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.
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.
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
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