## 4 Results and Discussion
### Results of Bolts Pull-Out Testing  
(1) Load-displacement relationship  
In the pull-out testing stage, the load and bolt displacement were measured during the loading process. The load value was imposed by the heart-piercing hydraulic jack, and the bolt displacement was calculated by subtracting the deformation value of the bolt measured on the rebar strain gauge from the value of the displacement meter. The average value of three specimens was taken for each group to obtain the load-displacement curves in the pull-out testing stage, as shown in Figure 11.  
Figure 11: Schematic diagram of load–displacement curves of each group. (**a**) LB-A. (**b**) LB-B. Figure 10: Failure modes in LB-B. (**a**) Bolt fracture failure. (**b**) Specim splitting failure. The following outcomes can be seen in the above load-displacement curves:  
When the anchorage length of the bolt was 15 d, the maximum load applied to the bolts of LB-A and LB-B reached 163.5 kN and 128.34 kN, respectively. Both LB-A and LB-B failed to reach the ultimate load of the high-strength bolts, while LB-A experienced bond-slip failure and LB-B exhibited splitting failure. However, the ultimate bond load between the bolt and the CBGM increased in LB-A due to the constraint of the square steel tube. When the anchorage lengths of the bolts were more than 18 d, both experimental designs showed a significant yield stage on the load-displacement curve. The slip changed less noticeably when the load was increased before the bolts yielded, while the slip increased significantly until the load applied to the bolts reached the limit load, before fracturing when the load exceeded the yield load. Based on the analysis of the load-displacement curves, it can be concluded that the load-displacement curves for bolt fracture failure are similar, regardless of the bolt anchorage lengths. The load is transmitted quickly to the bolt's bottom during the loading process, which prevents the bolt from slipping if it has a small anchorage length. On the other hand, when the anchorage length is large, the bonding property between the bolt and the CBGM plays a more significant role in preventing the bolt from slipping. The ultimate load of the broken bolts is basically the same in LB-A and LB-B, as can be seen in Equation (2), where the calculation expression of the average bond stress is written as follows:  
\[\overline{\tau}_{\text{max}}=\frac{\overline{F}_{\text{max}}}{\pi d\underline {l}_{\text{a}}} \tag{7}\]  
where \(\overline{\tau}_{\text{max}}\) is the average bond stress of the broken bolts (MPa), while \(\overline{F}_{\text{max}}\) is the average load of the broken bolts (kN). It can be seen from Equation (7) that the average bond stress decreases as the anchorage length increases, which is consistent with Ref. [27]. (2) Comparative analysis of the maximum load values  
The maximum load comparison chart of the different anchorage lengths is drawn to compare and analyze the differences between test experimental designs, and the results are shown in Figure 12.  
The following results can be seen in the above diagram, which compares the maximum loads:  
When the anchorage lengths were more than 300 mm in LB-A and LB-B, there was no bond-slip failure between the bolts and the CBGM, which showed that the connection behavior and anchorage performance between the bolts and the CBGM were reliable, regardless of whether the square steel tube acted as a constraint. The load applied to the bolts could reach their yield load, which then fractured upon reaching the ultimate load. The ultimate load of each specimen is basically the same. The bond-slip failure of high  
Figure 12: Schematic diagram of comparison of each specimen maximum load. strength bolts and the cone failure of the concrete occurred in LB-B when the anchorage length was 240 mm, though it only had the bond-slip failure of high-strength bolts in LB-A. Although the loads applied to the bolts failed to reach their yield and ultimate loads, the load at the time of failure was slightly greater than that of LB-B.