Patent Description:
Top layers based on bitumen and chippings are known per se. A known example is the European patent <CIT> granted to the present applicant. The aforementioned European patent discloses a method for treating an asphalt construction with a liquid agent, in particular an asphalt construction which comprises a top layer of very porous asphalt concrete (ZOAB), in which a liquid binder is sprayed onto the top layer of the asphalt construction so that the liquid binder penetrates the top layer and forms a binder sealing layer in the top layer in an area at the location of the bottom side of the top layer.

<CIT> relates to a process for the production of structured and/or coloured bituminous coverings for roads and paths. Such coverings are made from a grain-graded mineral mixture and consist of bitumen as a binder. The mixed material contains between <NUM> and <NUM> parts by weight of a coarse grain (grit) of approx. <NUM> to approx. <NUM>, which represents the colouring and structuring component. According to the process the mortar embedding the coarse grain together with the binding agent (bitumen) is removed to a maximum depth of <NUM>% of the diameter of the coarse grain on the surface of the covering by treatment with high-pressure water. The layer exposed by high pressure water treatment has a depth of between about <NUM> and about <NUM>, and the high-pressure water jet has an absolute pressure of more than <NUM> bar, wherein the amount of water per m<NUM> of surface to be removed is between about <NUM> and <NUM> litres with an exposure time of at least <NUM> seconds and a water temperature between ambient temperature and about <NUM><NUM>C.

<CIT> relates to a method of anti-skid adaptation of wearing course of the roadway wherein the base of the wearing course, without its destruction, is at first deeply cleaned and roughened to unify the properties of the macrostructure of the wearing course, after which the upper surface of the wearing course adapted in this way is fixed by means of binder and subsequently during the binder setting a sanding with abrasive is carried out, wherein the adhesion of the binder is defined by the force required for tearing the binder from the wearing course.

Porous or open layers, such as ZOAB (Very Porous Asphalt Concrete), have been used as a top layer or upper layers of a road surface for years. The advantages thereof are known and provide significant advantages both for the road user and the environment. Rainwater is absorbed into the porous layer, so that no water splashes or sprays are created when it rains which could hamper the view of the road user. In addition, the porosity absorbs and reduces the driving noise of vehicles, so that there is less noise pollution as a result of road traffic in the vicinity of the road. A two-layer very porous asphalt concrete is a particular form of a porous road surface. In this case, two layers of porous asphalt on top of each other are used; the bottom layer is a coarse-grained layer with large pores which allows for satisfactory water flow in and through the layer and, on top thereof, a relatively thin fine-grained top layer is provided which is optimized to reduce traffic noise.

Runways for aircraft have particular requirements. Not only is it essential that the functional properties are retained, including skid resistance and texture depth, but the costs and the operational opportunities and risks have to be considered, including the availability of runways. By coating a runway with a so-called antiskid top layer, it becomes so skid-resistant that a landing aircraft can use the rapid-exit taxiway more often. In addition, the runway also remains skid-resistant in severe weather conditions due to the high amount of texture, the reduced risk of hydroplaning and the intended surface condition 'dry when wet'. The skid-resistant top layer is not porous - but there is positive texture combined with slope in the track so that the water flows over the surface to the side. The differences between the tops and the valleys of the infill material provide water-retaining capacity. The skid-resistant top layer increases the directional stability and results in an aircraft having a shorter braking distance. As a result thereof, an aircraft can use the rapid-exit taxiway much more often. Thus, the runway is used more efficiently, and more flight movements are possible. Even if an aircraft is relatively heavy, it is still able to land safely in adverse weather conditions.

European application <CIT> discloses a top layer for roads provided with a water-permeable road surface, consisting of a water-draining binder layer between a road bed and the road surface, wherein the road surface is composed of a porous supporting structure made of aggregate materials and bitumen or polymer bitumen as binder and a reaction resin surface with open pores which reinforces the porous supporting structure and is filled in with sand and/or coarse aggregate materials having different properties, wherein fibres, in particular cellulose fibres or rubber fibres, are added to the bitumen or polymer bitumen. In order to adjust the desired viscosity during processing, the reaction resin preferably has a filler content of <NUM> to <NUM> % by mass, preferably <NUM> to <NUM> % by mass, with barium sulphate, calcium carbonate, sheet silicates (such as talcum or mica), silica, aluminium oxide, diatomaceous earth, magnesium silicate or organic and inorganic fibres being mentioned as suitable fillers for the reaction resin.

