## 4 Hybrid Fiber Composition
### Hybrid Combinations of Other Fiber Types  
The addition of twisted ST fibers with a length/diameter of 25/0.7 mm at 1.5% combined with carbon fibers of 7/0.01 mm at 1% resulted in a decrease of 8.6% in the compressive strength of UHPC compared to fiber-free concrete [84]. This drop suggests that the total fiber content of 2.5% was not evenly distributed and may have a negative impact on hydration processes, preventing a positive effect of hybrid fiber on the compressive strength of the UHPC. The highest strength was achieved with concrete with 1.5% ST, while increasing the ST content to 2.5% led to a reduction in strength. Straight steel fibers (15/0.6 mm) at 1% were combined with carbon fibers (20-30/0.018 mm) at 1% to assess their impact on the compressive strength of UHPC [85]. A 13.6% increase in strength was observed compared to UHPC without fibers and a 4.6% increase relative to UHPC with 2% ST fibers. These results indicate a positive hybridization effect of ST and C fibers, with an even fiber distribution within the UHPC matrix and proper hydration of the components. Compared to previous results [84], it can be seen that the combination of longer C fibers with shorter straight ST fibers is beneficial in improving the compressive strength of concrete. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \multicolumn{2}{c}{**Hybrid Fibers**} & \multicolumn{8}{c}{**Properties (Effectiveness \%)**} & \multicolumn{1}{c}{**Ref. **} \\ \hline
**Part A** & & \multicolumn{3}{c}{**Part B**} & \multicolumn{3}{c}{**Properties (Effectiveness \%)**} & \multicolumn{1}{c}{**Ref. **} \\ \hline
[PERSON] et al. [85] proposed another type of hybrid combination in UHPC, consisting of 1% C fibers (20-30/0.018 mm) and 1% G fibers (6-12/0.012 mm). This concrete achieved a compressive strength of 13.2% higher than fiber-free UHPC and 3.4% higher than UHPC with 2% ST fibers. A positive hybridization effect of C and G fibers was recorded. This may be due to the higher tensile strength and elastic modulus of C fibers compared to ST fibers, as well as the strong adhesion of G fibers to the UHPC matrix [210]. UHPC with the addition of basalt fibers measuring 12/0.013 mm and polypropylene fibers of the same size (12/0.013 mm) was investigated in [155]. The samples were prepared with a total fiber content of 1% or 2%. For all hybrid mixtures, a decrease in compressive strength was observed, ranging from 10.8% to 18.1%, increasing with the content of PP fibers. The results showed that increasing the combined amount of PP fibers and B fibers to 2% caused a greater reduction in strength compared to the mixture with a fiber content of 1%. The B and PP fibers tended to reduce the workability of the concrete by absorbing small amounts of water, leading to the formation of pores. Twisted steel fibers with a length/diameter of 25/0.7 mm at 1.5%, combined with carbon fibers (7/0.01 mm) measuring at 1%, resulted in a 22.7% increase in flexural strength compared to UHPC without fibers [84]. This result was identical to that of UHPC with 1.5% ST fibers, suggesting that the addition of short C fibers does not significantly improve flexural strength. An additional increase in the ST fiber content to 2.5% led to an additional 50% increase in flexural strength. Straight steel fibers (15/0.6 mm) at 1% were added to UHPC with C fibers (20-30/0.018 mm) at 1%, resulting in an increase of 34.9% in flexural strength compared to UHPC without fibers and causing a slight 0.9% decrease compared to UHPC with 2% ST fibers [85]. A positive effect of the hybridization of C and ST fibers on flexural strength was observed, contrasting with the findings in [84]. This can be attributed to the use of longer C fibers, which played a crucial role in improving the flexural strength of the UHPC compared to the fiber-free UHPC. Another type of hybrid combination involved C fibers (20-30/0.018 mm) at 1% and G fibers (6-12/0.012 mm) at 1%, added to the UHPC mix [85].