## 3 Results and Discussion
### Single-Factor Analysis of Bond Strength
#### 3.3.3 Effect of Concrete Type on Bond Strength  
As shown in Figure 8, the slurry type had less influence on the concrete bond stress. For CCRC, the ultimate bond strength was reduced by approximately 4% compared to OC under the same conditions. It can be seen that the dosing of rubber granules harmed the bond strength, but a small dosage had little effect on the strength reduction. For BFC, the ultimate bond strength was improved by approximately 3% compared to OC under the same conditions. This shows that the incorporation of basalt fibers had a positive effect on the bond strength. For RBFC, the ultimate bond strength was improved by approximately 2% compared to OC under the same conditions. This shows that the optimal test method for mechanical properties had a positive effect on its bond strength. However, due to the negative effect of the rubber granules, the overall bonding properties were not significantly improved.  
The microstructure of the interface between rubber granules, basalt fibers, and concrete can be studied to explain the experimental phenomena of variations in their effect on the bond strength of concrete. Figure 9 shows a detailed view of an RBFC specimen with pull-out damage. It can be seen that the concrete in front of the rib is crushed and adheres  
Figure 8: Bond stress–slip curves of each group: (**a**) 60-12-CCRC and OC, (**b**) 80-14-RBFC and OC, and (**c**) 100-16-BFC and OC.  
Figure 7: Relationship between ultimate bond strength and rebar diameter.  
to the rib in front of the rebar. Typical areas of the concrete where the pull-out damage occurred were sampled, and the specimens were microscopically analyzed using a tungsten filament scanning electron microscope (SEM), as shown in Figure 10.  
The microscopic characteristics of the interface between rubber granules, basalt fibers, and concrete are shown in Figure 11. From Figure 11a, it can be seen that the interface cracks between the rubber granules and the cement stones are more pronounced, and there are obvious flaws. This is because a rubber granule is a water-repellent waterproof material that has different properties than a cement stone. When the hydration reaction of cement occurs to produce cement stone, the rubber granules do not form a strong bond with the cement stone, so the bond strength is reduced. It is also due to its hydrophobicity, which significantly reduces the surface tension of water [38]. Its hydrophobicity allows the concrete to introduce uniform, stable, and fine air bubbles around the rubber granules during the mixing process. These closed-cell air bubbles reduce the bond strength, and the number of capillaries allows them to improve the frost resistance, toughness, and impact resistance of the concrete. From Figure 11b, it can be seen that the fibers are uniformly distributed within the cement paste and had a good hydration reaction with the cement stone, resulting in the fibers being firmly embedded within the cement stone. When the  
Figure 10: Tungsten filament scanning electron microscope (SEM).  
Figure 9: Detailed view of an RBFC specimen with pull-out damage.  
concrete is subjected to external forces that cause microcracking, the presence of fibers can effectively inhibit the development of microcracking. In addition, the fibers can alleviate the tendency to develop into larger cracks under pressure and act as anchors during the development of microcracks. To some extent, this increases the compressive strength and enhances the bond strength.