American publication <CIT> discloses a runway which is composed of a gradation layer and a surface layer which is paved onto the gradation layer and is composed of concrete or reinforced concrete.

It is an object of the present invention to provide a method for applying a top layer comprising bitumen and chippings (road metal, stone chippings) onto a substrate, wherein it is possible to obtain the desired physical properties of the top layer which has been applied in this way.

It is another object of the present invention to provide a method for applying a top layer comprising bitumen and chippings onto a substrate, wherein the top layer is suitable to be used for a runway.

It is yet another object of the present invention to provide a method for applying a top layer comprising bitumen and chippings onto a substrate, wherein the application may be performed under a wide range of weather conditions.

It is yet another object of the present invention to provide a method for applying a top layer comprising bitumen and chippings onto a substrate, wherein the resultant construction is immediately fully operational after opening.

The present invention thus relates to a method for applying a top layer comprising bitumen and chippings onto a substrate, which method comprises the following steps:.

wherein step vi) is repeated if the texture depth measured in step v) is less than the reference value, wherein step vi) is terminated if the texture depth measured in step v) is equal to or greater than the reference value, wherein in step vi) the water pressure is in a range of <NUM> bar - <NUM> bar, the amount of water is in a range of <NUM>-<NUM>/m<NUM> and the speed of travel is in a range of <NUM> - <NUM><NUM> per minute.

By applying such a method, one or more objectives are achieved. The inventors have found that the water-jetting technique, i.e. step vi), is a suitable technique for removing the bitumen skin in a controlled manner, that is up to the desired depth and to the desired degree, in which case the water pressure and the amount of water to be used and the speed of travel of the water jet play a part. It is desirable for these parameters to be adjustable and monitorable.

According to the present method the step of measuring the texture depth of the cooled composition applied onto the substrate is done before the step of water-jetting is carried out, wherein the step of water-jetting is repeated if the texture depth measured in step v) is less than the reference value. In the situation where the texture depth measured in step v) is equal to or greater than the reference value, the step of water-jetting is terminated. From the present method it is clear that the zero measurement (the initial measurement of the texture depth of the cooled composition applied onto the substrate) is an essential feature because the process conditions of the next step, i.e. the step of water-jetting, are based on objective data and not on experience or visual assessment. This will result in a well-defined, reproducible and structured top layer.

The inventors have thus found that it is desirable to remove a sufficient degree of the bitumen skin. This is understood to mean that some amount of bitumen may remain behind on the surface, but this remaining amount of bitumen has to be so little that no more bitumen film is formed. This is of particular importance because the bitumen becomes hot to such a degree that it becomes liquid due to the heat generated by the friction of the aircraft tyres and due to the cavities between the pebbles at the surface being opened further. The desired degree of removal of bitumen skin is, in particular, determined by the speed of travel and water pressure of the equipment which is used for water-jetting. The depth up to which the bitumen skin is removed is influenced by the angle of the water-jetting. The water pressure is in a range of <NUM> bar - <NUM> bar, the amount of water is in a range of <NUM>-<NUM>/m<NUM> and the speed of travel is in a range of <NUM> - <NUM><NUM> per minute. A water pressure of <NUM> bar, an amount of water of <NUM>-<NUM>/m<NUM> and a speed of travel of <NUM><NUM> per minute/<NUM><NUM> per hour may be mentioned as a suitable base setting for the equipment which is used for water-jetting. The indication "m<NUM>" relates to "per linear metre" and m<NUM> relates to square meter surface. It is possible that one or more settings may be adjusted on the basis of the measured texture depth after a first operation of the water-jetting has been performed. However, for quality reasons and reproducibility it is preferred to measure the texture depth. Thereafter, a second operation of the water-jetting may be desirable, for example using a water pressure of <NUM> bar and a speed of travel of <NUM> per hour, but other settings are also possible. If, for example, the texture depth is less than the required texture depth, not only the pressure used for water-jetting may be increased but the speed of travel and the amount of water as well.

