## 3 Results and Discussion
### Result of Recovery Stress Transfer Test for Cement-Based Mortar Bar  
Behavior of the cement-based mortar bar containing the Fe-SMA was investigated to calculate a compressive strength on surrounding concrete transferred from the Fe-SMA. As mentioned previously, in the recovery stress of the Fe-SMA, the Fe-SMA with the activated SME under constrained strain  
Figure 12: Bonding strength and slippage responses.  
Figure 13: Bond strengths on different temperature and pre-strained values: (**a**) Different recovery temperatures on L300-PS4 specimens; (**b**) Different pre-strained values on L300-T160 specimens.  
by the cured concrete generated the recovery stress which would be used as the pre-stressing force. The measured elongation on the surface of the cement-based mortar bar by the strain gauge was used to calculate the measured compressive stress, \(\sigma_{\text{measured}}\). The measured strength of the cement-based mortar bar was computed with the equivalent modulus of elasticity and obtained strains, as expressed in Equation (2) below.  
\[\sigma_{\text{measured}}=\frac{\Delta l}{l_{o}}E_{eq} \tag{2}\]  
in which \(E_{eq}\) is an equivalent Young's modulus of elasticity and \(l_{o}\) is the original length of the Fe-SMA.  
In addition, the expected compressive stress, \(\sigma_{\text{expected}}\), was calculated with the recovery stress from the thermal mechanical properties of Fe-SMA. The expected compressive strength of the Fe-SMA is calculated by Equation (3) below.  
\[\sigma_{\text{expected}}=\frac{\sigma_{r.s.}\times A_{\text{Fe-SMA}}}{A_{\text{ eq}}} \tag{3}\]  
where \(A_{\text{Fe-SMA}}\) is an area of the Fe-SMA strip, \(A_{\text{eq}}\) is the cross-sectional area of mortar bar, and \(\sigma_{r.s.}\) is the recovery stress by the SME of the Fe-SMA at the 160 \({}^{\circ}\)C of the recovery temperature.  
Figure 14 shows comparisons between the expected compressive stress from the Fe-SMA (black solid line) and the measured compressive stress from the cement-based mortar bar (red solid line) on different recovery temperatures. During the activation of the SME, the difference of two compressive stress responses was remarkably shown. This is because of the thermal expansion effect on the mortar bar. The heating for the activation of the SME was transferred to the surrounding cement mortar bar, which makes the cement-based mortar bar expand. Because the mortar strain was assessed from the strain gauge which was attached on the surface of the specimen bar, a strain could be measured that was higher than the surface strain of the Fe-SMA.  
Figure 14: Compressive stress on different recovery temperatures: (**a**) recovery temperature of 100 \({}^{\circ}\)C; (**b**) recovery temperature of 130 \({}^{\circ}\)C; (**c**) recovery temperature of 160 \({}^{\circ}\)C; (**d**) comparison of expectation and measurement of compressive stresses.  
The compressive stresses of the cement-based mortar bar and the Fe-SMA increased by 25% with the increased activation temperature. At higher recovery temperature than 160 \({}^{\circ}\)C applied to the mortar bar, the high heat of the Fe-SMA was transferred to the mortar surrounding the strip, which generated longitudinal cracks along the Fe-SMA. Furthermore, the recovery temperature higher than 160 \({}^{\circ}\)C delays a formation of ettrinigite at the interface between the cement and fine aggregate, or destroys the formation of ettrinigite, which generates crack development [33].  
According to the results of the recovery stress transfer test, it is confirmed that the pre-stressing force applied on the concrete member would be estimated by the recovery stress of the Fe-SMA. Furthermore, these results obtained from these tests suggest that the optimum recovery temperature is 160 \({}^{\circ}\)C for the introduction of the pre-stressing force on the concrete.