Evaluation of wearing course mix designs on sound absorption improvement of porous asphalt pavement

This paper describes a laboratory procedure to study the sound absorption characteristics of porous asphalt pavement wearing course mix designs, and estimate their contributions to tire-pavement noise reduction. The effects of the following mixture properties were considered: percent porosity of mixture, degree of clogging occurred, and layer thickness. Four porosity levels of porous asphalt were studied: 12%, 16%, 20% and 25%. The mix design with 20% porosity was tested for the effects of clogging and specimen thickness respectively. A dense graded mix was included to serve as a reference for comparison. The effectiveness of the sound absorption and tire-pavement noise reducing capability of the mixtures were evaluated using the laboratory data and field measurements of trial sections. An approximate noise reduction calculation based on laboratory sound absorption measurements is proposed for comparison of different mix designs. The study found that changing the percent porosity from 25% to 12% caused some changes in the frequency characteristics of sound absorption, but made negligible changes in their contributions to tire-pavement noise reduction. On the other hand, clogging of the pores in the mixtures resulted in noticeable decreases in both the sound absorption coefficients and tire-pavement noise reductions. The analysis estimates that about 23-33% of the total tire-pavement noise reduction achieved by the porous asphalt mixtures can be attributed to their sound absorption capability. Finally, changing the thickness of the asphalt mixture tested from 63 mm to 200 mm had no significant effect on sound absorption and tire-pavement noise reduction.