Thin Layer Pavements – Long Time Effect of Noise Reduction

In 2003 the Danish Road Directorate (DRD) started a co-operation with local administrations and asphalt contractors to develop noise reducing thin layer pavements and to construct such pavements on test sections. This article contains a status of the noise measurements carried out in the period from 2003 to 2010.

Test Sections
In 2010 the Danish Road Directorate has a total of 44 test sections with thin layer noise reducing pavement distributed on six road stretches. In 2003 and 2004, the first test sections with the first generation of thin noise reducing pavement were established at two urban roads Kongelundsvej and Udbyhøjvej and at highway M10 near Solrød.

On the basis of the experiences from these test sections, an optimisation of the noise reducing properties was carried out. In the following year new test sections were constructed, in 2006 at highway M64 in Herning and in 2007 at the urban road Kastrupvej. Finally, on the background of the experiences from the first Herning experiment, a further optimisation of thin layer pavement was performed and another test section was constructed in 2008 at highway M68 also in Herning.

At each site a reference pavement was also constructed as a dense asphalt concrete (DAC11t). Comparisons are made between the noise levels of the test pavements and the reference pavement, so that the reference pavement and the thin layer pavement have the same age and have been exposed to the same traffic and weather conditions.

Figure 1: Detailed SPB noise measurements attest section with optimized noise reducing thin layers in Copenhagen.

Detailed road side SPB noise measurements have been carried out on the test sections with the thin layer noise reducing pavement. On the basis of these noise measurements, it can be concluded that these noise reducing thin pavements have a noise reducing effect, also when they become old and worn down. In addition to that, the noise levels rise with time, as the pavements age. This applies for all the pavement types examined. However, the amount of noise increase depends on the type of pavements.

The reference pavements
The tendency for the dense reference pavements (DAC11t) is as follows:

1. For passenger cars on urban roads with a shown speed of 50-60 km/h, the increase is 0.3 dB/year
2. There is not a sufficient amount of data to evaluate the increase of noise from heavy vehicles on urban road.
3. For passenger cars on main roads/motorways with a speed of 80-110 km/h, the increase is 0.4 dB/year
4. For multi-axle heavy vehicles on main roads/motorways, the increase is 0.2 dB/year which is half of the increase for passenger cars

Frequency analyses show an increase in the vibration generated low frequency noise (500-1600 Hz). This indicates that the road pavement becomes rougher on the surface, but does not get a more dense surface structure. One hypothesis could be that the aggregates on the surface are exposed and thus the surface becomes rougher.

Noise reducing thin layers urban road

Figure 2: Development of pass-by noise measured according to the SPB method on the thin layer test sections at the urban test road Kongelundsvej.

As regards noise reducing thin layer pavements for urban roads (50-60 km/h), the tendencies are as follows:

1. The best first generation pavements are SMA6+ on Udbyhøjvej with an average noise reduction of 1.7 dB during the measuring period of seven years
2. The average noise reduction for the first generation pavements is a little more than 1 dB during the measuring period of seven years
3. The best second generation pavement is an open graded asphalt concrete (OGAC6) with an average noise reduction of 4.2 dB during a measuring period of three years
4. The average noise reduction for the second generation pavements is a little more than 2 dB during the measuring period of three years
5. The average annual noise increase is 0.5 dB/year for the first generation pavements and 0.7 dB/year for second generation pavements
6. Since the annual increase of the reference pavements is 0.3 dB/year, the noise reducing properties decrease over time

Frequency analyses indicate that the thin layer pavements gets rougher and less open surface structure over the years.

Noise reducing thin layers main roads/motorways
For noise reducing thin layer pavements on main roads/motorways (80-110 km/h) the tendencies are as follows for passenger cars:

1. The best first generation thin layer pavements are an open graded asphalt concrete (OGAC8) with an average noise reduction of 2.0 dB during a measuring period of six years
2. The average noise reduction for first generation pavements is over 1.3 dB during a measuring period of six years
3. The best second generation thin layer pavement is a thin semi porous asphalt (DA6) which has an average noise reduction of 3.4 dB during a measuring period of two years
4. The average noise reduction of second generation pavements lies at 1.2 dB during a measuring period of two years
5. The average annual increase of noise is 0.6 dB/year for the first generation pavements and 0.7 dB/year for second generation pavements
6. Since the annual increase of the reference pavements is 0.4 dB/year, the noise reducing properties decrease over time

The annual increase in noise from heavy vehicles on motorways is around half of the increase for passenger cars for the reference pavements – about 1/3 for the thin layer pavement.
There were not many heavy vehicles on urban roads when measurements were made, therefor results from measurements for such vehicles have been excluded in this report. It has been planned to make a separate study of heavy vehicles on urban roads and main roads.

Perspective
It is the intention to continue these measurement series throughout the total lifetime of the test sites to the extent that it is economically possible. In that way it will be possible to create unique knowledge about the development of noise on the many different pavements from “cradle to grave”. This knowledge can be used to:

1. Examine and analyse why and how noise levels rise on various types of pavement
2. Optimisation and improvement of noise reducing thin layer pavements
3. Calculation of the noise reducing effect during the lifetime of the pavements
4. Use when choosing noise reducing pavements
5. Development of the so called Danish “SRS” system for noise classification of noise reducing road pavements
6. Development of acoustic ageing models for road pavements in Pavement Management Systems
7. Improve noise emission data for the noise calculation model NORD2000 which is the official Danish calculation model

Contact: Hans Bendtsen hbe@vd.dk DRD, Denmark

Contact: Lykke Møller Iversen lykk@vd.dk DRD, Denmark