A road surfacing of the general kind stated is disclosed in EP-A-0 028 238 by the present inventor. It is made up of balls or chips of rubber or some other rubber-like material, and the individual particles thereof are bonded together such that communicating spaces are formed between the particles, thus forming air-permeable channels throughout the road surfacing.

It has turned out that this road surfacing achieves the desired objectives regarding noise reduction, but its inferior strength and durability as well as difficulties in laying it out on longer road sections as compared to a normal road surfacing containing stone ballast material and bitumen makes it uneconomical for public use.

It would be desirable, thus, and it is the object of the present invention, to combine the durability of a normal road surfacing and the teachings of EP-A-0 028 238 to achieve a road surfacing being durable as well as having at least the noise reduction properties of the road surfacing according to EP-A-0 028 238.

To meet this desire, the present invention proposes a road surface material having a ballast material including granules of stone and rubber and a bitumen binder adhering to the

granules of stone and rubber, the granules of stone having particle sizes ranging from 0.5 to 24 mm, and the granules of rubber having particle sizes ranging from 0.5 to 8 mm.

It should be noted that the largest stone particle sizes (e. g.

> 11 mm) are intended for lower levels of a multi-layer road surfacing in order to provide good draining properties, and that the smaller particle sizes (about 0.5-11 mm) are those providing the road surfacing with its noise reducing properties. In other words, in a multi-layer road surfacing, there may typically be an ordinary substratum comprising macadam, and an upper, or, surface layer according to the present invention including stone particles ranging from about 0.5 to about 11 mm.

The present invention also proposes a road surfacing for reducing rolling noise, which includes particles bonded together so as to form between them air-permeable channels, and which is characterised in that the particles comprise granules of stone and rubber, and that a bitumen binder adheres to the granules of stone and rubber, the granules of stone having particle sizes ranging from 0.5 to 24 mm, and the granules of rubber having particle sizes ranging from 0.5 to 8 mm.

It is desirable that there is no contact between adjacent granules of stone, but that there shall always be a rubber particle between each two stone particles.

It should be mentioned that CH-A-410 031 discloses a road surfacing containing fine granule macadam and bitumen, which is mixed with rubber. However, the wording of this document cannot be interpreted otherwise than that the layer of macadam mentioned is completely soaked with bitumen and rubber granules, thus rendering a road surfacing layer completely

dense i. e. , having no communicating voids or channels therethrough, thus possibly making this prior art road surfacing flexible but not porous. It is well known that a road surfacing being elastic (flexible) but not porous is more noisy than an ordinary (hard) road surfacing.

The contents of rubber material used in the road surface material and the road surfacing of the present invention is about 8-40% by volume of the entire ballast material, preferably about 10-25%. Furthermore, the rubber contents are 6-25% by weight of the entire road surfacing composition.

When manufacturing a road surface material according to the present invention, a bitumen, preferably a polymer modified bitumen (PMB) polymer modified with, e. g. , SBS (Styrene- Butadien-Styrene) with a high vinyl content, is heated to at least 150°C, preferably to about 170°C-190°C. SBS, typically 1 % to 6 % of the total bitumen weight, could also be added as powder directly into the hot blender unit or directly into the main plant mixer. Rubber, for instance used tires, are ground to form rubber granules (grains) having particle sizes ranging from approximately 0.5 to approximately 8 mm. The heated bitumen is fed together with the rubber granules (at ambient temperature) into a heated blender unit, where the rubber granules are mixed into the bitumen. The mixture is heated to a temperature in the range of about 150-190 °C and is kept in the blender unit for typically 20-40 minutes, thereby achieving evaporation of possible water contents (moisture) in the rubber as well as a certain degree of melting of the surface regions of the rubber granules in order to modify the surface chemistry (electrical dipole characteristics) of the bitumen/rubber interface, thereby improving the bonding

between the bitumen and the rubber granules. Another achievement is that the rubber is saturated with bitumen.

