## 2 Numerical Model
### Exemplary Results  
Selected results of numerical simulations are presented below. In the first stage, in order to better illustrate our results, the temperature distribution on the surface of the sample is presented in a system that depicts the entire simulated area (including the antenna). Figures 7 and 8 show the temperature distributions represented on the surface of the specimen, which solely encompassed CFRP concrete reinforcement, for two distinct forms of excitation: continuous (Figure 7) and modulated (Figure 8). The presented results depict the temperature distribution for three cases, namely, without the defect (Figures 7a and 8a), the defect located at the center of the sample (Figures 7c and 8c), and the defect located outside the center of the sample (Figures 7b and 8b). These results were obtained after a heating duration of 100 s. It can be easily seen that in all cases, the sample was heated unevenly, with the maximum temperature around its center (Figures 7a and 8a). This, of course, was related to the specific radiation pattern of the selected antenna. However, it was possible to distinguish between a non-defective and a defective sample due to the fact that the area around the defect was heated to a greater extent. We present a more detailed analysis below. Figure 6: Utilized mesh. Figure 7: The raw results of the numerical modeling for the case of continuous excitation presented for the chosen time step (t = 100 s): (**a**) the case without the defect, (**b**) the case with the defect located away from the sample’s center, and (**c**) the defect located in the center of the sample. The following section provides a broader overview of the results. The presented Figures 9 and 10 depict thermograms that were selectively chosen for both continuous and modulated excitation, respectively. The complete thermogram of the sample, both without a defect and with a centrally located defect, was reconstructed through mirror reflection (Figure 9a,b and Figure 10a,b). It should be noted that in the case of a system featuring an off-center defect, the aforementioned operation was not feasible. Consequently, in this case, we solely present one-half of the surface, as it was simulated in COMSOL (Figures 9c and 10c). The results of the study are shown in the form of unprocessed temperature distributions, namely, thermograms, in which the temperature values were standardized to a singular temperature scale. The outcomes for three specific time intervals are demonstrated, namely, at 50 s, 100 s, and 150 s, which corresponded to the end of the heating process. The data indicate that the temperature under continuous excitation was notably higher compared with that under modulated excitation, with maximum values of 18.88 \({}^{\circ}\)C and 16.11 \({}^{\circ}\)C, respectively. However, it is noteworthy that in both instances, the defect became apparent at the 50 s mark and the qualitative outcomes were similar. For instances of continuous heating, the central region of the sample experienced a considerably higher degree of heating compared with its off-center counterpart. This is a crucial factor to consider when the defect is located away from the center. The enhanced detectability of the off-center defect under modulated excitation was readily apparent. Below, the time-temperature characteristics illustrating the temporal fluctuations in the mean temperature within a specified region are shown. For this purpose, specific domains of interest were chosen for each case. For samples without the defect or a central defect, the designated region of interest (ROI) was the area in the middle that encompassed the defect. When dealing with a specimen that presented an off-center flaw, the ROI corresponded to the specific area that covered the defect. Figure 11 depicts specific regions for each case.