## 3 Health Monitoring and Test Results
Figures 12 and 13 show the damage indices of the sensors of RCW1 at various drift ratios under the various excitation frequencies of 100 Hz and 1000 Hz. According to these figures, the damage indices and damage state of each sensor in various frequencies have an increasing trend with the drift ratio increasing. In Figure 13, the damage indices of the sensors of S1, S4, S3 and S6 at the drift ratio of 1.0% are relatively higher than those of the sensors of S2 and S5. It can be observed that the damage state in the two sides of the wall are more serious than the center area. Additionally, the results have a high correlation with the final failure pattern of the specimen including the concrete crush in the bottom region of the boundary columns. However, the damage indices obtained from the excitation frequency of 100 Hz, as shown in Figure 12, cannot identify the location with the serious damage. The experimental results have demonstrated the effectiveness of the proposed damage index that can be  
Figure 11: Regions and sensor locations for damage detection. Figure 10: Cracking patterns of the specimen RCW2 at **(a)** drift 0.25%, **(b)** drift 0.5%, **(c)** drift 0.75% and **(d)** final failure mode at drift 1.0%. used to evaluate the damage state of an RC wall structure. Additionally, the piezoceramic transducers have the ability to identify the serious damage location using the excitation waves with high frequency. The loss of energy of a propagating stress wave that is caused by cracks or damage can be interestingly investigated in the frequency domain. For single-frequency excitation waves, the Fourier amplitude is concentrated around the designated excitation frequency in the frequency spectrum, so variation in the Fourier amplitude of received signals in the frequency spectrum can reveal the severity of damage in terms of the damage index that is defined in Equation (2). Figure 14 plots the time history of received sensor voltage and its corresponding Fourier amplitudes of sensor S4 under excitation by actuator A1 (frequency 5 kHz). The Fourier amplitudes around the frequency 5 kHz clearly decrease as the drift ratio of the specimen increases, indicating again that the transmission energy of the stress waves is attenuated by cracks and damage. Figure 14: (**a**) Time history of received sensor voltage and (**b**) corresponding Fourier amplitude of sensor S4 for excitation frequency 5 kHz. \[D.I.=\frac{\left|E_{h}-E_{i}\right|}{E_{h}} \tag{2}\]0 Damage index at each drift ratio for RCW 1 (excitation frequency 100 Hz). \[D.I.=\frac{\left|E_{h}-E_{i}\right|}{E_{h}} \tag{2}\]1 Damage index at each drift ratio for RCW 1 (excitation frequency 1 kHz). interface failure or cracks between two structural members (one existing and one added) are important and affect the effectiveness of the retrofit. Furthermore, interface failure or cracks cannot be observed during renovation as part of retrofitting work. The experimental results in this work demonstrate that PZT sensors can be used to identify interface failure or cracks between two structural members in retrofit construction and help engineers to determine the degree of damage to a structure as a result after an earthquake. On account of the advantages of piezoelectric sensors, such as their active sensing, low cost, availability in various shapes, simplicity of implementation and ability to detect slight cracks from high-frequency responses, piezoceramic-based sensors have greater potential for the SHM detection of damage in RC wall structures than conventional SHM systems. For convenience of implementation, future work should elucidate the relationship between crack patterns, widths or lengths and the damage indices that are obtained using PZT sensors. Conceptualization, W.-I.L. and F.-P.H. ; Methodology, W.-I.L. and F.-P.H. ; Formal Analysis, C.-K.C., C.-E.H. and W.-I.L. ; Investigation, W.-I.L. and C.-K.C. ; Resources, F.-P.H. ; Writing-Original Draft Preparation, W.-I.L. ; Writing-Review & Editing, W.-I.L., C.-K.C., C.-E.H. and F.-P.H.