## 3 Experimental Process
### Preparation and Curing of Specimens  
(1) Design of specimens  
The experiment was carried out according to the relevant provisions of GB/T 50152-2012 [20]. The grade 10.9 threaded bolts with nominal diameters of 16 mm were used to perform the mechanical property test, and the choice of the bolt was considered based on the practical application of the engineering and structural requirements of the new connection method. The cubic concrete foundation platform with a side length of 800 mm was made of C40 concrete, and the steel bars in the concrete were configured according to the construction requirements. The CBGM was wrapped by the square steel tube with a side length of 60 mm in LB-A, and the CBGM in LB-B was wrapped by the reserving grouting hole with a side length of 60 mm in the concrete. There were four different anchorage lengths--240 mm, 300 mm, 400 mm, and 500 mm--in these two experimental designs. The depth of square steel tube and reserved grouting holes were the same as the anchorage lengths of high-strength bolts. Among them, each structural form had three identical specimens. The detailed geometric parameters of the specimens are shown in Table 4, and the specific structural designs are shown in Figure 6.  
Preparation of specimens  
The grouting hole and the square steel tube were reserved in their designated positions according to the requirements of the experimental design, and the steel bar was configured according to the constructional requirements when the concrete foundation platform was cast. After 28 d, the bolt with the rebar strain gauge was inserted into the reserved hole or the square steel tube at the designed anchorage lengths. Next, the CBGM was cast into the reserved grouting hole or the square steel tube. Lastly, the specimens were cured again for 28 d. The casting process used for the specimens is shown in Figure 7.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Experimental** & **Grouping** & **Diameter of Bolt** & **Anchorage** & **Inchorage** & **Anchorage Mode of Bolts** \\
**Design** & **Number** & **(_d_/mm)** & **Length (_l_/mm)** & **Inchorage** & **Anchorage Mode of Bolts** \\ \hline  & LB 1-1 & 16 & 240 & 15 & Bolts are directly embedded into \\ LB-A & LB 1-2 & 16 & 300 & 18.75 & square steel tubes with a reserved \\ LB 1-3 & LB 1-3 & 16 & 400 & 25 & section of 60 mm \(\times\) 60 mm \\  & LB 1-4 & 16 & 500 & 31.25 & Bolts are directly embedded into \\ LB-B & LB 2-1 & 16 & 240 & 15 & Bolts are directly embedded into \\ LB-B & LB 2-2 & 16 & 300 & 18.75 & holes with a reserved section of \\ LB 2-3 & LB 2-3 & 16 & 400 & 25 & 60 mm \(\times\) 60 mm \\  & LB 2-4 & 16 & 500 & 31.25 & 60 mm \(\times\) 60 mm \\ \hline \end{tabular}
\end{table}
Table 4: Specim types and geometry parameters.  
Figure 6: Schematic diagram of the specimens (unit: mm). (**a**) Profile of LB-A. (**b**) Profile of LB-B. (**c**) Top view of LB-A. (**d**) Top view of LB-B.  
Experimental loading device  
The heart-piercing hydraulic jack that we chose had a maximum load capacity of 1000 kN, a hole diameter of 55 mm and a maximum extension of 100 mm. According to the GB/T 50152-2012 [20], the loading rate of the pull-out testing experiment was controlled using the following formula:  
\[V_{\mathrm{F}}=0.03d^{2} \tag{6}\]  
where \(V_{\mathrm{F}}\) is the loading rate (kN/min).  
The loading rate used to test the specimens was 0.5 kN/s, and the loading method adopted hierarchical loading, with the level increasing by 5 kN per stage. The corresponding data of the displacement meter and the rebar strain gauge of the high-strength bolt were recorded after loading for 10 min, and the next level loading was then proceeded. The loading device diagram is shown in Figure 8.