## 4 Hybrid Fiber Composition
### Steel--Basalt Hybrid Fiber
#### 4.2.2 Mechanical Properties
Furthermore, its high tensile strength delays crack propagation, slows the crack widening rate, and reduces tensile stresses that exceed the strength of the concrete matrix [184]. [PERSON] et al. [185] also confirmed that basalt fibers controlled the advancing cracks and had a positive effect on the bridging. The modulus of elasticity of the UHPC mix was determined with the addition of 0.5% basalt fibers (12 mm long) and 1.3% straight steel fibers (13 mm long) in [75]. The results showed a 22.2% increase in modulus compared to the control mix without fibers and an 11.4% increase compared to the mix with 1.8% straight steel fibers (13 mm long). This indicates a positive effect of hybridization with basalt fibers. The modulus of elasticity of concrete with 2% straight steel fibers (13 mm long and 0.2 mm diameter) achieved a higher value when 0.5% of the basalt fibers (30/0.02 mm long/diameter) were replaced by an equal amount of shorter basalt fibers (12/0.02 mm) [76]. Compared to the reference mix, the modulus increased by 20% for the longer basalt fibers and by 15% for the shorter ones, suggesting effective cooperation between the fibers and the matrix, regardless of the length of the fiber. The results of the static properties test of UHPC reinforced with steel and basalt fibers are analyzed in Table 8.  
The results of the compressive strength and flexural tensile strength tests of UHPC with the addition of steel and basalt fibers are presented in Figure 5. All programs used steel fibers of 12 or 13 mm in length and basalt fibers of 12 mm (except for a mixture with 2.5% fibers). The compressive strength results ranged from 105 to 152 MPa (Figure 5a). An increase in compressive strength was observed as the steel fiber content increased to 1.5%. For UHPC with 2.5% hybrid fibers, a positive effect was observed when the fiber length increased from 12 to 30 mm. In Figure 5b, it can be seen that the flexural tensile strength increased by 31 to 77% with the increase in the amount of 12 mm straight steel fibers replacing 12 mm basalt fibers for a total fiber amount of 2%. The highest strengths were achieved for short hybrid fibers with 1.5% ST and 0.5% B. Increasing the fiber content to 2.5% resulted in a decrease in strength, but the results were still 76-110% higher than the fiber-free UHPC. Although several studies confirm the positive impact of basalt fibers on the mechanical performance of UHPC, particularly in terms of crack resistance and residual strength after thermal exposure, there are also reports of conflicting findings regarding their influence on compressive strength. For example, [PERSON] et al. [70] observed a noticeable reduction in compressive strength with increased basalt fiber content, attributed this to agglomeration tendencies and reduced workability that compromised fiber dispersion. In contrast, [PERSON] and [PERSON] [75] reported strength improvements when basalt fibers were added in moderate amounts, suggesting that the rough surface texture of these fibers enhances the mechanical interlock with the matrix. These contradictory outcomes likely arise from differences in fiber dimensions, fiber-to-matrix ratios, and the homogeneity of fiber distribution. Furthermore, the effects of mixing procedures, superplasticizer dosage, and curing regimes can  
substantially influence the interaction between basalt fibers and the cementitious matrix. Such variables highlight the importance of contextual interpretation of the test results and suggest that the performance of basalt fiber-reinforced UHPCs is highly sensitive to mixing design and processing conditions.