In an alternative method for heat treating rubber granulate, cold and dry rubber granulate is fed into a plant mixer and bitumen heated to a certain over-temperature resulting in a suitable end temperature of the finished mixture is added.

Mixing continues until the rubber is saturated with bitumen, preferably 2-10 minutes.

A stone ballast material having particle sizes ranging from about 0.5 mm to about 11 mm (for a majority of particles) is heated to about 200°C and fed into a mixer, and the bitumen/rubber mixture is added thereto at a temperature of about 190°C. After proper mixing, the road surface material is ready for transportation to a road construction site at a temperature of 140°C to about 170°C.

In order to increase the amount of rubber granulate, a portion not more than 100 % of the amount of rubber granules wetted by the bitumen, could be added dry directly into the main plant mixer.

When the rubber granulate contains a large proportion of natural rubber, as the case may be when winter and friction tires are used as raw material for the granulate, it has been found that the rubber granulate soaks excessive amounts of bitumen that may lead to disintegration of the road surfacing composition. In order to overcome this problem, the present invention proposes to pre-coat the rubber granulate. This may be done: 1) either with a suitable polymer material forming a film coating protecting the granulates from absorbing bitumen or bitumen fractions; or

2) with warm bitumen or bitumen as an emulsion, such that the rubber is saturated in advance with bitumen and, thus, cannot soak more bitumen in the finished road surfacing. In case an emulsion is used, e. g. 20% aqueous emulsion is added resulting in 10% remaining bitumen on the rubber granules. This mixture is then kept warm and dry. After about 14 days the clods of rubber granules may be crumbled to granule powder again since the rubber has soaked and absorbed the amount of bitumen added.

Another way to improve the properties of the rubber granulate is to ensure that a maximum proportion of the rubber granulate is of the synthetic rubber type. This may be accomplished by rejecting winter and friction tires and only use summer tires.

The resulting granulate, principally consisting of synthetic rubber, has a limited bitumen soaking ability.

At the road construction site, the material is fed into an asphalt paving, or, surfacing machine at a temperature of about 150°C. During vibration, the asphalt paving machine lays out a material layer having a thickness of 20-150 mm. When the layer has reached a temperature of maximum about 130°C, but not less than 80°C, it is compacted by a roller compactor having rolls vibrating at a zero to peak amplitude of at least 0.7 mm and a frequency of at least 40 Hz, preferably a zero to peak amplitude of 0.8 mm and a frequency of 70-90 Hz. It would be desirable, however, to reach a zero to peak amplitude of 1.2 mm. It is preferred to let the roller cross four times over the layer. After this first compression follows a second compression at a vibration zero to peak amplitude of 0.3 mm and a frequency of 70 Hz. Also here, it is preferred to cross four times. Finally, a static packing or

compression is performed without vibration and with preferably two crossings.

The road surfacing may comprise two or more layers, a lowermost layer having the largest particles (typically 16- 24 mm) and an upper, surface layer having the smallest particles (typically 1-11 mm). Larger particles are beneficial for drainage and for creating an acoustically active air volume below a more fine-grained surface layer. The lower layers may comprise a traditional road surface material, i. e. , without rubber granules, or, it may include rubber granules in order to achieve a further increased elasticity of the entire surface design.

In order to explain more closely the structural composition of a road surfacing according to the present invention, reference is made to the accompanying drawing schematically showing a plurality of randomly oriented ballast particles. These ballast particles include stones S and granulated rubber particles R. Each rubber particle R is shown to be bound to two adjacent stone particles S by a layer of bitumen binder B adhering to the stone particles as well as to the rubber particles. Each rubber particle R serves as an elastic element between two stone particles S, thereby giving the road surfacing important elastic properties. Between the stone particles there remain voids V making the surfacing porous which is necessary for its sound absorption properties as well as beneficial for its drainage properties.

Even if here have been mentioned stone sizes within certain preferred ranges, this does exclude that those smaller stone particles (filler-like or dust-like), that are almost always present in road surface mixes, may be brought along and be mixed into the road surfacing material together with the preferred sizes without departing from the scope of the invention.