## 4 Discussion  
### Bond Stress between Mortar and Reinforcing Rod  
For IRBM and IRBM--F, where the bonding action between the filling mortar and rods was working, the bond stress (\(\tau_{s}\)) was calculated by Equation (1) [53], and its maximum value (\(\tau_{s,max}\)) was investigated. It was noted that \(\tau_{s}\) was calculated until the loading step, where the strain data could be reliable.  
\[\tau_{s}=\left(A_{r}\cdot\sigma-A_{r}\cdot\sigma^{\prime}\right)/\left(\pi \cdot d_{r}\cdot l\right)=\left(\sigma-\sigma^{\prime}\right)\cdot d_{r}/ \left(4\cdot l\right)\ (\text{MPa}) \tag{1}\]  
where \(A_{r}\) is the cross-sectional area of the SI rod, \(\sigma\) and \(\sigma^{\prime}\) are the stress of adjacent SI rods, \(d_{r}\) is the rod diameter, and \(l\) is the distance between adjacent SI rods.  
In the IRBM, the \(\tau_{s,max}\) between the rod and mortar was calculated to be in the range of 0.18-0.85 MPa, which were quite small values. From this result and the strain distribution of the rod, the deterioration of the bonding performance was initiated from the early stage of the loading, and the result was found similar to that of the previous study [49]. In order to understand the relationship between \(\tau_{s,max}\) and the bond splitting strength (\(Kf_{b}\)), \(Kf_{b}\) was calculated by Equation (2), according to the AIJ's guidelines [54]. It should be noted that the value calculated by the AIJ standard represented the bond strength between the concrete and longitudinal reinforcing bars and was actually different from that of this study. However, as a reference value, the evaluation of the bond strength between mortar and SI rods was attempted using the RC standard. As a result, \(Kf_{b}\) was calculated to be 2.43 MPa, far overestimating the experimental results. Nevertheless, considering that the maximum strength of the UBM specimen was 13.96 kN, the presence or absence of the SI rods greatly affected the maximum strength. In addition, even after the strength degradation, the IRBM maintained an almost constant residual strength up to an extremely large deformation. However, the UBM exhibited an abrupt brittle failure immediately after its maximum strength.  
\[Kf_{b}=\left(0.3\left(\frac{C+W}{d_{r}}\right)+0.4\right)(F_{c}/40+0.9),\text{ where }0.3\left(\frac{C+W}{d_{r}}\right)+0.4\leq 2.5\text{MPa} \tag{2}\]  
where \(C\) is the minimum value between \(3d_{c}\) (\(d_{c}\): smallest cover concrete depth) and longitudinal reinforcement spacing, \(W\) is the index exhibiting the effect of shear reinforcement, and \(F_{c}\) is the concrete compressive strength. In Equation (2), the value of \(C\) should be less than \(5d_{r}\) and \(W\) becomes zero because there is no shear reinforcement.  
In IRBM--F, \(\tau_{s,max}\) was estimated as 1.39-2.91 MPa, and the bond degradation was observed after those maximum values. Essentially, the calculation result of \(Kf_{b}\) was the same as IRBM. As a result, the \(Kf_{b}\) was 2.43 MPa, and the \(\tau_{s,max}\) in this specimen generally corresponded to \(Kf_{b}\). It was noted that \(Kf_{b}\) was a reference value, but it was verified by providing the brick members with better integrities and fixing conditions, and the bonding performances were also improved.