The invention is thus aimed in particular at an asphalt cover or top layer which is designed, after an aftertreatment, to function as the surface of a runway for aircraft. The asphalt cover or top layer obtained by the present invention is understood to be non-water-permeable, porous and draining, but with water-discharging and (limited) water-retaining properties due to the texture of the asphalt. In contrast to the very porous asphalt concrete explained previously in the present application, the cavities between the pebbles in the asphalt cover or top layer obtained by the present method are not connected to each other. The water-draining capacity of the asphalt cover or top layer obtained by the present method is influenced by the cross-sectional profile of the runway according to a specific standard (in particular EASA, European Union Aviation Safety Agency) and additional requirements.

According to the present invention step v) is performed before water-jetting the cooled composition applied onto the substrate (step vi)), i.e. a step v) in which the texture depth of the top layer applied in this manner is measured. The texture depth provides information about the water-retaining capacity of the pavement surface. The texture depth is an indication of the skid resistance of the top layer when it rains.

According to the present invention the texture depth measured in step v) is compared to a reference value, with the water-jetting of the cooled composition applied onto the substrate being repeated (step vi)) if the texture depth measured in step v) is less than the reference value. It is desirable for the water-jetting of the cooled composition applied onto the substrate (step vi)) to be terminated the moment the texture depth measured in step v) is equal to or larger than the reference value.

In one embodiment of the invention, it is desirable for the composition applied onto the substrate to be cooled down to a temperature of at most <NUM> before water-jetting of the cooled composition applied onto the substrate (step vi)) is performed. The present inventors have found that after the composition, also referred to as the cover layer, has been applied and compacted, the entirety has to cool down and the bitumen of the composition has to harden. This is also referred to as "setting" which involves a certain waiting time. If the water-jetting (step vi)) is started too soon, the mastic of the composition is still too soft, as a result of which the water-jetting will not be effective or may even be counterproductive, because the bitumen layer on the surface is not removed, but is pushed into the cavities between the pebbles of the composition. This does not result in the desired degree of skid resistance.

In one embodiment of the invention, the time between allowing the compacted composition which has been applied onto the substrate (step iii)) to cool down to a temperature of at most <NUM> and subsequently water-jetting the composition applied onto the substrate and cooled in this way (step vi)) is at least <NUM> hours, in particular at least <NUM> hours, preferably at least <NUM> hours.

According to an embodiment of the invention, the ASFT skid resistance of the cooled composition which has been applied onto the substrate is measured both before and after the water-jetting of the composition applied onto the substrate and cooled in this way (step vi)).

According to an embodiment of the invention, the composition mentioned in step i) furthermore comprises one or more constituents selected from the group comprising sand, polymers and limestone. The term polymers is also understood to include fibres, in particular natural and synthetic fibres.

According to an embodiment of the invention, the heating of the composition from i) also comprises mixing the constituents homogenously, so that a composition in which the constituents have been mixed homogenously is used in step iii) for applying the heated composition from ii) onto the substrate and compacting it.

It is therefore desirable for the texture depth to be measured before and after water-jetting, for example using the sand patch method and/or a digital texture depth meter. Measuring the texture depth before water-jetting indicates the starting situation and the measurement results after water-jetting are used to determine if the surface meets the requirements for the texture depth or if additional water-jetting is required to get the texture depth to the desired level. It is preferred to measure the texture depth using a digital texture depth meter, as will be explained below.

The present inventors have found that the surface of the applied top layer does not yet have the intended texture depth after applying and compacting, i.e. step iii). Therefore, an additional step, that is to say step vi), is required to remove the bitumen skin from the surface by means of water-jetting. Any bitumen skin which is still present may, for example, have an adverse effect on the skid resistance when heat is generated, in particular due to the tyres of aircraft during landing or take-off, or extreme heat during warm periods, such as the summer.

