In Europe, 17% of the population, or around 13 million people, are exposed to environmental noise levels of more than 65 dB(A), a level that can have negative health effects. Noise pollution arising from road traffic is one source of the problem that has received the most criticism from the public.
Above 30 mph, most of the noise caused by an automobile comes from contact between the tire and the road. This includes the impact of the tire on the road surface, compression of the air in front of the tire and expulsion at the rear, the suction effect and other factors. To reduce the noise nuisance arising from contact between the tire and the road surface, French road contractors have developed effective surfaces which can reduce the noise by several decibels, an important factor when it is considered that a reduction of 5 dB(A) decreases the acoustic energy by a factor of three. The performance of this surface coating is essentially obtained from a reduction in the particle size of the components of the surface layer and to an increase in the proportion of voids in the asphalt.
Until recently, road surface aggregates could be up to 14 mm in size to provide the roughness necessary to produce sufficient adherence for vehicles, but they were very noisy. Today, the sizes vary between 6 and 10 mm, and the percentage of voids, which trap sound waves efficiently, has risen from 5% to more than 20% of the surface layer.
For some time, roadbuilders in France have been seeing project books fill with orders from local authorities who, under pressure from residents living close to busy roads, are demanding a significant reduction in noise levels.
"The main market," explained Bernard Héritier, technical manager at SCR-Beugnet based in Velizy Villacoublay, "is clearly suburban roadways, where traffic moves between 40 and 70 mph, with a large number of vehicles."
Another source of concern is highway companies who, to justify tolls, must provide comfortable road surfaces that enable drivers to listen to their car radios in comfort.
"Noise has become a fundamental concern to our customers," claimed Pierre-Marie Spillemaecker, product and process marketing manager at Eurovia, located in Rueil Malmaison near Paris.
The very thin or ultra thin bituminous asphalt offered today by French manufacturers is a compromise between noise level and adhesion.
Layers of success stories
About 15 years ago, the availability of porous asphalt brought a significant improvement with regard to noise. But problems with the road surface clogging from dirt and surface wear stopped its development. Companies then started designing new road surface techniques.
For example, the French firm Gerland Routes, based in the city of Villeurbanne, is now offering Epsibel, a roadbed consisting of two layers of high porosity bituminous concrete with very differing aggregate size. The lower layer consists of an HP 10/14 porous asphalt, with 2.5% of bitumen 1.5 in. thick. The upper layer, 3/4 in. thick, consists of Microphone HP 4/6 made up of 4.5% highly cohesive modified bitumen and 0.8% of glass fiber. This thin layer helps reduce noise emission (69.7 dB), while the porous structure absorbs engine noise and other vibrations. A new concept of silent road surfaces, the product was awarded the Golden Decibel prize in 1997.
Another award winner in this competion (1995) in France is Colsoft asphalt, a thin or very thin bituminous asphalt developed by the French road surface developer Colas, located in Boulogne Billancourt near Paris, to combat noise nuisance.
Its granular formulation incorporates chopped rubber from scrap tires. In addition to low road noise it has a matte appearance, which improves visibility by reducing reflections from light sources. Measured according to the acoustic standard S 31-119 at Benfeld in eastern France, the Colsoft 0/6 coating produced a decrease of 8 dB(A), while retaining a high degree of roughness to provide tire grip.
The formulation of these noise-reducing asphalts is often discontinous: the mixture of grit and sand, with binding provided by the incoporation of organic fibers, offers better resistance to the mechanical stresses of traffic.
Porous asphalts, which are effective at noise suppression but have been accused of clogging, are experiencing renewed popularity. Third generation high-porosity asphalt, such as Accrophone PE from the Jean Lefebvre company, based in Vitry Sur Seine, has a void percentage of around 30% and is being used more and more on the French highways.