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.
Cleaning.
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.
Conclusion
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.