The sand patch method (RAW test <NUM>) is a method for measuring the texture depth. One protocol for determining the texture depth is as follows. The texture depth is tested on a dry, tack-free substrate which has been brushed clean beforehand. Fill a cylinder with a volume of <NUM>,<NUM> (± <NUM>) mm<NUM> and an internal diameter of <NUM> (± <NUM>) mm entirely with fire-dried round-grain sand with a grain size of between <NUM>- and <NUM>-mm. Skim off any excess of sand by using a flat slat. Then pour the measured amount of sand onto the spot on the pavement surface where the measurement will take place. If necessary, take precautions during the trial run to prevent the sand from being blow away. Distribute the sand over the surface using a flat brass disc having a diameter of <NUM> (± <NUM>) mm and coated with a hard rubber disc with a thickness of <NUM> (± <NUM>,<NUM>) mm on the bottom side and provided with a handle on the top side. Distribute the sand by rotating the brass disc which should in this case be held horizontally. Rubbing should result in a circular sand patch, with the depths in the surface being filled up to the highest points. Then measure, to an accuracy of <NUM>, the diameter of the sand patch in four directions at an angle of <NUM> degrees with respect to each other. Calculate the texture depth, to an accuracy of <NUM>, using the formula: <MAT> in which: TD is the texture depth in mm, Di is the measured diameter of the sand patch in mm. The texture depth is determined in at least <NUM> different spots.

Another method for determining the texture depth comprises using a digital meter. A digital meter uses a laser sensor to measure the depth at one location. The sensor has a specific diameter, for example <NUM> millimetres, with a circle having a certain circumference being measured, for example <NUM> millimetres. The associated circle is measured in its entirety, following which the mean value is determined for the average.

The digital meter has some advantages over the sand patch method, namely i) the speed of the measurement, ii) the objectivity of the measurement, iii) less susceptible to weather conditions (wind and water) and, iv) the digital meter has a GPS signal by means of which the measurements can be visualised later by means of a GIS environment and can thus be stored in a database. The rotating high-resolution laser of the laser-controlled texture depth meter scans the road surface at a high resolution. A commercially available laser-controlled texture depth meter is ELAtextur (brand name) which is used to determine the macrotexture of road surfaces according to the directives of EN ISO <NUM>-<NUM> and ASTM E1845-<NUM>.

It is possible to perform the texture depth measurements after the water-jetting according to the so-called Hammersley method. The Hammersley method uses a semi-arbitrary sequence which is repeated every 500x60 metres. Every 500x60 metres across the length of a substrate, <NUM> measurements are performed in order to arrive at a margin of error of less than <NUM> percent. If this is done, for example, across a length of a substrate of <NUM> metres, the total number of measurements is <NUM>.

It is also desirable to measure the ASFT skid resistance before and after water-jetting. The results before the water-jetting give an insight into the initial skid resistance, that is to say the initial situation in which the bitumen skin is still present. The skid resistance after water-jetting gives an insight into whether the skid resistance of the surface of the top layer meets the specified requirements, i.e. ASFT ≥ <NUM>, preferably ASFT ≥ <NUM>. A method for measuring ASFT is known, for example, from American publication <CIT>, which document, in particular the measuring method described therein, is deemed to be incorporated herein by reference.

A top layer comprising bitumen and chippings, which top layer is applied onto a substrate, has a texture depth ≥<NUM>, in particular ≥<NUM>, preferably ≥ <NUM>, particularly preferably >= <NUM>. In this case, the texture depth is measured according to a sand patch method which is described in more detail in the present description and/or by means of a digital texture depth meter, preferably a digital texture depth meter.

A top layer according to the present invention is characterized by a negative texture, to be understood as hollow spaces in the surface and hollow spaces which extend downwards in the surface. The top layers which are currently commercially available and used are described as top layers having a positive texture, to be understood as gravel which protrudes from the surface. The present texture which, in the context of the invention, is to be seen as a negative texture, ensures that the surface of the top layer is less susceptible to decreasing skid resistance and texture depth during its operational life.

