## 3 Results and Discussion
### Test Results and Bond Stress Calculation  
During the loading process, the bond stress at the bonding interface of rebar and concrete was not uniformly distributed. Considering the small length of the bonding  
Figure 4: Typical specimens with different damage forms. (**a**) Rebar pull-off damage. (**b**) Pull-out damage. (**c**) Splitting damage.  
section, the average bond stress was generally selected as the index in the statistical analysis of the influencing factors, and the bond strength was calculated according to Equation (1):  
\[\tau_{u}=\frac{F_{u}}{\pi dl} \tag{1}\]  
where \(\tau_{u}\) is the average bond stress, \(F_{u}\) is the maximum pull-out load value measured by the test force acquisition system, \(d\) is the rebar diameter, and \(l\) signifies the bonding length of the rebar.  
The damage forms, ultimate loads, and average bond strength statistics of the pull-out tests are shown in Table 7. The changes in specimen condition during the center pull-out tests were typically similar. In the pre-test section, the displacement of the rebar was approximately linear with the load, and the relative slip was small. Subsequently, the load rate increased slowly, and the slip increased until the maximum bond strength was reached. The experiment ended when a damage form occurred. Typical specimens with different damage forms are shown in Figure 4.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Group** & \begin{tabular}{c} **Bonding Length** \\ **(mm)** \\ \end{tabular} & \begin{tabular}{c} **Rebar Diameter** \\ **(mm)** \\ \end{tabular} & \
\begin{tabular}{c} **Ultimate Bond Stress** \\ **(MPa)** \\ \end{tabular} &
\begin{tabular}{c} **Damage Form** \\ \end{tabular} \\ \hline
60-12-CCRC-1 & 60 & 12 & 21.16 & Pull-out damage \\
60-12-CCRC-2 & 60 & 12 & 20.26 & Rebar pull-off damage \\
60-12-CCRC-3 & 60 & 12 & 19.59 & Rebar pull-off damage \\ \hline
60-12-RBFC-1 & 60 & 12 & 20.11 & Rebar pull-off damage \\
60-12-RBFC-2 & 60 & 12 & 22.73 & Pull-out damage \\
60-12-RBFC-3 & 60 & 12 & 22.16 & Pull-out damage \\ \hline
60-14-RBFC-1 & 60 & 14 & 20.87 & Pull-out damage \\
60-14-RBFC-2 & 60 & 14 & 21.27 & Pull-out damage \\
60-14-RBFC-3 & 60 & 14 & 20.50 & Pull-out damage \\
60-16-RBFC-1 & 60 & 16 & 20.25 & Pull-out damage \\
60-16-RBFC-2 & 60 & 16 & 18.77 & Splitting damage \\
60-16-RBFC-3 & 60 & 16 & 20.94 & Pull-out damage \\ \hline
60-16-BFC-1 & 60 & 16 & 20.53 & Pull-out damage \\
60-16-BFC-2 & 60 & 16 & 21.98 & Pull-out damage \\
60-16-BFC-3 & 60 & 16 & 18.18 & Splitting damage \\ \hline
80-12-CCRC-1 & 80 & 12 & 19.20 & Splitting damage \\
80-12-CCRC-2 & 80 & 12 & 17.38 & Rebar pull-off damage \\
80-12-CCRC-3 & 80 & 12 & 17.02 & Rebar pull-off damage \\ \hline
80-12-RBFC-1 & 80 & 12 & 19.49 & Pull-out damage \\
80-12-RBFC-2 & 80 & 12 & 16.53 & Splitting damage \\
80-12-RBFC-3 & 80 & 12 & 18.71 & Pull-out damage \\ \hline
80-14-RBFC-1 & 80 & 14 & 17.47 & Pull-out damage \\
80-14-RBFC-2 & 80 & 14 & 16.38 & Rebar pull-off damage \\
80-14-RBFC-3 & 80 & 14 & 18.15 & Pull-out damage \\ \hline
80-16-RBFC-1 & 80 & 16 & 16.54 & Pull-out damage \\
80-16-RBFC-2 & 80 & 16 & 15.15 & Splitting damage \\
80-16-RBFC-3 & 80 & 16 & 16.72 & Pull-out damage \\ \hline
80-16-BFC-1 & 80 & 16 & 16.34 & Pull-out damage \\
80-16-BFC-2 & 80 & 16 & 17.17 & Pull-out damage \\
80-16-BFC-3 & 80 & 16 & 15.57 & Pull-out damage \\ \hline
100-12-CCRC-1 & 100 & 12 & 16.35 & Pull-out damage \\
100-12-CCRC-2 & 100 & 12 & 15.31 & Splitting damage \\
100-12-CCRC-3 & 100 & 12 & 13.55 & Rebar pull-off damage \\ \hline \hline \end{tabular}
\end{table}
Table 7: Statistics of the data from the center pull-out test.