EP0795059B1 - A method of providing a road surface with an overlay - Google Patents

Description

The present invention relates to a method of providing a road surface with an overlay, normally for general road maintenance and restoration of good antiskid properties. By the laying out of a thin overlay it is economically possible to make use of high quality aggregates for maximum performance, and it is even possible to make a relevant selection between respective road sections more and less exposed to load and wear, e.g. with respect to binder dosage.

For works on the more heavily trafficked roads it is normal practice to make use for this purpose, of a so-called "thin hot mix asphalt". Despite of this preference, it is well known among experts that this hot technique implies a row of difficulties of both technical and environmental kinds, mainly caused by a vapourization of volatiles from the applied bituminous binder. Many attempts have been made to remedy the various problems, but so far without any fully acceptable result. The asphalt workers are still seen in the midst of the well known "blue smoke".

There do exist 'cold' alternatives, though with their own problems and limitations, based on the laying out of a layer of relatively coarse stones in a bed of a binder initially applied to the road surface by spraying. The stone layer is rolled, whereafter it stabilizes gradually. The coarse stones, although giving good strength when stabilized, are not too advantageous with respect to skid resistance, so for more qualified roads it is preferred to extend the method as in EP 0127470 A2 by spraying a binder on the top of the coarse layer and add a top layer of a less coarse stone fraction, better suited for skid resistance. In both cases, however, road users will experience problems caused by loose stones and sometimes even a surface surplus of liquid bituminous binder, unless the new surface is thoroughly rolled and allowed to stabilize through a considerable period of time.

This, of course, is not acceptable in practice, and while some compromise is possible for smaller roads, implying severe speed restrictions, it will be unacceptable in connection with highways and other heavily trafficked roads, e.g. also in city areas. The hot method, therefore, despite its problems and disadvantages, is still preferred for such more critical works.

According to the present invention it has been recognized that the discussed cold methods can be modified so as to be much better applicable even for the said more critical works. A basic idea is that the finer stone fraction is applied not as an individual full layer of stones on the top of the lower, coarser stone fraction, but only to the extent of filling out the upper voids of the lower layer of coarse stones, in association with another basic idea of applying the required binder to the finer stone fraction not as a bed underneath this fraction, but rather by a spraying of a low viscosity binder against the top side of this fraction, such that the binder, seeking rapidly downwardly, will very soon effect an initial binding of the finer stone fraction, which will also be mechanically stabilized in the upper voids between the underlying coarse stones. These, in their turn, will also be stabilized by the downwardly penetrating low-viscid binder which, however, may be an emulsion of a high viscid bitumen material, which may be adapted to 'break' relatively soon so as to cause an effective binding both of the lower coarse stone fraction to the former road surface and of the upper finer stone fraction to orin the upper voids of the lower, coarse stone fraction, without the finer stones being present above the top side of the coarse stone layer.

Thus, the finer stone will not behave as loose stones, and the binder will soon sink down from the surface, whereby the traffic on the road can soon be resumed. The major part of the surface area will be constituted by the smaller stones, which will provide a texture in favour of good skid resistance and low noise generation. The bituminous binder will effectively fill out the voids between the stones at least in the lower half of the overlay, thus providing for improved stress releiving and waterproofing.

It may still be preferred to start with a bottom layer of a bond coat of bituminous binder, preferably a polymer modified bituminous emulsion to ensure waterproofing and perfect bonding to the old surface as well as for temporarily securing the first chipping layer in place.

The first chipping layer of relatively coarse stones, e.g. 12-14 mm, is laid out in a single layer, preferably not fully compact, so as to cover some 95% of the surface area. A second layer of smaller nominally single sized chippings, e.g. 6-7 mm is then applied, taking care not to disturb greatly the first layer, the dosage being controlled to the effect that the second layer fills up the voids between the coarse stones without any substantial overheight therein. Thereafter the combined layers are compacted to interlock the second layer with the first layer and make the top sides of the two layers flush with each other.

Thereafter a low viscosity bituminous binder, preferably a polymer modified bituminous emulsion with good adhesion and breaking characteristics, is evenly applied to the surface such that the remaining voids in the first layer are filled and a surface texture is clearly visible. In this grouting stage the binder will flow down in contact with all the stones, thus producing an effective bonding of and in the combined layer.