The present invention also relates to a runway for aircraft which is provided with a top layer obtained according to a method as described above or which is provided with a top layer as described above.

Thus, the present invention relates to a runway for aircraft in which an ASFT skid resistance for the runway applies which is at least greater than <NUM>, preferably greater than <NUM>.

The present invention is explained in more detail by means of an example, but it should be noted that the invention is by no means limited to such an example.

In an example of the present method the composition comprising bitumen and chippings according to i) is characterized as a discontinuous graded composition.

In an example of the present method the proportion of the split with the largest grain diameter in the composition is at least <NUM> vol. %, preferably at least <NUM> vol. %, based on the total weight of the composition.

The term "discontinuous graded composition" is well known in this field and includes a composition in which one specific grade dominates. In other words, such a composition lacks a plurality of different grades wherein the particle size of each grade is close to the particle size of another grade. For example, <CIT> as discussed above typically discloses a continuous graded composition. In that context <CIT> discloses a composition of <NUM>-<NUM> parts by weight of grit <NUM>/<NUM>, <NUM>-<NUM> parts by weight of grit <NUM>/<NUM>, <NUM>-<NUM> parts by weight of grit <NUM>/<NUM> and <NUM>-<NUM> parts by weight of grit <NUM>/<NUM>. According to <CIT> the proportion of the split with the largest grain diameter in the mixture is between about <NUM> and <NUM> parts by weight.

A composition consisting of <NUM> % by weight of fine sand, <NUM> % by weight of Quarz Grauwacke <NUM>/<NUM>, <NUM> % by weight of modified bitumen, <NUM> % by weight of polyacrylonitrile polymer, <NUM> % by weight of average limestone filler was heated, applied onto a solid substrate and compacted using an asphalt spreader. Compacting usually takes place by means of a roller. After a substrate had been applied, the asphalt layer which had thus been applied was first cooled down to at least <NUM>. Furthermore, the asphalt layer had to set for at least <NUM> hours. In the period between <NUM> and <NUM> hours, the temperature on the surface of the asphalt was measured at least twice. During this <NUM>-hour period, the asphalt temperature was found to be below <NUM>, so that the water-jetting was started after the <NUM>-hour period had passed, with a water pressure of <NUM> bar and a speed of travel of <NUM><NUM> per minute/<NUM><NUM> per hour being used. After the water-jetting, the texture depth was found to have a mean value of <NUM>. The ASFT skid resistance of the asphalt layer had a value of <NUM>. The texture depth was measured using the commercially available laser-controlled texture depth meter ELAtextur (brand name) according to the directives of EN ISO <NUM>-<NUM> and ASTM E1845-<NUM>.

The same composition and equipment as in Example <NUM> were used, except that the water-jetting was performed in two separate steps. After a first stage of water-jetting, the texture depth was <NUM>, which value was increased, after a second stage of water-jetting, to a mean value of <NUM>. The texture depth was measured using a commercially available laser-controlled texture depth meter ELAtextur (brand name) according to the directives of EN ISO <NUM>-<NUM> and ASTM E1845-<NUM>.

Claim 1:
Method for applying a top layer comprising bitumen and chippings onto a substrate, comprising the following steps:
i) providing a composition comprising bitumen and chippings,
ii) heating the composition from i),
iii) applying the heated composition from ii) onto the substrate and compacting it,
iv) allowing the composition applied onto the substrate to cool,
v) measuring the texture depth of the cooled composition applied onto the substrate from iv), wherein the measured texture depth is compared to a reference value,
vi) water-jetting the cooled composition applied onto the substrate from iv),
wherein step vi) is repeated if the texture depth measured in step v) is less than the reference value, wherein step vi) is terminated if the texture depth measured in step v) is equal to or greater than the reference value, characterized in that in step vi) the water pressure is in a range of <NUM> bar - <NUM> bar, the amount of water is in a range of <NUM>-<NUM>/m<NUM> and the speed of travel is in a range of <NUM> - <NUM><NUM> per minute.