## 4 Results and Discussions
### Comparison Experimental Test with the Analytical Model and Numerical Parametric Study  
#### 4.4.1 Comparison Experimental Test with the Analytical Model  
In this section, the bond stress-slip curves obtained from experimental results are presented and compared with the numerical results in Figure 10. In addition, the experimental results and numerical results for maximum bond stress \(\tau_{\text{max}}\) and corresponding slip \(u_{1}\) are shown in Table 6. It is apparent that bond stress-slip curves achieved from the numerical results are well correlating with experimental results, especially for the hardening stage of the bond stress-slip curves. The results in Table 6 show that the FEM models can provide exact results for the maximum bond stress. While the numerical results of the corresponding slips are slightly larger than the experimental values. Therefore, the proposed numerical  
Figure 9: Comparison of experiment data and previous study.  
approach can be effectively used to model steel reinforcement bond-stress slip relations with the fly ash-based geopolymer concrete.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \(D_{b}\) & \(f_{c}\) & \(s_{1}\)\({}_{Exp}\) & \(s_{1}\)\({}_{TEM}\) & \(\tau_{\max}\)\({}_{Exp}\) & \(\tau_{\max}\)\({}_{FEM}\) & (\(s_{1}\)\({}_{Exp}\))/(\(s_{1}\)\({}_{TEM}\)) & (\(\tau_{\max}\)\({}_{Exp}\))/(\(\tau_{\max}\)\({}_{FEM}\)) \\ \hline
**(mm)** & **(MPa)** & **(mm)** & **(mm)** & **(MPa)** & **(MPa)** & \\ \hline
12 & 20.33 & 1.28 & 1.93 & 6.8 & 6.82 & 0.67 & 1 \\
16 & 20.33 & 1.17 & 2.28 & 8.65 & 8.68 & 0.51 & 1 \\
20 & 20.33 & 1.13 &  
As observed in Figure 10 and Table 6, the experimental results show that the bond-stress increases with the increase of concrete compressive strength. In addition, the bond stress for d12 reinforcing bars is smaller than those of d16 and d20 bars, which are close to each other. The experimental results of \(u_{1}\) range between 1.0 and 1.3 mm. This is in good agreement with the suggested value in fib model code 2010 [38]. The mean value of \(\tau_{\text{max}Exp}/\tau_{\text{max}FEM}\) is 1.00. The coefficient of variation (\(CoV\)) is only 0.01. This means that an excellent agreement between the proposed FEM model and experimental results is archived. For the simulation results of bond-slip at maximum bond stress, the mean values of \(s_{1 Exp}/s_{1 FEM}\) are 0.71, and the corresponding coefficient of variation is 0.2. These results indicate that the numerical results of the corresponding slips are slightly larger than the experimental values. This slightly small error could be attributed to the simplified modelling technique employed in this study. However, it can be seen that the results for the slips at maximum bond stress are relatively small, and the numerical results could be acceptable. In Figure 10, the full range of bond stress-slip curve proposed in [34] for common concrete is also depicted. The details of the bond stress-slip curve of the fib model are given as follows  
\[\begin{split}\tau_{b}&=\tau_{\text{max}}(s/s_{1})^{0. 4}\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad \quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quadAs shown in Figure 11, parametric investigations indicate that the bond strength increase considerably with the rise of concrete compressive strength, while a reverse trend is observed in case of the values of bond slips at maximum bond stress. For the influence of concrete cover, the results shown in Figure 12 points out that the thickness cover slightly influence the bond strength. These results are consistent with those observed in the experiments.