ACOUSTICAL PERFORMANCE OF PERVIOUS MACADAM SURFACES FOR HIGH-SPEED ROADS

A full-scale performance trial of pervious macadam laid on the A38 Burton bypass has been underway since September 1984. The trial, being carried out by the Materials and Construction Division of the Transport and Road Research Laboratory, will compare the durability and performance of a number of proprietary and nonproprietary materials made with a range of polymer-modified and unmodified bitumens. However, these surfaces are also good sound absorbers, so the opportunity was taken to monitor the noise emitted by vehicles traveling on these surfaces. Reporting results of the acoustical measurements covering the first 4 years of exposure to traffic is the primary concern of this paper. Other aspects of performance are also briefly discussed. The results show that the noise from vehicles running freely on pervious macadam is reduced when compared with equivalent nonporous surfaces by between 5.5 and 4.0 dB(A) on average. This result was not found to be influenced significantly by binder content variations in the range 3.7 to 5.0% or by the presence of binder additives. After 4 years of exposure to heavy traffic, the surfaces showed slight deterioration in noise performance, although improvements over conventional surfaces were still averaging 4.0 dB(A). This occurred despite a substantial fall in the hydraulic conductivity and the surface texture depth of the trial surfaces over the same exposure period. Even so, the stabilized surface textures remain high and acceptable for high-speed road applications, and the fall in hydraulic conductivity did not give rise to a concomitant fall in the spray suppression properties of the surfaces.