## 3 Test Results and Analysis
### Bonding Stress-Slip Curve  
Assuming that the bond stress is distributed evenly along the steel bars' embedded lengths, the average bond stress is calculated as described below:  
\[\tau=\frac{F}{\pi dl_{a}} \tag{1}\]  
where \(\tau\) is the bond stress between steel bars and concrete, \(F\) is the mean of the drawing force, \(d\) is the steel bar diameter and \(l_{a}\) is the embedded length of steel bars.  
The average bond stress is defined as the ultimate bond strength, \(\tau_{u}\), when the bond specimen is damaged. The corresponding free-end slip amount represents the peak slip amount. As illustrated in Figure 6, the average bond stress-slip curve demonstrates a consistent trend.  
Figure 6a shows that an increase in concrete strength results in a corresponding rise in mechanical biting force, leading to a steeper slope in the curve and a higher peak value of bond stress. Bond stress begins to decline as the bonding interaction weakens. Concrete strength also directly impacts the relative slip, with higher strengths corresponding to smaller relative slips and steeper curves. When the relative slip value of the steel bars and concrete exceeds one rib distance and the mechanical bite force is completely eradicated, the friction between the steel bars and concrete generates bonding stress. The strength of the concrete serves as a constraint on the steel bars, influencing the friction generated and consequently, the residual bonding stress.  
The curves of the different steel bar diameters are presented in Figure 6b. A higher bond stress of reinforced concrete is observed with increasing steel bar diameter; a larger diameter leads to a smaller surface area of ribs, thereby reducing the bond anchorage effect. Concurrently, the relative slip amount decreases with an increase in steel bar diameter. This decrease is primarily attributed to the reduced tensile deformation of the steel bars in the non-anchored section, resulting in a smaller slip amount at the loading end, and ultimately leading to a lower slip of the specimen.  
The curves for the different steel bar diameters are illustrated in Figure 6c. The thickness of the reinforcement protective layer dictates the restraint effect of concrete on reinforcement. A thicker protective layer results in a less pronounced crack development. When the thickness of the protective layer is minimal, the restraint on the expansion of cracks is not effectively inhibitory, allowing the cracks in the rib to propagate to the concrete surface prematurely, leading to a splitting-shear failure. Conversely, a thicker protective layer exhibits a more significant inhibitory effect on crack development, and accordingly, experiences a higher bonding stress.  
The variations in the curves of the different anchoring lengths are presented in Figure 6d. The figure shows that the bond stress reduces as the bond anchorage length increases. Generally, a longer anchorage length corresponds to a deterioration in bond performance, primarily due to the non-uniform distribution of bond stress along the anchorage length axis. The stress value at the loading end increases, while the stress value at the free end decreases. Furthermore, a longer anchorage length results in a lower average bond stress.