Mixing and surfacing apparatus

Description

Field of the Invention

[0001] The present invention relates to surfacing processes, in particular road-surfacing, involving the use of bitumen-based surfacing materials. More specifically, the present invention relates to apparatus for mixing bitumen emulsion and polymer-modified bitumen emulsion-based surfacing materials, and for applying such materials to the surface of a substrate, for example a road.

Background to the Invention

[0002] Slurries comprising polymer-modified bitumen emulsion and graded aggregates, commonly referred to as microasphalts, have widespread use in road surfacing. Such slurries typically also comprise a filler, cement and water in addition to that contained in the bitumen emulsion. Microasphalts are typically mixed at the point of application, although they may be mixed off-site, either by a batching process or by using a continuous flow mixing and laying machine, and are applied to road surfaces by spreading or screeding.

[0003] The repeated contact of bitumen emulsion with metal surfaces in the mixing and/or laying apparatus results in deposition of bitumen on those surfaces. This reduces the effectiveness of the mixing and laying operations, and consequently operations must be interrupted regularly to remove the deposited bitumen, resulting in a significant reduction in productivity.

[0004] Removal of the deposited bitumen is typically achieved, slowly, through the use of high pressure water jets or, when deposition is more severe, by burning off the bitumen. The latter method has the added disadvantage that it can cause damage to the mixing and laying apparatus, thereby reducing the life of that apparatus.

Summary of the Invention

[0005] It has now been found that deposition of bitumen can be prevented, or at least significantly reduced, by applying to the metal surfaces of the mixing and/or laying apparatus that contacts the bitumen emulsion in use, an electrical charge of the same polarity as the charge carried by the bitumen emulsion. If any bitumen is deposited on these surfaces, it is much easier to remove than in the currently operated processes.

[0006] Bitumen emulsions used in surfacing operations tend to be cationic in nature, and therefore positively charged, i.e. the bitumen droplets suspended in the emulsion have electropositive charges. These charged droplets repel each other, conferring stability on the emulsion as a whole. Accordingly, these droplets are also repelled by an electropositive charge applied to the metal surfaces of mixing and/or laying apparatus, reducing deposition thereon of bitumen from the emulsion.

[0007] Less frequently, an anionic bitumen emulsion may be mixed with aggregate for road surfacing. In this case, use of an electronegative charge on the surfaces of the mixing and/or laying apparatus will effectively prevent deposition thereon of bitumen from the anionic emulsion.

[0008] According to one aspect of the present invention, therefore, a surfacing vehicle for mixing and applying to a substrate a bitumen emulsion-based surfacing composition comprises a mixing unit for mixing together bitumen emulsion and aggregate to form the surfacing composition, a laying box for applying the surfacing composition, by trailing application, to a substrate, and means for electrically charging surfaces of the mixing unit and/or laying box which, in use, contact the surfacing composition, with an electrical charge of the same polarity as the charge carried by the bitumen emulsion.

[0009] The principle upon which the present invention is based may be applied to mixing and/or laying apparatus utilised separately, and to other bitumen emulsion-contacting parts in a combined mixing and laying apparatus.

[0010] According to a further aspect of the present invention, a method of surfacing a substrate, preferably a road, with a bitumen emulsion-based surfacing composition comprising a bitumen emulsion and aggregate, comprises mixing together the bitumen emulsion and aggregate in a mixing unit having surfaces which are electrically charged with an electrical charge of the same polarity as the charge carried by the bitumen emulsion, so as to repel the bitumen emulsion-based surfacing composition therefrom, and applying the surfacing composition to a substrate by trailing application from a laying box, wherein preferably the surfaces of the laying box are charged with an electrical charge of the same polarity as the charge carried by the bitumen emulsion, so as to repel the bitumen emulsion-based surfacing composition therefrom.

Description of the Invention

[0011] In the context of the present Application, all references to bitumen emulsion include bitumen emulsions which are polymer-modified. Furthermore, when, for example reference is made to a cationic bitumen emulsion-based surfacing composition this means a surfacing composition based upon a cationic bitumen emulsion.

[0012] Naturally, in order to apply an electrical charge to the bitumen-contacting surfaces of mixing and/or laying apparatus, the surfaces of such apparatus must comprise electrically-conductive material, usually a metal, such as steel or aluminium. This is conventional in the field.

[0013] An electrical charge can be applied to these surfaces by simply making an electrical connection, for example through a wire, between the surfaces to be protected and a power supply. In the case of stand-alone mixing apparatus or a combined mixing and laying apparatus the surfaces to be charged may be electrically-connected to the main power supply of that apparatus. In the case of a mixing and laying vehicle, it is preferred that the surfaces to be protected are electrically-connected to the appropriately-charged terminal of the battery of the vehicle.

[0014] It is essential that the metal surfaces to be protected from bitumen deposition are isolated electrically both from earth and other metal surfaces in the apparatus. This is achieved, simply, by interposing insulating material between the surfaces to be charged and earth and other metallic surfaces. Suitable insulating materials include rubber and plastic materials.

