## 3 Results and Discussion
### Bonding Test of Fe-SMA  
The slippage failures occurred in the all the specimens at the ultimate bonding strength, which occurred because of the less shear strength at the interface between the Fe-SMA and the cement-based mortar than the yield strength of the Fe-SMA. Furthermore, the feasibility of the Fe-SMA inside the concrete block was evaluated based on the below Equation (1). The feasibility index (_F.I._) is expressed in Equation (1).  
\[F.I.\ =\ \frac{\sigma_{\text{max}}}{f_{\text{ult.}}}\times 100 \tag{1}\]  
where, _F.I._ is a feasibility index, \(\sigma_{\text{max}}\) is the maximum tensile stress of the Fe-SMA from the bonding test, and \(f_{\text{ult}}\) is the ultimate tensile stress of the Fe-SMA from the material test. The bonding strength, slippage, and _F.I._ are given in Table 5.  
Figure 12 shows the bonding strength and slip deformation responses according to the bonding lengths of the Fe-SMAs inside the concrete blocks. The bonding strength and _F.I._ were linearly proportional to the bonding length. The bonding strength and _F.I._ are improved by an average of 8.8%  
Figure 11: Recovery stress after completed cooling down: (**a**) Activation temperature of SME; (**b**) Pre-strain value.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Specimen** & **Bond Strength (kN)** & **Slippage (mm)** & **F.I. (\%)** \\ \hline L250-PS0-T020 & 14.87 & 1.46 & 57.39 \\ L300-PS0-T020 & 18.14 & 2.03 & 62.29 \\ L350-PS0-T020 & 18.32 & 2.64 & 70.71 \\ L400-PS0-T020 & 19.82 & 3.87 & 76.48 \\ L450-PS0-T020 & 20.77 & 5.04 & 80.17 \\ \hline L300-PS4-T080 & 19.95 & 2.57 & 76.99 \\ L300-PS4-T120 & 21.06 & 3.45 & 81.82 \\ L300-PS4-T160 & 22.27 & 5.03 & 85.95 \\ L300-PS4-T200 & 23.38 & 6.64 & 90.19 \\ \hline L300-PS2-T160 & 22.47 & 6.56 & 86.73 \\ L300-PS4-T160 & 22.27 & 5.03 & 85.95 \\ L300-PS6-T160 & 22.16 & 3.43 & 85.51 \\ L300-PS8-T160 & 22.04 & 3.05 & 85.05 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Results of bonding tests.  
as the bonding length of the specimen increased from 250 mm to 450 mm. Because of linear regression analysis based on the bonding length and the _F.I._, the required bonding length of the Fe-SMA to introduce fully pre-stressing force on the concrete is over 600 mm.  
Figure 13 shows the bonding strengths and the slippage on different recovery temperatures and different pre-stained values of the embedded Fe-SMAs. The bonding strengths linearly increase by an average of 5.1% with the recovery temperature (see Figure 13a). With this result, it can be mentioned that the Fe-SMA at the high recovery temperature maintains the sufficient bonding strength. Additionally, there is no significant difference on the bond strengths with increase in the pre-strained Fe-SMAs, as shown in Figure 13b. This is because that the pre-strained Fe-SMAs have similar recovery stresses on different pre-strained values. However, the slippage occurrence of the Fe-SMA, referred as \"slippages\" in Figure 13, shows somewhat different results, compared to the bonding strength. In Figure 13a, after increasing the recovery temperatures, the slippages showed decreases. This might be closely related to the recovery stress. The higher recovery stress accounted for the tensile strength, which resulted in lower slippages. In Figure 13b, the slippages on the different pre-strained values of the Fe-SMAs happened at the deformation of around 2.6 mm. This is because the recovery stresses of those specimens were gathered around the average of 370.40 MPa.