## 5 Experimental Results
### Wavelet Packet-Based Structural Damage Indices  
During the test, SA-6 and SA-9 failed to function. The possible reason for the dead sensors may be that the wire broke during casting. Experimental results from the other sensors are shown below. Figure 10 shows the wave packet-based damage indices of all the functional SA sensors during the test. The damage indices of each SA sensor present generally increasing values, which correspond to the damage development of the RC column. Before Step 1000, values of the damage indices are close to zero due to small displacement and damage within the RC column. As the load increased after Step 1000, the values of damage indices increase. After Step 3500, damage indices of all SA sensors report values close to one, which means the RC column sustained a high level of damage. The results shown in the damage indices are in good agreement with the test record [45]. Other than the general increasing trend shown in the damage indices, several local increasing and decreasing cycles can also be found in the damage indices. Since the RC column was subjected to cyclic loading, the cracks in the RC column opened and closed correspondingly. The local increasing and decreasing values shown in the damage indices detected this opening and closing of the cracks during  
Figure 10: General damage indices throughout the test. Figure 9: Displacement history in \(x\)-direction of the column. the test. According to different cycles of increasing and decreasing shown in the damage indices, overlay plots of the displacement history curve and damage indices of each functional SA are shown Figure 11.  
In the first 1000 steps, the displacement of the RC column was very small, as shown in Figure 9 (0-1000 steps). Since no damage occurred during this time, the received signal of SAs did not change much. Therefore, the values of the local damage indices are also very small, as shown in Figures 10 and 11. It can be considered that the RC column was in good health during the first 1000 steps. From Step 1100 to Step 2100, the concrete column was subject to five initial simulated pseudo-dynamic loading cycles (1-5), as shown in Figure 9. When load increased, the bottom cracks opened and the structural damage increased. When load decreased, cracks closed corresponding to the decreased damage. During this period, the peaks and valleys between the damage indices and the displacement history as shown in Figure 11 are not clearly matching to each other, but the similar increasing and decreasing trends can still be found. The possible reason is the crack open-close phenomenon in this period is not clear, since the crack formed at the bottom of the concrete column is very small under the first several loading cycles. From Step 2100 to Step 3300, both the cycles observed in displacement history and the damage indices are in good agreement with each other. Since the highest value in each displacement cycle of the RC column was quite large, as shown in Figure 9, the major crack has existed at the bottom of the concrete column. As the concrete column was pushed to one side, the major crack opened and the values of the damage indices for each smart aggregate correspondingly increase. As the concrete column was further pushed back to its original position, the values of the damage indices decrease. Since the crack in this period is much larger than the previous step from 1100 to 2100, the propagating stress wave attenuates more than the previous period. The cycles (6-11) shown in the damage index are more clear than the previous cycles (1-5). In addition, the highest values of the damage index reached to one, which implies the concrete column was subject to severe damage in the maximum displacement of each cycle. After Step 3300, the RC column almost reached the ultimate capability. In both cases in which the crack opened and closed, most of the propagating stress wave energy was attenuated. SA sensors \"lost contact\" with the actuator.