## 7 Conclusions  
Based on the results presented in this study, the following conclusions can be drawn:  
1. The high-strength bolt and the CBGM experienced bond-slip failure without square steel tube constraining when the specimens were composed of pre-cast concrete and high-strength bolt connected to the CBGM in a reserve hole or square steel tube with an anchorage length of less than 15 d. The specimens experienced splitting failure when the square steel tube was used as a constraint. The failure loads of specimens in LB-A were slightly greater than that of LB-B, which implies that the square steel tube had a constraining effect on the CBGM and improved the bonding property between high-strength bolt and the CBGM. When the anchorage lengths were more than 18 d, the high-strength bolts in both LB-A and LB-B fractured at the same load level, which reached the ultimate load of the high-strength bolts. Before the bolt fractured, the tensile force increased with the increase in anchorage lengths, whereas the bond strength decreased.
2. The bond strength-anchorage length relationships between high-strength bolts and the CBGM were analyzed, as were the bond strength-anchorage length relationships for bolts with different anchorage lengths and the special connection mode. A formula for determining bond strength-anchorage length relationships was proposed.
3. The bond-slip curves between the fractured bolt and the CBGM were investigated, and the curves were divided into ascending and intensifying stages. The bond-slip constitutive relationship was also determined by fitting the experimental curves.  
Investigation, P.L.; writing--original draft preparation, W.Z.; conceptualization, J.M.; writing--review and editing, L.L.; formal analysis, Y.C.; methodology, Z.Y. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (grant numbers 52178182, 52108262 and U1934217) and the Science and Technology Research and Development Program Project of China railway group limited (Major Special Project, No.: 2020-Special-02, 2021-Special-08, 2022-Special-09; Major Project, No.: 2021-Special-02; Key Project, No.: 2021-Key-11, No.: 2022-Key-46). The authors have also received research grants from the National Science Foundation for Distinguished Young Scholars of Hunan Province (2022)J10075), the Natural Science Foundation of Hunan Province of China (2020)J5982) and the Hunan Innovative Province Construction Special Project (2020 CK4058).  
Not applicable.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy restrictions.  
The authors thank the National Natural Science Foundation of China, the Science and Technology Research and Development Program Project of China Railway Group Limited, the National Science Foundation for Distinguished Young Scholars of Hunan Province, the Natural Science Foundation of Hunan Province of China and the Hunan Innovative Province Construction Special Project for funding this research, as well as anonymous reviewers for their contribution to this paper. [PERSON], [PERSON] and [PERSON] also made great contributions.  
The authors declare no conflict of interest.