Two-layer porous asphalt for noise reduction

Traffic noise is one of the major environmental issues in Europe in general and specifically in the Netherlands with many urban areas close to major highways. For this reason the current policy in the Netherlands is to use Porous Asphalt (PA) 0/16 as a wearing course on all motorways to reach more noise reduction. Recently twolayer PA systems were developed. They are expected to reduce traffic noise in a better way compared to PA 0/16.
In this paper laboratory test results from a number of two-layer PA systems developed by Dutch contractors are described. In the laboratory compactibility, mechanical properties, durability and acoustic relevant properties of the products were determined in order to have a reference for the field tests. Compactibility was considered important, because the thin top-layer is expected to cause problems during adverse weather conditions.
The compactibility was tested with the gyratory compactor by compacting at several temperatures. Considerable difference in compactibility was observed. Ageing is not causing problems with relation to compaction. From the results it can be concluded that depending on the mix, even relatively low temperatures can give good compaction results. The resistance to ravelling was measured on slabs with a new test method developed by Dutch contractor Heijmans, the Rotating Surface Abrasion Test (RSAT). Results are given in the paper. In addition to the RSAT mechanical tests like direct tensile test (DTT) and semicircular bending (SCB) test were performed to relate these tests to ravelling in the RSAT. Considerable difference between mixes was observed. Acoustic relevant properties measured in the laboratory were absorption-coefficient of the two-layer system, flow-resistance and macro texture of the fine PA top-layer layer. Absorption of all mixes was very good in the relevant frequency area. The macro-texture measurements gave problems due to the dispersion of the laser signal due to the shining surface of the material.