Quiet roads : engineering aspects

Basic assumptions

Improvement of acoustic properties of conventional porous asphalt requires in general finer and more single-grained mixture. The use of aggregate 8/11 instead of 6/16, leads to better reduction of traffic noise. Aiming for finer mixtures seems to be in contradiction with the problems of pollution. This requires on the other hand coarse, single-grained mixtures. Because bigger stones will result into bigger water carrying voids, by which the discharge resistance will reduce. Water will run off more quickly and dirt will get less chance to form itself in the voids. Already formed dirt will erode sooner. On the other hand within a coarse mixture, with large voids on the surface, more dirt will penetrate into the surface layer.

Two layered construction

The mentioned considerations have led to the development of a two layered construction by Heijmans Civil Engineering, the so-called Twinlay. Twinlay is made up of a bottom layer of porous asphalt with a coarse single-grained aggregate (11/16) and a thin top layer of fine porous asphalt (4/8). Because the stone particles of the top layer can "settle" into the coarse texture of the bottom layer (4.5 cm), a relatively thin top layer can be applied (2.5 cm). The two- layered porous asphalt concept has, in comparison with conventional open graded asphalt concrete, the following advantages: the fine top layer offers acoustic advantage through the porosity and the fine surface texture. The top layer prevents coarse dirt or temporary a large amount of dirt from entering the construction (sieve effect). Dirt which nevertheless penetrates in the open structure is absorbed at the top of the top layer. From there it can be removed by vacuum cleaning. The difference in air flow resistance between the top and bottom layer has a positive effect on the self-cleaning capacity caused by traffic. The bottom layer has a higher discharge capacity compared to conventional porous asphalt through which the sideways discharge of water is improved considerably. Given the fine grain size of the top layer mixture, it is important to keep porosity as high as possible and the flow resistance as low as possible. This can be reached by leaving the sand fraction from the mixture. It is than however necessary to use a binder with -in the warm phase - a very high viscosity like rubberised bitumen. This improved type of binder does not only compensate the lack of sand in the mixture; the high viscosity allows also the use of a high percentage of bitumen (6.5 % ) without causing drop of binder in the warm phase. This higher bitumen percentage is essential for durability. In order to conform the service life of both layers, rubberised bitumen is also used as a binder in the bottom layer.

Mechanical resistance

Because the gradation of the aggregate of the top layer is finer compared to conventional porous asphalt, it has in general a better resistance against loads caused by running traffic. In comparison with dense mixtures (dense asphalt concrete, stone mastic asphalt) the mechanical properties are less (this applies to porous asphalt mixtures in general). The application of the two- layered porous asphalt concept is therefore not recommended on crossings. Here it is advised to use stone mastic asphalt. For that matter the low vehicle speeds as well as the larger distance to buildings make it acceptable to leave out noise reduction near junctions. Besides: also noise screens have to be interrupted near junctions. The most recent development is the application of a very fine open graded asphalt mixture (2/4) as a thin top layer (1.5 cm) in a two layered construction. This mixture with crushed stone 2-4 mm is especially developed for a better mechanical resistance against turning traffic. Experiences are still limited but the indications are positive. A additional positive effect of this very fine top layer is the better noise reduction, even in comparison with the normal two layered construction. If experiences remain favourable, also in relation to clogging, this will become the second version of two layered porous asphalt.


Roads are exposed to pollution. In case of an open graded road surface, pollution will penetrate and block the pores. With this the specific advantages of an open road surface are lost. To prevent this, maintenance is required. From porous asphalt it is known dirt settles itself through the total thickness of the layer. With the two layered-construction the pollution stays limited to the thin top layer. The filter action of the fine top layer prevents dirt from reaching the bottom layer. The sieve-effect of the top layer also retains coarse dirt or temporary a large amount of dirt, which will) run off over the surface with the water. Visual observation proved dirt doesn't enter the top layer for more than 1 to 1.5 cm. The dirt concentrated in the top layer can easily be removed with a special cleaning technique. This is done by a vacuum cleaning method; water under pressure (up to 120 bar) is sprayed onto the surface. The rotating movement of the spray nuzzles makes sure that water enters the top layer from all directions. Directly behind the spray bar the water, containing dirt, is sucked up and recycled for again entering the circuit. Cleaning the two- layered porous asphalt in this way is much more effective compared to conventional porous asphalt, because the dirt is concentrated in the upper part of the top layer. Tests have shown a totally clogged Twinlay-surface can be cleaned with good result. Depending on the severity of pollution, cleaning is required once or twice a year. After cleaning, the functional properties of the two- layered constructions are almost the same as the zero situation. In order to determine the severity of pollution, a permeability test can be performed with the instrument of Becker.

