## 4 Experimental Results and Discussion  
### Mechanical Properties  
The measurement results of the compressive strength and splitting tensile strength of concrete incorporated with mixtures of different fiber contents are shown in Figure 3a,b, respectively. Figure 3a shows that compressive strength can be improved by the incorporation of an appropriate amount of hybrid fibers. The compressive strength of BF5 PF5 is the highest, 44.43 MPa, which is 5.06% higher than that of the reference concrete (BF0 PF0). However, with an increase in the fiber content, compressive strength becomes lower than that of the reference concrete; BF5 PF15 shows the lowest compressive strength, which is 22.63% lower than that of the reference concrete. These findings are in agreement with the results reported by other researchers [40; 41]. [PERSON] et al. demonstrated that the compressive strength and splitting tensile strength of concrete are improved when the content of hybrid fibers was lower than 0.1%, but degraded when the content of hybrid fibers was higher than 0.1% [40]. The incorporation of BF and PF into concrete can reduce microcracks and prevent the expansion of macro cracks, which consequently improves the internal structure of the concrete. The transverse deformation of concrete under compression is restrained, and the fibers can bond strongly to the matrix. When stress is transferred from the matrix to the fibers, the fibers consume energy because of the deformation, which consequently delays the destruction process of the concrete; as a result, the compressive strength of the concrete improves. However, when the fiber content is too high, the fibers are not uniformly dispersed, and they overlap and agglomerate in the matrix, as shown in Figure 4a; this consequently worsens the bonding effect between the fibers and the matrix [42]. As a result, zones with poor cohesion and weak structures are formed that ultimately nullify the reinforcing effect of the fibers on compressive strength, and this leads to a reduction in compressive strength. Therefore, accurate control of the fiber content of concrete is necessary. As observed in Figure 3b, the improvement in splitting tensile strength shows a trend similar to that of compressive strength. The test results show that hybrid fibers can effectively improve the splitting tensile strength. The splitting tensile strength of BF5 PF5 is the highest, 4.01 MPa, which is 9.56% higher than that of the reference concrete. When the BF content is fixed at 0.05%, splitting tensile strength decreases rapidly with an increasing PF content. BF5 PF15 has the lowest splitting tensile strength, which is 23.22% lower than that of the reference concrete. When the content ratio of BF to PF is 1:1, and the total fiber content is higher than 0.2%, the splitting tensile strength is lower than that of the reference concrete. This result is in agreement with previously reported results [4; 43]. BF and PF are evenly dispersed or intertwined in the concrete matrix to form a three-dimensional support network, as shown in Figure 4b. When concrete is subjected to loading, the fibers intersect the cracks  
Figure 4: Scanning electron microscopy (SEM) images of fibers. (**a**) Distribution of BF in basalt–polypropylene fiber-reinforced concrete (BPFRC). (**b**) Distribution of BF and PF in BPFRC. Figure 3: The compressive strength and splitting tensile strength of different mixtures. (**a**) Compressive strength. (**b**) Splitting tensile strength. transfer the load to the upper and lower surfaces of the cracks, and therefore, the cracks can continue to bear the load. In addition, the fibers can also share part of the tensile force on the section, reduce the stress concentration factor at the micro-cracks in concrete, increase the ultimate tensile strain of concrete, and prevent the formation and propagation of cracks, all of which lead to an improvement in splitting tensile strength.