## 4 Hybrid Fiber Composition
### Steel--Basalt Hybrid Fiber
#### 4.2.2 Mechanical Properties  
The ultra-high performance concrete (UHPC) with the mix addition of steel fibers (diameter/length of 16.5/0.2 mm at 0.33% and 19.5/0.2 mm at 0.67%) and basalt fibers (12/0.012 mm at 0.5%) exhibited lower compressive strength than the mix with 1.5% steel fibers [143]. The results suggest that basalt fibers negatively affect the compressive strength of concrete, as these fibers tend to agglomerate, creating spaces where there are no fibers. Upon considering the results, it can be inferred that the addition of an appropriate amount of basalt fibers is crucial to improving the compressive strength. Incorrectly chosen fibers, regardless of type, length, or content, can have the opposite effect of what is intended. As fiber content increases, the reinforcement effect decreases due to uneven fiber distribution in concrete and weak bond strength due to inhomogeneity of the mix [170]. Furthermore, exceeding a certain fiber content, which depends on the specific properties of the mix, causes the fibers to overlap, leading to an increase in the pores within the concrete. This can create weak interfaces and gaps [180,181]. A hybrid fiber UHPC with 2% fiber addition was tested to determine its compressive strength as the percentage of fiber content changed [70]. A decrease in compressive strength was observed as the amount of basalt fibers (12/0.45 mm) increased compared to steel fibers (12/0.3 mm). Compared to the fiber-free mixture, the compressive strength increased from 37.6% to 50.5%. However, because of the lower stiffness of the basalt fibers, the compressive strength of the UHPC decreased when the basalt fibers replaced the steel fibers. [PERSON] and [PERSON] [75] presented the results of the compressive strength tests for a hybrid mix with 1.3% steel fibers (13/0.2 mm) and 0.5% basalt fibers (12/0.02 mm). The compressive strength was found to increase by 17.3% compared to the fiber-free mix. However, the compressive strength of the hybrid fiber mix was lower compared to the 1.3% steel fiber mix, indicating a negative impact of the basalt fibers. The reason for this could be the uneven distribution of fibers in the concrete matrix. Other results of studies on the hybrid combination of steel fibers (13/0.2 mm) and basalt fibers (12/0.02 mm) also indicate that the compressive strength is primarily determined by the percentage of steel fibers [142]. The mixture with 2% steel fibers achieved a compressive strength of 148 MPa, while replacing 0.5% of steel fibers with basalt fibers resulted in a reduction of 8%. Despite the reduced compressive strength, the combination of these two types of fibers showed positive properties. While samples with 2% steel fibers and those without fibers experienced spalling at temperatures between 500 and 1000 \({}^{\circ}\)C, samples with basalt fibers maintained integrity, and it was possible to determine the compressive strength test. A mixture with 2% steel fibers (13/0.2 mm) and 0.5% basalt fibers (30/0.02 mm) achieved a higher compressive strength than an identical mix in which the basalt fibers mentioned above were replaced with basalt fibers of 12/0.02 mm with the same volume content [76]. Longer fibers, due to their lower quantity, were more evenly distributed in the matrix and effectively bridged cracks, leading to a positive effect on increased strength.