# Flexural Behavior of Self-Prestressed RC Slabs with Fe-Based Shape Memory Alloy Rebar
## 3 Experiment Results and Discussion
### Load-Deflection Relationship
#### 3.4.3 Reactivation of Fe-SMA Rebar  
As mentioned earlier, the prestressed concrete member with the SMA rebar has the advantage of being able to recover the prestress force (recovery force), even though it is a bonded prestressed concrete in which the Fe-SMA rebar and the concrete work integrally. To verify this, a reactivation experiment of Fe-SMA rebar was performed on the SL-3R specimen. This test was performed by repeating the following steps four times. (1) The Fe-SMA rebar of the specimen was activated in the same way as described in Section 2.3. (2) The specimen was loaded up to 70% of the ultimate load of the activated specimen (however, the last (fourth) loading was carried out until the specimen was destroyed). Finally (3), the load applied to the specimen was reduced to 0 for about 1 min.  
Figure 13 shows the time-upward displacement relationship in the middle of the specimen during Fe-SMA activation and reactivation. A camber of 0.314 mm occurred during the first activation of the SL-3R specimen without preloading. In the SL-3R, which was loaded up to a load corresponding to 70% of its ultimate load, a camber of 1.08 mm, 0.954 mm, and 0.987 mm was generated during the second, third, and fourth activations, respectively. These values were 344%, 304%, and 314% larger than the camber generated in the first activation, respectively. The camber was larger in the second, third, and fourth activations than in the first activation because the flexural stiffness of the cross section was greatly reduced due to the cracks generated in the specimen as a result of the pre-loading.  
Figure 12: Comparison of \(P_{\sigma r,exp}\) and \(P_{\sigma r,theory}\).  
Figure 14 shows a comparison of the load-displacement relationship between the SL-3R, where the Fe-SMA rebar was activated four times, and the SL-3A. As shown in Figure 13, in the SL-3R specimen, where a load of 70% of the ultimate load was applied, residual deflection occurred even when the load was removed, and a portion of the deflection was recovered by the activation of the Fe-SMA. After the fourth Fe-SMA rebar's activation was completed, the specimen was loaded until it was destroyed. As can be seen from Figure 14, the load-displacement relationship of SL-3R was similar to that of specimen SL-3A, which was activated once, regardless of the number of activations. The ultimate load of SL-3R, which had undergone four activations, was only 1.8% different from that of the SL-3A. Therefore, it can be concluded that the effect of the number of activations on the flexural behavior of the member is very insignificant.  
It is considered that the prestress force of the concrete reinforced with Fe-SMA rebar can be recovered through reactivation even if the prestress force is reduced due to various reasons, such as drying shrinkage and relaxation, unlike conventional bonded prestressed concrete. Therefore, it is judged that the self-prestressing method through Fe-SMA rebar can solve the disadvantages of the low strength problem of unbonded prestressed concrete and the difficulty of retensioning bonded prestressed concrete.