## 4 Hybrid Fiber Composition
### Steel--Polymer Hybrid Fiber
The flow time of a hybrid mixture with 1.5% steel fibers (length/diameter 13/0.2 mm) and 0.5% polyvinyl alcohol fibers (8/0.038 mm) indicates the negative impact of PVA fibers on the fluidity of the mixture [139]. A mixture with identical composition but with 3% steel fibers achieved a shorter flow time. The results show that the addition of 0.5% PVA fibers reduces the fluidity by more than 1.5% of the steel fibers, partly due to the high water absorption of the PVA fibers. [PERSON] and [PERSON] [76] observed a decrease in the diameter of the slump flow for mixes containing straight steel fibers with a length/diameter ratio of 13/0.2 mm and five types of non-metallic fibers. The highest flow was achieved by the UHPC mix with 2.5% steel fibers. The replacement of 0.5% steel fibers with non-metallic fibers generally had a minor effect on flow. Only for the PVA fibers with the smallest diameter was a noticeable reduction in flow diameter observed. This was related to the significantly higher volume of PVA fibers compared to those of the other mixes, which strongly influenced the binding of the matrix. [PERSON] et al. [42] noted that a mix with a total addition of 3% hybrid fibers (straight steel fibers with a length/diameter ratio of 13/0.2 mm and polyoxymethylene fibers of 12/0.2 mm) achieved a slightly lower slump flow diameter than a sample containing a single type of fiber. Compared to the control sample without fibers, the diameter of the slump flow was less by 18-23%. The addition of polyoxymethylene fibers in the amount of 1% or 2% to, respectively, 2% or 1% steel fibers led to the formation of denser skeletons, creating resistance during movement of the mixed components. Although a single type of fiber affected fluidity in the same way, the combination of both types of fiber indicated a mutual interaction between them and the other components of the mixture. \begin{table}
\begin{tabular}{l c c c c c c c c c} \hline \multicolumn{2}{c}{**Hybrid Fibers**} & & & & & & & & **Properties (Effectiveness \%)** & **Ref.** \\ \hline
**Part A** & & & & & **Part B** & & & & \\ \hline
**Type** & **1\({}_{t}\) (mm)** & **d\({}_{t}\) (mm)** & **V\({}_{t}\) (\%)** & **Type** & **1\({}_{t}\) (mm)** & **d\({}_{t}\) (mm)** & **V\({}_{t}\) (\%)** & & \\ \hline \multirow{3}{*}{\
\begin{tabular}{c} hooked- \\ end \\ steel \\ \end{tabular} } & \multirow{3}{*}{30} & 0.9 & 0.5 & \multirow{3}{*}{plastic synthetic} & \multirow{3}{*}{30} & 1.5 & \multirow{3}{*}{flowability = 158 mm (\(-\)12.2\%)} & \multirow{3}{*}{[150]} \\  & & & & 1.0 & & & 0.9 & 1.0 & flowability = 160 mm (\(-\)11.1\%) & \\ \cline{1-1} \cline{5-10}  & & & & 1.5 & & & 0.5 & flowability = 161 mm (\(-\)10.6\%) & \\ \hline \multirow{3}{*}{
\begin{tabular}{c} hooked- \\ end \\ steel \\ \end{tabular} } & \multirow{3}{*}{50} & 0.5 & \multirow{3}{*}{1.0} & 0.5 & \multirow{3}{*}{polpropylene} & \multirow{3}{*}{12} & 0.025 & \multirow{3}{*}{slump = 111 mm (\(-\)7.5\%) 2} & \multirow{3}{*}{[154]} \\  & & & 1.0 & & & 0.05 & & slump = 95 mm (\(-\)18.1\%) 3 \\ \cline{1-1} \cline{5-10}  & & & & 1.5 & & 0.1 & slump = 88 mm (\(-\)24.1\%) 3 & \\ \hline \end{tabular}  
* comparison to a mixture with 1% straight steel fibers; 2 water/binding material ratio = 0.3; 3 water/binding material ratio = 0.25.  
\end{table} 
Table 9: _Cont._[PERSON] et al. [150] presented the results of the slump flow of a hybrid mix containing hooked steel fibers with a length/diameter ratio of 30/0.9 mm and plastic fibers of 30/0.9 mm. As the proportion of synthetic fibers increased, the diameter of the slump flow decreased. A mixture containing hooked-end steel fibers with a length/diameter ratio of 50/1 mm and polypropylene fibers of 12/0.025 mm showed a decrease in slump with increasing fiber content [154]. When the water-to-binder ratio was lower and the fiber content remained the same, a further reduction in fluidity was observed. The reduction in slump was caused by significant adhesion forces that developed between the fibers and the moving matrix.