[0015] The surfaces to be protected may comprise only the inner walls of the mixing and/or laying apparatus. However, if, as is conventional, the mixing and/or laying apparatus includes paddles and/or augers whose surfaces comprise electrically-conductive material, an electrical charge may also be applied to their surfaces, to prevent bitumen deposition thereon.

[0016] As mentioned above, the present invention may be applied to stand-alone mixing apparatus, for example footway slurry mixers and cold mix plants, which may be static or mobile. The invention is particularly beneficial, however, when applied to mixing and laying vehicles, which typically comprise hoppers, containing aggregate and other materials, for example fillers, fibres, etc., to be mixed with a bitumen emulsion from an on-board tank, a mixing unit where the aggregates and bitumen emulsion are mixed, and a laying box from which the resultant surface composition is applied, by trailing application, to the surface of a substrate, usually a road. Vehicles of this type normally utilise a strike plate or screed plate to control the depth of surfacing composition applied from the laying box to the surface of the substrate. If desired, an electrical charge may also be applied to the strike plate or screed plate.

[0017] The present invention may also be applied solely to a laying box, or to other isolated parts of a mixing and laying vehicle which, in use, come into contact with bitumen emulsion, or bitumen-coated aggregate.

[0018] The present invention may also be applied to static and mobile bitumen emulsion storage tanks. Tanks for storing cationic bitumen emulsions should be charged positively, and tanks for storing anionic bitumen emulsions should be charged negatively.

Claims

1. A surfacing vehicle for mixing and applying to a substrate a bitumen emulsion-based surfacing composition, the vehicle comprising

a mixing unit for mixing together bitumen emulsion and aggregate to form the surfacing composition,

a laying box for applying the surfacing composition, by trailing application, to the substrate, and

means for electrically charging the surfaces of the mixing unit and/or laying box which, in use, contact the surfacing composition, with an electrical charge of the same polarity as the charge carried by the bitumen emulsion.

2. A surfacing vehicle according to claim 1, for mixing and applying a cationic bitumen emulsion-based surfacing composition, wherein the charging means applies an electropositive charge to the surfaces of the mixing unit and/or laying box.

3. A surfacing vehicle according to any preceding claim, wherein the charging means charges the surfaces of both the mixing unit and the laying box.

4. A surfacing vehicle according to any preceding claim, wherein the laying box comprises walls and at least one paddle and/or auger therebetween, and the charging means charges both the surfaces of the walls and the at least one paddle and/or auger.

5. A surfacing vehicle according to any preceding claim, which further comprises a strike plate for controlling the depth of the surfacing composition applied from the laying box to the substrate, and wherein the charging means additionally charges the strike plate.

6. A surfacing vehicle according to any preceding claim, wherein the surfaces to be charged by the charging means are electrically-insulated from other metallic parts of the vehicle.

7. A surfacing vehicle according to any preceding claim, which is powered by a battery which is electrically-connected, through an appropriately-charged terminal, to the surfaces to be charged, and which comprises the charging means.

8. A surfacing vehicle according to any preceding claim, which is a road-surfacing vehicle.

9. Apparatus for mixing a bitumen emulsion-based surfacing composition, comprising

a mixing unit for mixing together bitumen emulsion and aggregate to form the surfacing composition, and

means for electrically charging the surfaces of the mixing unit which, in use, contact the surfacing composition, with an electrical charge of the same polarity as the charge carried by the bitumen emulsion.

10. Apparatus according to claim 10, for mixing a cationic bitumen emulsion-based surfacing composition, wherein the charging means applies an electropositive charge to the surfaces of the mixing unit.

11. A laying box suitable for use with a surfacing vehicle, by means of which a bitumen-based surfacing composition is applied to a surface by trailing application, and which comprises within the walls of the laying box at least one paddle and/or auger, and means for charging the walls and/or at least one paddle and/or auger with an electrical charge of the same polarity as the charge carried by the bitumen emulsion.

12. A laying box according to claim 13, for applying a cationic bitumen emulsion-based surfacing composition, wherein the charging means applies an electropositive charge to the walls of the laying box and/or the at least one paddle and/or auger.

13. A method of surfacing a substrate, preferably a road, with a bitumen-based surfacing composition comprising a bitumen emulsion and aggregate, the method comprising

mixing together the bitumen emulsion and aggregate in a mixing unit having surfaces which are electrically charged with an electrical charge of the same polarity as the charge carried by the bitumen emulsion, so as to repel the bitumen emulsion-based surfacing composition therefrom, and

applying the surfacing composition to a substrate by trailing application from a laying box, wherein preferably the surfaces of the laying box are charged with an electrical charge of the same polarity as the charge carried by the bitumen emulsion, so as to repel the bitumen emulsion-based surfacing composition therefrom.

14. A method of surfacing a substrate according to claim 13 with a cationic bitumen emulsion-based surfacing composition, wherein the surfaces of the mixing unit, and preferably, the laying box, are electropositively charged.

15. A bitumen emulsion storage tank having metallic inner walls and means for charging the walls with an electrical charge of the same polarity as the charge carried by the bitumen emulsion.

16. A storage tank according to claim 15, wherein the charging means applies an electropositive charge to the inner walls of the tank.

17. A storage tank according to claim 16, which contains a cationic bitumen emulsion.