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then creates noise. This noise is in the frequency area less than 1000 Hz. The more even the road pavement, the less noise.
• Noise from air pumping occurs when a tyre goes over a pavement. As the rubber blocks of the tyre hit the road surface, they are compressed, and thus the air between the rubber blocks is pushed out. When the rubber blocks are released from the road surface, air is sucked in again. These quick movements in the air result in high frequency noise over 1000 Hz. If the road pavement has an open structure the air will to a greater extent be pressed into the pavement and be sucked up again, which reduces the level of noise produced.
When comparing noise levels, it is always necessary to have a reference pavement against which the noise level can be compared. The layer thickness has in fact no direct influence on the noise level from noise reducing pavements. But as mentioned above, there is an indirect effect by using a little maximum aggregate size of 6 or 8 mm. Since as a rule of thumb, pavements usually are constructed three times the size of the aggregate in thickness, noise reducing pavements with an open structure will usually have a small layer thickness of typically 15-25 mm. This can give a noise reducing effect. However, not all thin pavements are noise-reducing.
Noise reduction
In France, researchers and asphalt producers have worked for a number of years to develop noise reducing thin layer pavements. Thin pavements are divided into two classes depending on the voids in the pavement mix. They have a thickness of 20-30 mm. Also a type of ultra thin pavement is being considered with a thickness of 15-20 mm. In order to obtain an efficient sealing, an emulsion layer is usually put underneath the thin pavement. Modified bitumen is used in places where there is a risk of shear forces. According to the French Road |
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Research Laboratory, LCPC, thin pavements are resistant to rutting, they reduce splashing from rain and have a high friction. It would seem that pavements with 6 mm aggregate have a better friction than pavements with 10 mm aggregate.
Noise reductions were measured in France in relation to a dense asphalt concrete with 10 mm maximum aggregate size, which basically corresponds to the Danish reference pavement. Noise is measured at 80 km/h and 90 km/h. In general, noise reductions are measured on thin layer pavements of 2-4 dB, which is less than can obtained on a drainage asphalt, which is 4-6 dB. It can be seen that the best noise reduction can be obtained on pavements with the smallest aggregate and for Class 2 pavements with the largest possible voids. The noise reduction is in general somewhat greater for cars than lorries.
Hans Bendtsen
Literature:
1. The DRI-DWW Noise Abatement Programme —Project description. Technical Note 24, 2005.
Danish Road Directorate, Danish Road Institute
2. International experiences with thin Iayer pavements.
Technical Note 29, 2005. Danish Road Directorate, Danish Road institute
3. French experience on noise reducing thin Iayer.
Technical Note 28, 2005. Danish Road Directorate, Danish Road Institute
Article specially written for Nordic Road & Transport Research by Senior Researcher Hans Bendtsen, Road Directorate Danish Road Institute
hbe@vd.dk. |
