Chapter Two: Design

Denmark

In Denmark, safety is the top priority. Although noise is a regulated property, pavement performance is still a very high priority. Therefore, life-cycle costs determine the use of most noise-abatement remedies.

Although surface texture (expressed as mean profile density (MPD) has an effect on noise, the significance of this property on overall pavement noise emission is uncertain. Pavement temperature also has an effect on noise, with hotter pavements generally being quieter than cooler pavements. Also, as the air voids increase in porous pavements, the noise generally decreases. Denmark requires porous mixes to be at or above 18 percent air voids.

The Danes have measured noise stability on several experimental pavements for several years and will continue this measurement as long as the pavements perform safely. The results are shown below in tables 2 and 3.

Note: DA refers to drainage (porous) asphalt and the following numeric refers to the aggregate size; i.e., DA8 is a porous asphalt mix with an 8-mm maximum aggregate size. AB refers to asphalt base.

The drainage asphalt (DA) with the smallest chip size (8 mm) and the highest percentage of built-in voids (over 22 percent) had the best noise reduction (3 to 4 dB) and retained its porosity. Thin open layers are being placed experimentally in urban areas to determine noise-reduction capacities while extending service life and reducing maintenance costs.

Single- and double-layer porous mixes and thin surfacing have all been used as noise-reducing pavement mixes. The porous mixes have the greatest potential to reduce noise by more than 3 to 5 dB, but have experienced performance problems (clogging, durability, etc.). The thin mixes are more cost effective and appear to be more durable, but provide only limited noise reduction (1 to 3 dB). (See figure 1 for examples of two-layer drainage asphalt.)

Figure 1. Examples of two-layer drainage asphalt.

From left to right,DA5 + DA16, DA8 + DA16, and DA5 +DA22.

The Danes have completed three case studies in which they compared the cost of PA, noise barrier, and sound insulation for three road categories: city street, ring road, and freeway. They concluded, “Compared to noise barriers and façade insulation, porous asphalt gives a much higher noise reduction per invested Euro.” However, they added this disclaimer: “The test section is only 3 years old, and it therefore is still to be proven that the pavement can maintain the noise reduction throughout its entire lifetime.” (Figure 2 illustrates the use of 8-mm porous asphalt surface in Copenhagen.)

Figure 2. Oster Sogade in Copenhagen with PA8 (8-mm porous asphalt surface).

The Netherlands

“…the cost effectiveness of silent road surfaces in general, and that of two-layered porous asphalt especially, is very high. In other words: the investment per reduced decibel is much lower for such a road surface than for sound barriers or façade isolation.”—G. G. van Bochove, Heijmans Infrastructuur

All new roadway construction must not exceed a noise level of 50 dB and the noise levels of all existing roadways that are reconstructed must fall below 55 dB. Any widened roadway must not exceed the current noise limits (stand-still principle). Safety is still a top priority, and skid resistance is required for all new surfacing. Consideration is being given to requiring a skid warranty, and future projects may include an acoustical warranty. All pavements are now warranted for 3 years and are awarded on a low-bid basis.

In a recent comparison test of dense asphalt concrete, PA (single-layer), TLPA, and thin top layer (Microflex 0/6), TLPA was quieter at all speeds tested (30 to 130 kilometers per hour (km/h), as much as 4 dB quieter than the next best mixes (thin layer and porous single layer) at high speed (130 km/h), and as much as 9 dB quieter than conventional dense-graded asphalt. TLPA, therefore, is especially interesting for the main highway system where traffic speeds are higher and sound reduction the greatest (see figures 3 and 4).

Figure 3. Two-layer porous asphalt (TLPA).

Figure 4. Experimental two-layer porous mix on A28.

Another consideration noted by the Dutch was that these porous mixes do not perform as well as conventional dense mixes when there is more braking, acceleration, and turning, or “wringing” actions, as might be expected in urban areas. These considerations make a case for the following application classifications:

• National highways—PA or TLPA
• Inner-city roads—thin, semi-dense top layers

The current thin gap-graded asphalt pavements are achieving 8 to 10 years of life, while the previous dense-graded mixes lasted 10 to 12 years. Aggregate size, void structure, binder properties, skid resistance, and mix durability are all considered critical mix properties. The porous mixes have the greatest potential to reduce noise by more than 3 to 5 dB (at greater than 30 to 55 km/h). The thin mixes are more cost effective and appear to be more durable, but provide only limited noise reductions (1 to 3 dB). However, the thin single-mix layers work better in urban (low-speed) conditions than the two-layer systems.

Although porous pavements are slightly more expensive, they produce a 50 percent cost efficiency compared to the same reduction of noise by barrier. Current barrier costs are estimated at €400 to €500 per square meter. The high costs are associated with the extensive foundation structural support needed in the Netherlands.

France

“The difference in the noise between the reference pavement and the porous pavement increases as the distance from the pavement increases. The reverse is true for noise barrier walls.”—Michel Berengier, Laboratoire Central des Ponts et Chaussees

France uses several different pavement designs for noise reduction. The following techniques are employed:

• Use separate structural and surface characteristics.
• Use best-quality aggregates.
• Adjust pavement dressing to noise characteristics.
• As a general rule, use smaller grain size for quieter pavements.
• Use smaller aggregate size for best adhesion (skid).

In the past, France has used thicker surfacing (5 to 8 centimeters (cm) and continuous grading to ensure good waterproofing of the pavement. Today, France has separated the structural function from the surface function. Therefore, very thin (2-to-3-cm) and ultrathin (1.5-cm) mixes were developed to improve the surface characteristics (skid resistance, noise). The ultrathin mixes developed in France 10 years ago are not used as much today because these mixes cost as much as the very thin mixes. The grading composition of mixes being used today is 0/6 mm and 0/10 mm gap graded. These surface mixes are usually 25 to 30 mm thick with 5.7 to 5.9 percent asphalt. France has not experienced problems with these mixes. Figure 5 illustrates levels of tire-road noise with different surface mixes.

