## 3 Numerical Simulation and Laboratory Testing
### Results of Experiments and Simulations  
Figure 13 shows the comparison between simulated and experimental B-scan results. Cracks, voids, and impermeable solids exhibit significantly different reflection characteristics in the profile. Figure 13a,d shows the reflection of cracks, with longer diagonal cracks  
Figure 11: The setting of prefabricated defects in concrete beams: (**a**) non-dense material, void, and (**b**) cracks generated during the experiment.  
Figure 12: (**a**) GSSI GPR equipment and (**b**) measurement process.  
appearing more clearly in the cross-section than vertical cracks. A wider crack may result in more pronounced reflections (if the crack is not filled with other media), while a narrower crack may have weaker reflection effects, and reflections may not even appear in the profile. Cracks appear as line reflections on radar profiles, usually narrower and less noticeable than voids.  
Figure 13b,e shows the reflection of a PVC pipe with a diameter of 40 mm. PVC pipes usually exhibit strong reflection at the interface under the action of radar waves, forming a downward opening reflection profile. Due to their larger surface area and significant interface reflection effect with concrete, large-diameter PVC pipes usually produce stronger reflection signals, and the boundaries of the reflections are usually clearer and easier to identify.  
Figure 13c,f shows the reflection profile of a square wooden box. Non-dense solid defects exhibit higher reflection intensity than cracks and PVC pipes in radar profiles. However, due to their less dense internal structure, irregular scattering and absorption are observed on both sides of the profile line, resulting in faster attenuation of signal intensity.  
During the process of processing simulation and experimental signals, A-scan signals were extracted at the vertices of each defect reflection contour and conducted spectral analysis (see Figure 14). In the time domain signal, whether it is a simulated signal or an experimentally measured signal, the reflection of non-dense solids within the 1-2 ns interval has the largest amplitude response, indicating that their internal regions have higher reflectivity, which may be due to their larger dielectric constant or richer internal structure. The 3-4 ns interval is the reflection at the bottom interface of the concrete beam, where the reflection amplitude of the crack is higher than the other two groups; this is because the crack runs through the entire concrete beam. From the spectrum, it can be seen that 1.6 GHz electromagnetic waves have similar propagation characteristics in different concrete beams, and the reflected signals of three different types of defects have the same main frequency. The amplitude value of non-dense solids at the main frequency position is slightly higher than the other two sets of data. Due to the interference of background  
Figure 13: Radar profile features of different defects (Figure (**a–c**) are simulation results, while Figure (**d–f**) are experiment results).  
noise in the experimental data, we can draw the fitting curve of the experimental data and see that with the increase of frequency, both simulation and experimental signals indicate that the reflection amplitude of non-dense solids is significantly higher than that of voids and cracks. This indicates that air has a significant absorption and attenuation effect on the high-frequency components of electromagnetic waves. Meanwhile, there is a good correlation between the intensity of spectral amplitude values and the reflection intensity of different defects displayed in B-scan images.