## 4 Hybrid Fiber Composition
### Steel--Basalt Hybrid Fiber
#### 4.2.2 Mechanical Properties
\begin{table}
\begin{tabular}{c 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\({}_{\text{f}}\) (mm)** & **d\({}_{\text{f}}\) (mm)** & **V\({}_{\text{f}}\) (\%)** & **Type** & **1\({}_{\text{f}}\) (mm)** & **d\({}_{\text{f}}\) (mm)** & **V\({}_{\text{f}}\) (\%)** & \\ \hline \multirow{3}{*}{straight steel} & \multirow{3}{*}{12} & \multirow{3}{*}{0.3} & 0.5 & \multirow{3}{*}{basalt} & \multirow{3}{*}{12} & \multirow{3}{*}{0.45} & 1.5 & flowability = 175 mm (\(-\)43.6\%) & \\  & & & & & & & 0.5 & flowability = 200 mm (\(-\)35.5\%) & \\ \cline{1-1}  & & & & & & & 0.5 & flowability = 215 mm (\(-\)30.7\%) & \\ \hline \multirow{3}{*}{straight steel} & \multirow{3}{*}{13} & \multirow{3}{*}{0.2} & 2.0 & \multirow{3}{*}{basalt} & \multirow{3}{*}{12} & 0.02 & 0.5 & flowability = 410 mm (\(-\)34.9\%) & \\ \cline{1-1}  & & & & & 30 & 0.02 & 0.5 & flowability = 415 mm (\(-\)34.1\%) & \\ \hline \end{tabular}
\end{table} 
Table 7: Influence of hybrid fibers on flow of UHPC mixes. The flexural tensile strength of UHPC with a total fiber content of 2%, comprising steel fibers (12/0.3 mm) and basalt fibers (12/0.45 mm), decreased as the basalt fiber content increased from 0.5% to 1.5% [70]. Basalt fibers exhibit a better anchoring effect due to their rough structure, but compared with steel fibers, their mechanical properties do not contribute as much to increasing the concrete strength. Basalt fibers inhibited the development of microcracks, but the maximum strength value was mainly dependent on steel fibers. A positive hybridization effect occurred in the mixes when 0.5% and 1% basalt fibers were mixed with 1.5% and 1% steel fibers, respectively. The UHPC flexural strength of the mixture with 2% steel fibers (13/0.2 mm) achieved a higher value with the addition of basalt fibers (30/0.02 mm) than with basalt fibers (12/0.02 mm) [76]. Compared to the reference mix, the strength increased by 75.5% for the shorter basalt fibers and by 110% for the longer fibers, indicating a high level of synergy between the fibers and the matrix. These results are consistent with those of [PERSON] et al. [64], who found that longer basalt fibers led to a greater increase in flexural tensile strength. The axial tensile strength of a UHPC mix with 1.3% steel fibers (13/0.2 mm) and 0.5% basalt fibers (12/0.02 mm) was studied by [PERSON] and [PERSON] [75]. Compared to control samples, the mixture achieved a strength that was 30.8% and 4.3% higher than the sample with 1.8% steel fibers, indicating a positive hybridization effect. The increase is attributed to the mechanism that occurs during the interaction of force with the hybrid fiber matrix. As the tensile stresses increase to the level at which the fiberless matrix will fail, the load is transferred by the fibers that bridge the cracks. Furthermore, compared to ordinary concrete, the onset and propagation of cracks is delayed. The mixture with two types of straight steel fibers (16.5/0.2 mm at 0.33% and 19.5/0.2 mm at 0.67%) along with basalt fibers (12/0.012 mm at 0.5%) showed a 14.7% higher axial tensile strength compared to the control mixture [143]. Furthermore, the strength was higher than in mixes with 1.5% steel fibers (1% fibers of 19.5 mm and 0.5% fibers of 16.5 mm), indicating a positive hybridization effect with basalt fibers. The results suggest that the partial replacement of steel fibers with basalt fibers can increase the strength value at which the first crack occurs, as well as the ultimate strength. Basalt fibers, unlike other types of fiber, are composed of minerals with a composition similar to that of cement, leading to strong chemical bonds between the matrix and the fiber [182]. Due to their density being close to that of concrete, basalt fibers can improve the mechanical properties and durability of concrete without significantly increasing its weight [183].