France has discontinued the use of the 0/14 mm mixes because of inferior performance in terms of adherence (not enough microtexture), and because they are too noisy. In situ noise is measured 2 to 3 months after construction to determine compliance with noise criteria (60 dB during the day and 55 dB at night). If it does not meet requirements, then corrections are made. For PA mixes, the French have measured a 1-dB increase over a 6-year service life.

Figure 5. Tire-road noise.

ES = seal coat, BBSG = normal dense mix, BBUM = ultrathin surface,

BBTM = very thin surface, BBDr = porous mix

Safety issues are not compromised to achieve noise reduction. Skid enhancement and spray reduction are deemed more important than noise reduction. Rutting resistance, shear resistance, smoothness, and cracking are considered as critical as noise reduction. Service life performance issues are traded for improved safety and noise enhancement issues, but a program of performance enhancement continues. After much experimentation, the French have found that the 0/6-mm extreme gap is most effective at reducing noise and increasing skid (see table 4).

Note: The plus (+) sign indicates an improved characteristic over conventional surfacing mixtures. The minus (-) sign indicates a negative improvement. The number of signs associated indicates a relative degree of change.

These noise-reducing mixes are considered sacrificial layers and are not given any structural value in pavement design. However, the French estimate that these mixes have about half the modulus of dense-graded mixes.

Although the service lives of the current mix formulas are anticipated to exceed 15 years, older mixes are being recycled after 10 to 12 years.

Aggregate size, void structure, binder properties, skid resistance, and mix durability are all considered critical mix properties. Gap grading seems to increase raveling potential, but the addition of 7 to 10 percent sand mortar has helped resist raveling. The wearing course is replaced within 10 to 15 years. The French also placed PA on one continuously reinforced concrete pavement (CRCP).

Lightweight aggregates (expanded clay) have been used for skid resistance in areas with few good natural aggregates. These synthetic aggregates may provide slight improvements in noise reduction. Rubber has been used in the mix binder and has reduced noise by up to 1 additional dB. The optimum rubber content is 1 to 2 percent rubber. Rubber has also been used in the very thin mixes.

Italy

About 35 percent of the Autostrade (1,868 km) was surfaced with PA by the end of 2003, but most of this PA is composed of 0/16 mm aggregate size. The new formula tends toward a smaller maximum aggregate size of 0/11 mm or perhaps even 0/8 mm.

A minimum level of skid resistance is required on all new pavements and is monitored annually under the pavement management program. Ride quality is also a highly regulated attribute. Currently, performance and pavement life are not as good as with conventional dense-graded mixes, but performance research is a continuing program.

Some of the first porous mixes were placed in the late 1980s. A properly designed mix constructed with due care has about an 80 to 90 percent life performance, compared to a quality dense-graded mix. Aggregate size, void structure, binder properties, skid resistance, and mix durability are considered critical mix design properties. The porous mixes are specified to achieve a minimum of 23 percent air voids, and most are constructed at about 30 percent air voids.

United Kingdom

Paving contracts require the use of quiet surfaces, but contractors are allowed to decide what type of surfacing products to use for the roads they build. More than 32 approved proprietary surfacing systems meet safety and noise requirements. The approval process is known as the Highway Product Approval System, or HAPAS. To obtain HAPAS approval, the product must be proven to perform in an extensive range of quality tests, including skid resistance, drainage, and durability, with an optional test of noise generation. In addition, the products must perform in situ as indicated during HAPAS testing for at least 3 years. The British are considering an extension to this warranty period. If the product does not meet the specified minimum noise or skid requirement during the warranty period, it is removed and replaced. If it fails other requirements, the product may be permitted to remain in place at no pay. Noise-reduction properties are compared to the performance of HRA using a noise prediction algorithm. To obtain approval as a quiet pavement mixture, it must provide at least a 2.5 dB noise reduction (compared to HRA).

The British experimented with EAC, which was reported to achieve a 3 dB reduction compared with a standard brushed finish with minimum texture (1.2 mm). The British reported a 10 percent cost increase using EAC surface. Thin-layer bituminous surfacings have replaced EAC as a more cost-effective way to reduce noise. Current policy does not allow concrete pavement to be used as the finished surface. Any new concrete pavement is considered a supporting base with a required quiet pavement surfacing. Even so, 40 percent of new roads are CRCP (almost exclusive use of CRCP in England) with a thin surface layer. The public has responded favorably to the use of noise-reducing surfaces and is especially impressed by improvement in ride comfort.

The thin surfacing mixes are generically similar to SMA, but are proprietary formulations using modified binders and a closely controlled aggregate mix. The mixes are not as difficult to construct and maintain as the PA mixes, and have reported service lives of 12 years compared to the HRA that typically lasted 15 years. The primary differences in PAs and thin surfacings are cost (thin surfacings are about half the thickness and so cheaper and quicker to lay) and texture. PAs and thin surfacings are both negatively textured, having a relatively smooth running surface. Where PAs have interconnected voids below the surface, the aggregate particles in thin surfacings are embedded in binder. The primary failure mode of the thin surfacing mixes is raveling after the binder has started to oxidize.

The British allow the same structural design value for thin surfacings as for conventional asphalt mixes. Porous asphalt mixes are assigned 50 percent structural credit. The top layer contains the quality of aggregate needed to provide skidding resistance. The layer thickness for porous asphalt is 50 mm and for thin surfaces 20 to 35 mm. This material is reusable in future construction when the pavement is recycled.