# Comparative Test on the Bond Damage of Steel and GFRP Bars Reinforcing Soft Rock Slopes
## 3 Results
### Bond Strength of the Bar Interface under the Ultimate Load  
Table 8 shows the corresponding characteristic and phenomena of the specimens when they reached the ultimate state under tensile load. Figure 14 shows the failure mode of the bar during the test. Figure 15 shows the load displacement curve of the steel and GFRP bars under the soft rock slope excluding the bar body deformation of the free section. Figure 13: Curve of the bar anchorage depth. (**a**) Steel Bar. (**b**) GFRP Bar. As shown in Table 8, the ultimate load of the steel bar was 230 kN. Compared with the strength of the steel bar itself, the ultimate stress of the steel bar body is small, therefore  
Figure 14: Failure mode of the bar body. (**a**) Failure mode of the GFRP bar (**b**) Failure mode of the steel bar. Figure 15: Load displacement curve of the bar excluding the bar body deformation of free section. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Specimen** & **Anchor Hole** & **Ultimate** & **Ultimate** & **Top Displacement** & **Bond** & **Ultimate State Test** \\
**Type** & **Diameter** & **Tensileload** & **Stress** & **of the Bar** & **Strength** & **Phenomen** \\  & **/mm** & **/kN** & **/MPa** & **/mm** & **/MPa** & \\ \hline Steel Bar & 110 & 230 & 450 & 42 & 0.59 & Under pressure, the load of the bar did not increase. The top displacement of the bar continued to increase. The bar and the grouting body showed traces of slip. the failure mode of the steel bar was pulling-out failure. The ultimate load of the GFRP bar was 270 kN. Relative to the strength of the GFRP bar itself, the ultimate stress of the GFRP bar body is large, so the failure mode of the bar was fracture failure. When the anchoring system was damaged, the top part of the steel bar body moved 42 mm, and the GFRP bar body moved 39 mm. At the same time, it is observed that compared with the GFRP bar, the grouting body of steel bar is more seriously damaged. It shows that under the condition of soft surrounding rock, the anchoring system of GFRP is more intact, which will make it harder for water to enter the rock mass through cracks, resulting in the softening and disintegration of soft rock when encountering water. To further prove the above conclusion, the bar body deformation of free section is subtracted from the displacement at the end of the bar body, so as to obtain the curve of load displacement of anchored part in Figure 15.  
Figure 15 shows that when the tensile load reached 200 kN, the gradient of the load displacement curve of the steel bar decreases rapidly and is close to zero, indicating that the deformation increased sharply, and the interface was about to reach the limit state. When the load reached 230 kN (not exceeding the strength of the bar itself), the strain measured at the deepest part of the bar was 10 \(\mu\epsilon\). It shows that the interface between the bar and the grouting body was very likely to produce relative slip. Combined with the analysis in Section 3.3, the damage between the steel bar and the grouting body is extensive when the force is small. The sudden change of the gradient of the load displacement curve also occurs when the tensile load of the GFRP bar was 100 kN. This is because the length of the grouting body mobilized by GFRP bar in the early stage was small. The sheer force was concentrated and reached the limit state more quickly. Part of the bond force was transformed into friction, and the deformation of the bar body in the anchorage section continued to extend downward, resulting in large deformation. However, due to the good deformation coordination between the GFRP bar and the grouting body, the grouting body interface is not completely destroyed, while the bar body itself had reached the ultimate strength. Figure 16 shows schematic diagrams of two kinds of bar failure.