Replacement top layer

The two- layered construction is a relatively new product. Therefore the service life cannot be specified accurately. Depending on intensity of use (situation; traffic load) an estimation of 7 to 10 years can be given. Ravelling can be noted as the main pavement distress. The two layered structure offers an opportunity of only replacing the top layer. From a viewpoint of intervention as well as costs this is an interesting option because only road sections with a high traffic intensity will have to be replaced. The bottom laver, the possible dense parts (for instance cycle paths) and the drainage system will remain intact. The service life of these road sections is, compared to the open top layer in general longer. Replacing the two- layered construction top layer has been tested in practice. By means of a very fine milling technique the thin top layer has been removed. After this the bottom layer has been thorough cleaned by means of the above-mentioned vacuum cleaning-method. Then by using a special adhesive technique, the top layer is applied again. On the treated section acoustic measurements took place. From this it can be concluded acoustic specification doesn't alter from this intervention.

Behaviour in Winter Conditions

Just like conventional porous asphalt the two- layered construction requires adjusted salting operations. Preventive spreading a big amount of salt is not advisable because the salt is eventually carried off with the meltwater. For that matter use in practice has shown black ice causes less problems with the two- layered construction , compared to conventional porous asphalt. The fine structure of the top layer fixes the salt nearer to the surface.

Urban roads

Not only the higher noise reduction, but also the improved discharge of water makes the layered porous asphalt concept suitable tor urban application. For newly-built areas the new concept offers, in combination with a specially for this purpose designed drain along the kerb an excellent solution. The drain construction is made up of ceramic elements with openings on top and the side. This newly developed "Keradrain" can be combined with an adjusted combination gully. The drain can easily be flushed from gully to gully. The discharge capacity of the side intakes (with open graded asphalt placed in front) amounts to 2 litres per hole per minute. This is amply sufficient, even during heavy rainfall. Advantages of this drain are: Easy placement of small elements (length 33 cm) on the first bottom layer of asphalt concrete (compare normal tile). Therefore the drain can be placed in the last phase, just before laying the asphalt layers and after the finishing of sidewalks, car parks and verges. The drain is universal and can be used along kerbs, but also at bus stops, car parks, entrances and side roads. The drain profile can be continued at these locations. The surface water of the road sections behind the drain (parking place, bus stop) can run off to the drain and be carried off by the openings on top. The drain profile satisfies strict requirements on strength and can be used in situations with high traffic loads, even with traffic driving over the drain. For any adjoining cycle paths or parking places it is advised to change the top layer of the two- layered construction for a dense mixture; for instance stone mastic asphalt 0/6 or a tack coat. On these sections the open mixture has no functions and besides dirt which piles up on these locations. For any adjoining cycle paths or parking places it is advised to change the top layer of the two- layered construction for a dense mixture; for instance stone mastic asphalt 0/6 or a tack coat. On these sections the open mixture has no functions and besides dirt, which piles up at these locations, will not penetrate into the asphalt. Also the self-cleaning ability of porous asphalt on these locations, by the lack of the sucking effect of car tires, is almost zero. Dirt can - with the water - run off the dense top layer to the side, from where it enters the Keradrain by the openings on top and will be carried off. Water in the bottom layer runs Keradrain by the openings on top and will be carried off. Water in the bottom layer runs under the dense strip and enters the Keradrain by the side openings. This system (covering of cycle strips) will remain intact for years, the bottom layer underneath the dense top layer remains clean through the filter action of the top layer at the driving lanes. Even overlayments of existing roads with the two- layered porous asphalt construction are possible. In this case the existing tiles along the kerbs can be removed and replaced by the Keradrain. In general it can be stated that the characteristic two layered structure can offer a solution

in many situations. The construction can be applied with almost every road construction design.


The two layered porous asphalt construction opens new possibilities for application of porous surfaces to reduce traffic noise. Experiences over the last five years shows with a normal cleaning frequency (once a year) a remaining open surface. Reduction of traffic noise both at high and low car speeds (urban areas) and even with heavy trucks can be maintained over years. The most critical point is the mechanical resistance of the top layer in relation to turning traffic. On surface areas with heavy traffic loads ravelling (loss of stone at the surface) occurs. The newest development in two layered construction: a very fine mixture (crushed stone 2-4 mm, filler and rubberised binder), applied in a thin top layer (15 mm) perhaps gives an answer to this problem. Preliminary sound measurements on test tracks showed very positive results When the mechanical resistance against traffic forces is also better (indications say so), this will be the future trend in two layered porous asphalt construction.