## 2 Materials and Methods
### Monitoring Process  
This experimental work implemented a wireless SHM system based on the fundamental principles of the EMI method employing piezoelectric sensors. The PZTs were selected due to their unique properties of generating a surface electric charge under mechanical stress and, conversely, undergoing mechanical deformation when exposed to an electric field. Consequently, any changes observed in the electrical impedance of the implemented PZTs are extracted from variations in the mechanical impedance of the monitored area. In addition, PZT's voltage responses are recorded at multiple loading levels to assess the component's structural integrity (Figure 8). Thus, fluctuations in these measurements  
Figure 7: Positions and notations of the PZTs of beam “B200b-J”. within a specific excitation frequency range indicate potential structural degradation of the monitored element. In the proposed approach, PZT patches were affixed to the interfacial between the existing concrete of the initial beams and the inner layer of the applied mortar using epoxy resin. Thus, the efficacy of the retrofitting scheme was investigated. The incremental imposed loading leads to beam deformations, which, in turn, develop stress on the interfacial part of the jacket. With the development of stress due to the load subjection, the shear deformation leads to crack formation within the specimen. As a result, different effects are induced as the jacket's structural mechanisms resist overcoming the imposed incremental loading. Hence, the development of stress in the interfacial surface and possible slippage between the U-shaped jacket and the initial concrete beam are expected due to the loss of bonding conditions. Further, a PZT patch bonded on the interfacial surface could detect the development of cracks in the concrete and any potential jacket's slippage. For this reason, six PZTs were epoxy-bonded to the interfacial surface of the \"B-Js\", as shown in the close-up of Figure 3.  
In addition, custom-made and designed impedance analyzers, referred to as \"WiAMS\" (Wireless Impedance/Admittance Monitoring System), are implemented to acquire the EMI responses from the PZTs (Figure 8). A WiAMS device is assigned to capture the EMI responses of a PZT patch within a specified frequency range of 10 to 250 kHz, featuring a frequency resolution of 1 kHz. The acquired EMI responses are subsequently uploaded to an online database for further data analysis (Figure 8). Consequently, the EMI response of each PZT patch in the specimens' pristine (undamaged) state is recorded to establish a baseline. Furthermore, to assess the efficacy of the retrofitting method, the EMI responses of the attached PZT patches are reassessed after completing each loading level, corresponding to the relative damage state/condition. As mentioned above, the EMI measurements could indicate the structural integrity degradation reflected by the monitored area's mechanical impedance changes and demonstrated in voltage or/and frequency response curve shifting. The interaction between the PZT transducers and the host structure generates a voltage response composed of a  
Figure 8: Schematic illustration of the monitoring process. real component (conductance) and an imaginary component (susceptance). This response reflects essential structural properties and can be represented by the complex admittance of the PZT patch, as follows in Equation (1).