ICSV17, Cairo, Egypt, 18-22 July 2010 1 ESTIMATION OF THE POROSITY OF ASPHALTS BY SURFACE IMPEDANCE MEASUREMENT Jörn Hübelt, Christian Kühnert, Uta Figula, Hochschule Mittweida, University of Applied Sciences, Technikumplatz 17, D-09648 Mitt- weida, Germany, joern.huebelt@hs-mittweida.de Road surfaces made of porous asphalt have several advantages: e.g. the high amount of air void reduces noise and drains water. These effects depend strongly on the porosity that is achieved in the laying process. By now the usual way to determine the porosity is to measure it on cores taken from the road surface. But this method is destructive and the cores can be taken from few positions only, which is not sufficient for statistics. Furthermore, the porosity can be determined only after the construction work is finished. So it is not possible to influence the air void inbuilt during the laying process. In the paper a new method for the determination of the porosity without these disadvantages is presented. The approach bases on acoustical properties that can be measured at the road surface without damaging it. In connection with the Phenomenological Model the material properties can be calculated. So it is possible to compute the porosity immediately during the laying process. 1. Introduction Open graded porous asphalt reduces the traffic noise of cars at medium speed significantly. It acts as acoustic absorber but also reduces the generation of noise by suppressing the air pumping of the tyre-road system. When driving on porous asphalt the pumped air is pressed into the open chan- nels and not set into vibrations by being compressed and expanded between road and tyre surface. The sound absorption as well as the ability to drain water from the surface depends strongly on the porosity that is achieved in the laying process. In order to obtain the best effect it must have values between 10 % and 30 %, depending on the type of asphalt. The porosity is influenced by the rolling process. The difficulty consists in finding the best time to stop the rolling, i.e. to detect if the porosi-