## 4 Results and Discussion  
### Experimental Phenomena and Failure Characteristics  
According to the above-outlined loading method, 36 specimens were loaded until damage to the concrete and the CBGM, as well as the fracture of bolt or pull out of the CBGM, occurred. The failure modes of pull-out testing are shown in Table 5.  
Figure 8: Loading device. (**a**) Loading device diagram. (**b**) Heart-piercing hydraulic jack. Figure 7: Casting process used for the specimens. As can be seen from Table 5, in LB-A, the high-strength bolts with anchorage lengths of more than 300 mm were concentratedly broken at the strand-tapered anchorage, and at these locations, the high-strength bolt necking phenomenon also occurred, though there were no damage to the concrete and the CBGM. During the initial loading stage of the specimens, concrete, CBGM and high-strength bolt did not significantly change, though the high-strength bolt was stretched with extremely small changes caused by the heart-piercing hydraulic jack, and a significant necking phenomenon happened due to the increase in load. Finally, the high-strength bolt instantly fractured and created a broken acoustic emission when the load rose beyond a certain extent. When the high-strength bolt had an anchorage length of less than 300 mm in LB-A, it pulled out of the CBGM, and it did not experience fracturing or necking. During the loading process, the concrete and the CBGM did not experience significant changes or damage, though the high-strength bolts pulled out with a dull loud band when the load increased beyond a certain extent. The forms of destruction are shown in Figure 9.  
In LB-B, it was observed that the high-strength bolts with anchorage lengths of more than 300 mm were broken on the outside of CBGM. The location at which the high-strength bolt necked and fractured was concentrated at the strand-tapered anchorage, though the concrete and CBGM remained undamaged. When the high-strength bolts had anchorage lengths of less than 300 mm, the bolts slipped but did not fracture, and there were a large number of cracks on the concrete and CBGM surface, with some fragments falling off. The failure modes are shown in Figure 10. \begin{table}
\begin{tabular}{c c c c c} \hline
**Experimental** & **Grouping Number** & \begin{tabular}{c} **Diameter of Bol** \\ **(_d_/mm)** \\ \end{tabular} & \begin{tabular}{c} **Anchorage Length** \\ **(_l_/mm)** \\ \end{tabular} & \
\begin{tabular}{c} **Failure Mode** \\ **Failure Mode** \\ \end{tabular} \\ \hline LB-A & LB 1-1 & 16 & 240 & Bolt pulled out of CBGM \\ LB 1-2 & LB 1-2 & 16 & 300 & Bolt fracture failure \\ LB 1-3 & 16 & 400 & Bolt fracture failure \\ LB 1-4 & 16 & 500 & Bolt fracture failure \\ \hline LB-B & LB 2-1 & 16 & 240 &
\begin{tabular}{c} Bolt pulled out of CBGM and cracks \\ occurred in concrete \\ Bolt fracture failure \\ Bolt fracture failure \\ Bolt fracture failure \\ Bolt fracture failure \\ \end{tabular} \\ LB 2-3 & 16 & 400 & Bolt fracture failure \\ LB 2-4 & 16 & 500 & Bolt fracture failure \\ \hline \end{tabular}
\end{table} 
Table 5: Failure modes of the test specimens. Figure 9: Failure modes in LB-A. (**a**) Bolt fracture failure. (**b**) Bolt pull-out failure. The experimental phenomenon showed that when the high-strength bolts had anchorage lengths of more than 18 d, the bolt bond-slip failure and splitting or shear failures of the specimens could not be observed during the process of high-strength bolt pulling. The reason for this outcome may be that the CBGM was extruded when the load applied to the bolt reached the ultimate tensile load; thus, the CBGM tended to expand outward. However, the internal hoop and square steel tube had a hoop effect in the core area of the CBGM. Therefore, the bonding force between the high-strength bolt and the CBGM effectively prevented the bolt from slipping.