## 5 Conclusions  
The experiences gained from the analysis of the 240 m length experimental section of a residential street in southern Hungary show that it is useful to perform GPR measurements after the laydown and compaction of each layer on the surface of the road. This procedure may require more measurements, but these additional measurements provide the opportunity to determine the relative compactness or the air void content of each layer separately. In our study, PaveScan was used to determine the relative permittivity of asphalt layers quickly and quantitatively. A strong correlation was found between relative permittivity and the air void content of the asphalt mixtures, similar to previous studies. In this article, a novel empirical model is suggested for this task. Our new logistic model is less sensitive to the limitation of fewer calibration data points being available than other models in the literature. This offers the opportunity to adapt the laboratory-parameterised model to the actual conditions based on a few field core samples. Based on systematic data acquisition, an empirical model can be developed to estimate the air void content values at a \(\pm\)0.5% accuracy. For this purpose, either drilled samples or gyratory specimens obtained from the asphalt mixtures being tested can be successfully used. In the present work, the latter method was chosen for the non-destructive road condition assessment. The air void content computed based on the recommended empirical model can be used to derive a relative compactness (RD) map, which highlights the problematic parts of asphalt layers. Maps of the relative compactness values measured after the first roller pass provide a good comparison basis for assessments that are performed later. The compaction data produced are not of better quality than those determined from drilled core samples but the large amount of data obtained still makes them more useful. In summary, it can be stated that among existing non-destructive road diagnostic methods, only the GPR technology and the Intelligent Compaction (IC) technology are able to provide continuous determination of the quantitative compactness of asphalt mixtures. Moreover, the GPR technology is suitable for the determination of layer thicknesses in road pavement structures as well. Therefore, it can be recommended for quality control of construction work. In future research, the combination of GPR and UAVs as non-destructive devices should be investigated, specifically for the quality control of asphalt compaction work and the hot asphalt mix behind the compaction roller during pavement construction. Conceptualization, P.P., I.F. and C.T. ; Methodology, P.P. ; Validation, E.A. and A.M.A. ; Formal analysis, G.S. ; Investigation, I.F. ; Resources, E.A. and A.M.A. ; Data curation, C.T. ; Writing--original draft, P.P., A.H. and C.T. ; Writing--review & editing, P.P. and A.H.; Visualization, P.P. ; Supervision, G.S. and I.F. ; Project administration, G.S. All authors have read and agreed to the published version of the manuscript. **Funding:** This study was conducted under the framework of the project TKP2021-NVA-13, which was implemented with the support provided by the Ministry of Culture and Innovation of Hungary through the National Research, Development and Innovation Fund, financed under the TKP2021-NVA funding scheme. The GPR system applied in this study was provided by RODEN Engineering Office as part of the project 2018-1.1.2-KFI-2018-00029, which was supported by the National Research, Development and Innovation Office of the Ministry of Innovation and Technology through the National Research, Development and Innovation Fund. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** The data presented in this study are available in article. **Acknowledgments:** We would like to thank [PERSON], [PERSON], and [PERSON] for organizing and supporting the GPR measurements and for their useful advice.