Federal Highway Research Institute


Testing of a modified mix design of porous asphalt

The high void content of porous asphalt surface layers results in good acoustic effectiveness.

Porous asphalt surface layer


Porous asphalt surface layers are used in Germany to reduce Tyre/Road-Noise directly at the source of origin the road surface. The mixture contains coarse aggregates > 2 mm, filler and polymer-modified bitumen as binder without the addition of fine aggregates. This results in a high void content for a good acoustic effectiveness and guarantees the required acoustic durability of the layer. From the construction point of view problems like reduced stability and lack of process control could occur.


A group of experts of the German Asphalt Association suggested a modified mix design after carrying out a study in the laboratory. This changed mix design with the addition of five percent of fine aggregates is tested practically on the duraBASt. Two different mix designs with a maximum aggregate size of 8 mm are used in an industrial scale. The first variant is based on the specifications of the technical regulation, the second one is based on the suggestions of the German Asphalt Association. Combined with two different layer thicknesses this leads to four variants in total.


The objective of the project was to check the production and the laying process of the modified mixture and to monitor and analyse the properties of the finished layer. For this characteristic values like void content and absorption are determined. The improved stability of the layer was tested with the help of a scuffing device.


Asphalt-technological characteristic values - such as void content - as well as acoustic characteristic values - such as the absorption coefficient - were determined. The surface wear test was used to prove the improved durability.

The pictures show a measurement of the road surface Measurement of the geometric shape of the road surface (surface texture) with the three-dimensional measurement system T3DM-k of BASt

The picture shows the removal of drill cores Removal of drill cores to determine the layer properties of the OPA


Overall, the results allow the conclusion to be drawn that porous asphalt surface layers with good acoustic behaviour when new can be paved if the sand addition is precisely adhered to. Laboratory tests predicted that this asphalt mix variant would have a higher service life than the variant without sand addition. The acoustic service life could not be predicted within the framework of the project.

Logos of the project partners