## 2 Materials and Methods
### PZTs' Installation  
Small and thin PZT patches, with dimensions 10 \(\times\) 10 \(\times\) 2 mm, in different configurations were placed in the retrofitted beams. Further, the PZTs were strategically positioned in the following locations: (a) embedded within the core of the initial beam, (b) externally epoxy-bonded to the jacket's outer surface, and (c) on the core-outer surface shell IZ. Notably, two PZTs were referenced at the same point, with one placed on the front surface and the other at the back. Thus, the total sensors' resources for retrofitted beam \"B500-J\" consisted of four pre-installed PZTs from the initial beam \"B500\", configurated as \"Smart Aggregates\". The orientation of the polarization direction of the embedded transducers is considered a crucial factor in their monitoring effectiveness and is depicted with their embedded angle in the relevant figures. Henceforth, this configuration is denoted as \"SA\". Additionally, six PZTs were epoxy-bonded on the IZ between the initial concrete surface and the inner mortar layer of the U-shaped jacket, henceforward referred to as \"J\". Furthermore, two epoxy-bonded PZTs have been attached to the jacket's external surface, further denoted as \"X\". The configuration, positions, and types of the employed PZTs are illustrated schematically in Figure 5.  
In addition, Figure 6 depicts the configuration, the positions and the notations of the attached PZTs of beam \"B200a-J\". Thus, three pre-installed \"Smart Aggregates\" of the initial beam \"B200a\" were \"survived\". One PZT of the left span has been destroyed due to the extensive shear cracking development in the left span of the initial beam. Furthermore, six PZTs are epoxy-bonded on the interfacial between the concrete surface and the mortar layer of the U-shaped jacket. Moreover, two epoxy-bonded PZTs have been attached to the jacket's outer surface. Figure 5: Positions and notations of the PZTs of beam “B500-J”. Figure 6: Positions and notations of the PZTs of beam “B200a-J”. Further, Figure 7 presents the positions, the notations, and the configuration type of the employed PZTs. Thus, three pre-installed \"Smart Aggregates\" PZTs were saved from the initial beam \"B200b\". One PZT of the left span was destroyed due to the extensive cracking formation developed in its \"neighborhood\", during testing of the initial beam. In addition, six PZTs were epoxy-bonded on the interfacial surface of the U-shaped jacket. Moreover, two PZTs have been attached with epoxy to the jacket's outer surface. At this point, it is noteworthy to be mentioned that all the embedded PZTs have been coated with epoxy-resin before their installation to enhance their water-proofing resistance and to develop a protection layer, primarily for the tasks of the casting process and later for the potential cracking development. Moreover, the initially embedded PZTs had been further coated with cement-based mortar, forming the configuration of a \"Smart Aggregate\", initially to improve their cohesion with the casting concrete, avoid forming voids, and increase their resistance under an aggressive environment due to subjected load and cracking propagation or/and the penetration of chemical agents. As depicted in Figures 2 and 3, the destroyed embedded PZTs of the initial beams B200a and B200b were positioned directly to the defected span area, where extensive shear cracking formation had developed. Moreover, the repair works for removing the defective concrete section triggered further damages to the \"infrastructure\" of the patches, cutting the connected wires. Thus, the PZT's functionality can be assured in a long-term period under normal operational conditions for the monitored element. Whenever severe circumstances occur near its monitoring area, it directly reflects the transducer's functionality, potentially affecting the installation parameters (coating materials, epoxy-resin, wires, PZT patch).