Preferably and in particular where heavy traffic is to be resumed quickly, the treatment is finished by applying a thin top layer of still smaller chippings, e.g. sand, in order to prevent contact of the binder with the traffic tyres in the initial stages and to aid the quick build up of skid resistance.

The actual quantities of chippings, their grading and size, can be used to calculate the voids in the layers and to dictate the binder addition to achieve the desired texture, given the traffic weight and speed, the degree of site difficulty and the characteristics of the existing road surface.

The chippings in the second layer are preferably pre-coated with bituminous binder or chemically coated in order to ensure that the applied bituminous binder or binder emulsion will penetrate the underlying layer and not break so quickly as to form a thick coating on the surface aggregate. Also, the overlay will then be better stabilized should water be present, particularly when freeze-thaw cycles occur.

Generally, the use of hot binders such as cutback bitumen would not allow for satisfactory grouting or penetration of the binder into the voids, unless the viscosity is undesirably low, and would not, therefore, provide the same durability as when using emulsion binders, which may be of low viscosity, but nevertheless be based on high viscosity bitumen. However, the invention will comprise also the use of hot binders, which may present advantages in terms of costs on lightly trafficked roads.

Normally, the stones in the first layer should not be coarser than 20-24 mm and not finer than some 8 mm. For primary stones of a given fraction, e.g. 14 mm, the secondary stones should preferably be only one fraction smaller, e.g. 8-10 mm, so that these stones will still be 'relatively large' and not liable to be deposited on top of each other.

The invention is not limited to the use of any special aggregate or binder qualities, so the method may be widely varied in this respect, as long as satisfying results are obtained, subject to different requirements.

If desired, both of the chipping layers may be pre-coated. Instead of virgin chippings, other materials may be used, e.g. recycled asphalt, graded into the respective fractions. Of course, a certain content of fine fractions may be tolerable in both chippings, and in connection with graded aggregates the fine fraction may be used in the final top layer, when used.

The first layer may be a low cost aggregate such as gravel stones with low skid resistance properties, followed in the second layer by a higher skid resistant chipping, thus maximizing the resources of higher cost quality aggregate. Also, various artificial aggregates may be used, e.g. slag or calcined bauxite, cementious mortar or cement concrete, formed or crushed, or any suitable waste product, with or without pre-coating.

Although the method is well usable under cold conditions, the invention will of course comprise even the use of heated aggregates and/or binder.

For some applications it may be advantageous to make use of geotextiles or other fibre reinforcement between the layers, including grids prepared from metal or polymer compounds, especially between the existing road surface and the first layer.

The binder may be selected from many usable substances such as cutback bitumen, foamed bitumen and modified versions of these with additives or polymers or fibres or filler, added to the binder or during construction, and all emulsion variants thereof. For the emulsions, any suitable breaking control system may be used. Further binder examples will be thermo-setting binders and other resin binders including polyurethane, acrylic, polysulphide, expoxide, and all two component binder systems.

Different binders may be selected for the respective grouting application and bond coating, where the latter is used. A cost saving may be obtained by using unmodified bituminous binder in one or more applications.

Instead of small chippings or sand for the said top layer, when used, it will be possible to use a slurry surfacing as a final wearing surface or a fine slurry seal, possibly applied as a spray. A suitable slurry will be a stone fraction of 0.2-0.3 mm in a fast breaking bituminous emulsion.

The invention will be applicable also in connection with the construction of new roads and on cement concrete, on airfields, car-park decks etc., and even for providing waterproof layers in buildings. Aggregates may be coloured, if desired.

In the following some examples of the method are given:

Example I:

Overlay on highways and other heavily trafficked roads:

a. Laying out of 13-14 kg/m² stone material, fraction 12-16 mm.

b. Rolling with pneumatic tyred roller.

c. Laying out 6-8 kg/m² bitumen coated stone material with high polishing resistance, fraction 8-12 mm, for complete filling out of texture in first layer.

d. Rolling with steel roller.

e. Spraying out 3.5 kg/m² of a 70% polymer modified cationic bitumen emulsion.

f. Laying out 5-7 kg/m² stone material, fraction 2-5 mm.

g. Rolling with steel roller and optionally pneumatic tyred roller.

h. Opening road for traffic with limited speed through approximately 2 hours, then cleaning road surface with suction sweeper.

