to longer fibers of 10/0.2 mm [137].