Pavement FRFs and noise: A theoretical and experimental investigation

Traffic and tyre/road noise affect human health. In order to mitigate the tyre/road noise, the knowledge of the frequency-based mechanics of pavements (e.g., pavement Frequency Response Functions, FRF) could be a crucial factor, because the remaining factors have been investigated in depth. Consequently, the main objective of this study was to investigate the relationships between road acoustic response (herein called RAR) and FRFs in case of bituminous mixtures added with different percentages of crumb rubber (CR). Laboratory tests and analyses were carried out. In particular, during preliminary tests, the system (i.e., impact hammer, microphone, analog-to-digital converter cards, and software) was set up using both cylindrical specimens and rectangular slabs, and a new testing procedure for both sample types (based on the standards EN29052-1 and ISO 7626–5) was defined. Subsequently, the new procedure was applied to four sets of cylindrical samples consisting of dense asphalt concretes (i.e., AC6d), where different percentages of crumb rubber (CR; i.e., 5%, 29%, and 100%) were added (dry method). Results demonstrate that even if many parameters affect the FRF estimates, FRFs and RAR are related especially in the range 0.4–3.2 kHz, and that CR content can contribute to lowering the mechanical impedance, the dynamic stiffness, and the acoustic response of the asphalt concrete mixtures used in this study. By referring to RAR-based versus Close Proximity method (CPX)-based inferences and analogies, the relationship between the RAR herein defined and measured and the on-road CPX emerges as mainly unknown and probably quite complex. Further investigations are required to have a higher accuracy of the results and to better understand CR actual impact on tyre/road noise and remaining properties.