Permeable friction courses: area-based vs. line-based surface performance and indicators

The objective of this study is to investigate acoustic absorption and surface texture in porous friction courses. The in situ measurement of sound absorption properties of road surfaces permits to derive acoustic information on different types of road surfaces. When the extended surface method is used, the method is based on the analysis of several square meters of surface, under several hypotheses and conditions. In contrast, the characterization of pavement texture (in spectral or aggregate terms, by using surface profiles) essentially refers to line segments. The relationship between the indicators which can be derived from the two different approaches (area-based or line-based) poses several practical and theoretical issues in terms of interpretation and surface homogeneity. In pursuing the above objects, experiments were carried out in order to: i) study the variability of in situ acoustic measurements in normal and non-normal conditions; ii) study pavement surface texture in terms of aggregate and spectral indicators, by varying measurement direction. The acoustic absorption coefficient was measured according to the ISO 13472-1, Acoustics - Measurement of sound absorption properties of road surfaces in situ – Part 1: Extended surface method, 2002. Surface texture was investigated according to the standards ISO 13473-1 and 4. A dependence of the absorption coefficient on the angle was found in terms of both absolute maximum and maximum point. On average, an increase of the maximum acoustical absorption and a reduction of the maximum frequency were found. The assessment of surface texture indicators permitted to derive that the dependence on angles was not affected by texture-related issues or systematic variations.