Since 2010 double-layer porous asphalt (DPA) pavements have been used on motorway E4 through the Swedish city Huskvarna. The pavement has been a great success despite the challenge to use porous pavements in a country where studded tyres are used in wintertime, resulting in excessive surface wear and subsequent clogging of pores. In this paper, seven interesting trials on this road related to the noise reduction of the pavement are reported:

1. Paving DPA hot-on-hot: Commonly, when paving DPA, it is considered necessary to do this “hot-on-hot” which means that the top layer must be applied while the bottom layer is still hot. This project has shown that it works fine to pave the two layers in two different days.

2. Rejuvenating the surface may almost totally clog the pores in the top layer. In an attempt to extend the technical lifetime, the slow lane was rejuvenated by application of a Fog Seal. This filled the remaining porosity which resulted in a great loss of noise reduction.

3. The effect of the bottom layer is analysed in relation to the top layer. One part of the section had a single-layer PA, the performance of which could be compared to another section where the same PA layer had been laid on a bottom layer, thus creating a DPA. It appeared that 2/3 of the noise reduction is due to the bottom layer of the DPA.

4. Reusing the bottom layer: On one part of the new pavement laid in 2017, only the top layer of the old DPA section was milled-off and then repaved with a new top layer, while the bottom layer was reused. The noise reduction which is lost by reusing the old bottom layer is only around 1 dB (of the initial 7-8 dB), or 0.5 dB as an average.

5. Steel slag has been used instead of stone aggregate in the top layer in one trial. The performance of this trial is analysed in comparison to the conventional aggregate.

6. Grinding off the peaks in the surface has a favourable effect on both noise reduction and rolling resistance. This is a way to produce an “extra negative texture”.

7. End-of-life noise reduction: After a few years, the top layer is clogged; yet, the pavement provides a considerable noise reduction. Surprisingly, 1-3 dB of noise reduction remains even when there is full clogging. Reasons for this are discussed.