## 3 Test Results and Analysis
### Bonding Properties between Ordinary Steel Bars and Different Concretes  
Figure 7 depicts the failure morphologies at the interfaces between steel bars and ordinary reinforced concrete (OC) and hybrid fiber-reinforced concrete (HPRRC) specimens. The ultimate failure mode of the conventional concrete specimens predominantly involves splitting failure (S-type), characterised by a loud splitting sound, swift load reduction, and the specimens being divided into two or three pieces. In contrast, the hybrid fiber-reinforced concrete specimens exhibited superior toughness and ductility, with their failure mode primarily manifesting as splitting-shear failure (S-S type). Upon concrete splitting failure, the load rapidly decreases but the specimens' primary maintain integrity and retain a certain degree of load-bearing capacity.  
The tensile behaviour of HPBRC significantly improves. Prior to loading, the stress between the cement-based matrix and the fiber was uniformly distributed (Figure 8a). Upon loading, the tensile load is transferred from the cement-based matrix to the fiber through the shear transfer mechanism, with the fiber bearing a portion of the load due to its high elastic modulus. The stress distribution along the fiber embedding length exhibited an uneven pattern, with higher shear stresses at the ends and lower in the middle (Figure 8b). The tensile stress was higher in the middle and lower at both ends. The stress distribution within the cement-based matrix is depicted in Figure 8c.  
The primary cause of the impairment of the bond between steel bars and OC is the radial component of oblique extrusion pressure \(q\), which induces cyclic tensile stress \(\sigma_{c}\) in the surrounding concrete. Once \(\sigma_{c}\) surpasses the concrete's ultimate tensile strength, the concrete fractures immediately, resulting in a rapid decline in bond stress (Figure 9a). The radial component \(q\) of transverse costal oblique compression pressure is jointly resisted by the annular tensile stress \(\sigma_{c}\) of the concrete and the shear stress \(\tau\) of the fiber and cement base (Figure 9b). As previously reported [11], adding fibers can effectively enhance the concrete's ultimate tensile strength, deflection deformation capacity and peak ductility.  
Figure 8: Tensile behavior and stress distribution of fibers in cement-based matrix. (**a**) Before loading; (**b**) after loading; (**c**) shear and tensile stresses of fibers.  
Figure 7: Failure modes of the specimens.  
The fiber shear stress shoulders a portion of the circumferential tensile stress of the concrete, leading to an increase in concrete strength. Consequently, the bond strength between steel bars and concrete is improved. The prominent impact of hybrid fibers on the damage to the bond between steel bars and concrete is the enhanced concrete ductility after cracking, with a better post-peak residual adhesion capacity than OC (Figure 10). Following the initiation of concrete cracking, the rapid progression of cracks is mitigated by the fiber bridging action, resulting in a relatively mild damage process. During the intercostal concrete crushing or cutting process, the hybrid fiber concrete can still provide better constraint through the bridge action. Thus, the residual bond of hybrid fiber concrete and steel bars exhibit superior performance.