Example Ia:

Same example, only starting with spraying out a first layer of 1.2 kg/m² of the bitumen emulsion and reducing item e to 2.3 kg/m²

Example II:

Overlay on roads with medium traffic load:

a. Spraying out of 1.2 kg/m² of a polymer modified, rapid breaking cationic bitumen emulsion with 3% rejuvenator, e.g. "MaxiCoat", Vejtek Int. A/S, Denmark.

b. Laying out 13-14 kg/m² of a stone material, fraction 12-16 mm.

c. Rolling with pneumatic tyred roller.

d. Laying out 6-8 kg/m² of recycled asphalt, fraction 8-12 mm for complete filling out of texture in first layer.

e. Rolling with tandem steel roller.

f. Spraying out 2.1 kg/m² of a 70% polymer modified cationic bitumen emulsion with 3% rejuvenator.

g. Laying out 5-7 kg/m² of recycled asphalt, fraction 0-6 mm.

h. Rolling with steel roller and optionally pneumatic tyred roller.

i. Opening road for traffic through 24 hours with limited speed and then, if necessary, cleaning road with suction sweeper.

Example III:

Overlay on roads in dwelling areas or generally in low-noise areas:

a. Spraying out 0.9 kg/m² of a 70% polymer modified rapid breaking cationic bitumen emulsion.

b. Laying out 11-12 kg/m² of a stone material, fraction 8-12 mm.

c. Rolling with pneumatic tyred roller.

d. Laying out 5-6 kg/m² of a bitumen coated stone material, fraction 5-8 mm, for complete filling out of texture in first layer.

e. Rolling with tandem steel roller.

f. Spraying out 1.9 kg/m² of a 70% polymer modified cationic bitumen emulsion.

g. Laying out 5-6 kg/m² of a stone material, fraction 2-5 mm.

h. Rolling with steel roller and optionally with pneumatic tyred roller.

i. Opening road for traffic for a few hours with limited speed, then cleaning road with suction sweeper, if required.

Claims (9)

1. A method of providing a road surface with a binder bonded overlay by a successive laying out of stone layers of coarse and finer particle sizes, respectively, characterized in that the first layer of the coarse stones is laid out, with or without previous application of binder to the existing road surface, with the stones distributed into a slightly loose layer structure, whereafter the finer stone fraction is laid out with a dosage so as to substantially fill out the upper voids of the coarse stone layer without any noticeable further layer build-up, whereafter the combined layer is compacted and a binder is applied to the resulting surface, this binder exhibiting an application viscosity low enough to condition a penetration of the binder to the lower voids of the combined layer, and the binder being applied with a dosage corresponding to a filling of both the lower and the upper voids up to a level slightly underneath the top level of the compacted layer structure.
2. A method according to claim 1, by which the finer stone fraction is laid out directly on the first layer, without any intermediate treatment thereof.
3. A method according to claim 1, by which a final, mainly temporary surface layer is applied after the application of the binder, viz. as a still finer stone fraction, e.g. sand, in dry condition or as a slurry, suspended in a bituminous emulsion.
4. A method according to claim 1, by which the finer stones finer are sized only a fraction smaller than those of the first coarse layer, for example 14:10 mm.
5. A method according to claim 1, by which both stones and binder are used in cold condition.
6. A method according to claim 1, by which at least one of the stone fractions are pre-coated with a bituminous substance or chemically coated so as to be hydrophobic.
7. A method according to claim 1, by which at least one of the stone fractions comprises crushed recycled asphalt.
8. A method according to claim 1, whereby the first coarse stone layer is laid out on a base layer of a binder of higher viscosity than that of the binder applied to the combined layer.
9. A method according to claim 1, by which the aggregates at least partly consist of artificial material such as crushed slag, calcined bauxite, cementious mortar or cement concrete.