## 4 Results and Discussions
### Comparison Experimental Test with the Previous Study  
In the previous work, [PERSON] et al. [47] proposed the formula to predict the bond strength:  
\[\tau_{u}=0.083045\sqrt{f_{c}^{\prime}}\bigg{[}1.2+3\bigg{(}\frac{c}{d_{b}} \bigg{)}+50\bigg{(}\frac{d_{b}}{l_{d}}\bigg{)}\bigg{]} \tag{15}\]  
where \(c\) is the minimum concrete cover (mm), \(f_{c}^{\prime}\) is the cylinder compressive strength of concrete (MPa), \(d_{b}\) is the bar diameter (mm), and \(l_{d}\) is the development length (mm)  
[PERSON] and [PERSON] [48] proposed an equation for predicting bond stress of geopolymer concrete:  
\[\tau_{u}=\sqrt{f_{c}^{\prime}}\bigg{[}2.07+0.2\bigg{(}\frac{c_{\text{min}}}{d_{ b}}\bigg{)}+4.15\bigg{(}\frac{d_{b}}{l_{d}}\bigg{)}\bigg{]} \tag{16}\]  
where \(c_{\text{min}}\) is smaller of the minimum concrete cover of \(1/2\) of the clear spacing between bars (mm)  
The experimental and predicted data of bond strength using the two above equations are presented in Table 5  
From Table 5 and Figure 9, it is evident that the calculated values from the previously proposed formulation show significant differences. In the case of expression developed by [47], the tendency revealed that the bond strength of specimens decreased when the bar diameter is higher. The remarkable dissimilarity in bond behavior might be generated by the different types of samples and rebars. This study employed the cylinder geopolymer concrete specimens while [47] operated with the beam specimens. On the other side, the equation's results by [45] showed a similar trend with [47]. That is, the bond strength of geopolymer concrete reduces when the bar diameter increases. Moreover, the experimental results have an unlike performance with [48]. The current bond strength of the group of GP2 and GP3 increases with the increased diameter of reinforcement from 12 mm to 16 mm, and the bond strength decrease with the increased diameter of rebars from 16 mm to 20 mm. While the calculated values from [48] decrease in accordance with the increase of rebars from 12 to 20 mm. In the case of GP1, the bond strength shows a similar trend with [48]. In the first stage, they decrease in accordance with the increase of the diameter of rebars from 12 mm to 20 mm.  
However, the bond strength slightly increases when that which was calculated from [48] keeps decreasing. The reason may come from the components of geopolymer concrete and curing conditions. This study used only fly ash on the geopolymer binder, while [48] used the combination of fly ash and slag as a geopolymer binder. Besides, the curing conditions used for this study was 60 \({}^{\circ}\)C for 4, 8 and 12 h, while the previous research was 70 \({}^{\circ}\)C and 24 h. With the different components of geopolymer concrete and curing conditions, the hardening of the geopolymer occurs in different ways. As a result, the microstructure of specimens shows different behavior. Even the specimens have similar compressive strength values. There is a need for studies to focus more on the effect of components of geopolymer binder and curing condition on the bond strength of reinforced geopolymer concrete in the future.