## 3 Results and Discussion
### Freeze-Thaw Resistance of HSC and Its Relationship with Expansive Agents and Fibers  
Figure 2 illustrates the influence of the single incorporation of expansive agent, fibers, and their combination on the freeze-thaw resistance of C80-grade HSC. The results indicate that C80-grade HSC possesses a small and non-connected pore structure, leading to extremely low porosity and permeability, as well as minimal freezing water content, thereby demonstrating high freeze-thaw resistance [20]. In the case of the reference HSC-1, a mass increase in the specimens was observed before 500 freeze-thaw cycles due to the continued hydration of cement inside the concrete, with a maximum increase of 0.8%. Moreover, the relative dynamic elastic modulus showed a certain degree of improvement due to hydration effects before 100 cycles. Beyond 500 cycles, surface spalling occurred in HSC-1 specimens, resulting in mass loss. However, its freeze-thaw life could still surpass 1200 cycles, aligning with the findings of [PERSON] et al. [21]. Figure 3 shows the variation in specimen mass and surface damage after 1250 freeze-thaw cycles for HSC with different mix ratios. The concrete specimens in Figure 3a-f exhibit surface sanding and spalling of the hardened cement paste. Additionally, HSC-2 in Figure 3b displays dropped corners, while HSC-6 in Figure 3f exhibits spalling of the mortar, exposure of the coarse aggregate and fibers, and end-breakage. Analysis of the graph reveals that for HSC-2, with a single incorporation of AEA expansive agent, despite having a smaller spalling amount on the surface of its specimens during the freeze-thaw process compared to the reference concrete HSC-1, its relative dynamic elastic modulus significantly drops below that of HSC-1 after 300 cycles. Through  
Figure 3: Mass variation and surface damage following freeze-thaw tests. Figure 2: The effect of expansive admixture and fibers on the freeze-thaw durability of HSC. calculation, its freeze-thaw life is determined to be 1120 cycles, which is 8% lower than that of the reference concrete. It is evident that the AEA expansive agent has a negative impact on the freeze-thaw resistance of HSC, attributed to the transformation from AFm to AFt within the concrete during the freeze-thaw process, leading to structural microcrack [22]. XRD analysis [19] indicates that HSC-2 with added expansive agent forms more sulfate-aluminate hydrate, and the characteristic peak intensities of AFm (0.846 nm) and AFt (0.971 nm) are 18% and 15% higher than those of the corresponding hydration products in HSC-1. For HSC-3, with a single incorporation of steel fibers, the structure's cracking resistance is limited during freeze-thaw due to the crack-arresting effect of the steel fibers. Consequently, its freeze-thaw resistance is significantly improved, and the freeze-thaw life can reach 1285 cycles, a 5% increase compared to the reference concrete. Regarding HSC-4, with a single incorporation of HEMPF fibers, despite the elastic modulus of these fibers being only 37% that of steel fibers, the crack-arresting effect in concrete depends largely on the average spacing between fibers and the number of fibers in a unit volume of concrete according to the fiber spacing theory [23]. Table 3 illustrates the fiber spacing and quantity in the two types of concrete, showing that the fiber spacing in HSC-4 is reduced by 76% compared to HSC-3, and the fiber quantity is increased by 2800 times. During freeze-thaw, the crack-arresting effect of HEMPF fibers becomes more pronounced, not only restricting the generation of freeze-thaw microcracks in the concrete structure but also suppressing the surface spalling of the specimens. This significantly enhances its freeze-thaw resistance, with a freeze-thaw life of 1295 cycles, a 6% increase compared to the reference concrete. Therefore, it is evident that high elasticity modulus fiber materials have a positive effect on the freeze-thaw resistance of HSC.