Ground-penetrating radar (GPR) is extensively used in structural engineering for the non-destructive evaluation of materials such as concrete and masonry. However, high-frequency GPR data often exhibit complex signal signatures that cannot be fully explained by conventional specular reflection-based assumptions. This study provides numerical insights into waveguide effects in structural GPR data, highlighting their role in generating non-intuitive responses, contributing to signal interpretation and translating them into novel diagnostic tools rather than mere signal interference.

Numerical simulations of structural configurations were conducted to reproduce electromagnetic wave propagation. The results demonstrate that, for specific combinations of geometry, material properties, and antenna frequency, structural components can behave as waveguides, promoting horizontal propagation of electromagnetic energy along layers, and interfaces. The study proves that waveguide effects can enhance GPR side visibility and provide, in specific cases, velocity estimation of the top structural layer.

To support the numerical findings, simulated responses were compared with real GPR datasets acquired from representative structural case studies, revealing strong agreement in observed signal behaviour and confirming the physical relevance of the identified mechanisms.
The study emphasizes the importance of recognizing waveguide-induced propagation in the interpretation of GPR data. By integrating numerical modelling with field observations, it contributes to improving the reliability of structural diagnostics and reducing the risk of misinterpretation in engineering assessments.