The Cologne ring-road system is an endurance test dreaded amongst motorists – since the busiest section of motorway in Germany lies between the A1, A3 and A4, with a high risk of congestion. And it’s not just the drivers’ nerves that are under strain, but the asphalt as well: long-distance traffic winds its way from here through Europe and branches out across Germany. There is particularly heavy traffic on the section of the A3 between the Leverkusen intersection and the Cologne-Mülheim junction.
To ease traffic, it was therefore decided to widen the six-lane motorway to eight lanes. The technical focus lay mainly on the asphalt bearing the brunt of the HGV traffic, which covers around 100,000 square metres of road surface. The main priority alongside high stability was that the asphalt also com-plied with more stringent noise-control requirements to protect the residential areas bordering on the motorway.
To reduce noise levels for local residents, a significant five-decibel reduction in the sound pressure level was required in the A3 project. Due to these requirements, the only option for the road surface was open pore asphalt (OPA). OPA is characterised by having interconnected voids in the surface layer. This dampens the sound of vehicles and prevents “hissing” on the road surface, since the air in the tyre tread is distributed via the interconnected voids. Such sound absorption can-not be achieved with the more commonly used stone mastic asphalt. The void content of stone mastic asphalt is usually under four per cent. Consequently, to achieve the required noise reduction, the asphalt needed to have a void content of at least 24%.