AU539587B2 - Process for tack coating in road surfacing - Google Patents

Description

Process for tack coating in road surfacing

The present invention relates to a process for tack coating between layers of road surfacing material such as asphalt concrete where an existing layer is swept clean and sprayed with a thin film of an asphalt tack coat, whereafter asphalt is laved out on top of the tack coat.

A road surface is built up layer by layer with different materials for the various functions of the road. in order to assure that no slippage occurs between the layers, it is necessary to bind the layers with a thin tack coat. Normally the tack coat is only laid between the uppermost asphalt layers and not between the gravel layers which have by themselves sufficient friction between them.

The tack coat normally consists of an asphalt which has been dissolved in a solvent, e.g. naphta, or of an asphalt which has been emulsified in water. In the former case one speaks of an asphalt solution, and in the latter case of an asphalt emulsion. The asphalt solution is however of less interest now and will presumably disappear completely from the market because of the fact that it contains solvents which can be used for other purposes, i.e. as motor fuels, and the fact that they create a hazardous working environment.

In conventional tack coating, the tack coat is left to dry, and the water evaporates, leaving a sticky film of asphalt. This process is quite disadvantageous, since a special work team and a separate unit are required for the tack coating plus the fact that a waiting period is required for drying.

The purpose of the present invention is to achieve a process for tack coating between layers of surfacing material, which does not have the above-mentioned disadvantages, which provides full adhesive effect immediately and which provides better adhesion than conventional tack coating.

This is achieved by means of the process according to the present invention, which is characterized in that the asphalt compound is laid out immediately after the existing asphalt concrete layer has been cleaned and tack coated, and in that the tack coat is dried and broken by the applied fresh and hot asphalt compound.

According to a preferred embodiment of the invention, an aqueous emulsion of an asphalt is used as the tack coat.

Further characteristics and advantages of the present invention will be revealed in the following detailed description.

Asphalt compound, which consists of gravel and asphalt, is made in asphalt plants. From there, the asphalt is driven in trucks to the surfacing site and is dumped into an asphalt laying machine or is laid out manually. The asphalt compund then has a temperature of about 150ºC. After this compound has been applied and rolled, it is called asphalt concrete.

In order to avoid slippage between the asphalt concrete layers, a tack coat is used, as was mentioned above, the composition of which is specified in construction standards.

According to the technique which is the only one used at present, the existing asphalt concrete layer is first brushed clean, whereafter the tack coat is spread out by a separate work team with a special separate unit. Then one waits about 1/4 to 2 hours until the water or solvent has evaporated, leaving a thick, sticky asphalt film. Only then is the asphalt compound for the next layer laid out and rolled.

This binding method only produces an adhesion on the "tops", i.e. a surface adhesion.

The reason for waiting until the water has evaporated is that according to prevailing wisdom up to now, full adhesive effect is not achieved immediately, and this is necessary to prevent the new layer from slipping during the subsequent rolling and under traffic loads.

Tests have now shown rhat it is not necessary to allow the tack coat to dry. Rather, in complete contrast to previous wisdom, a better result is obtained if the tack coat is not allowed to dry prior to application of the asphalt compound. According to the invention, the fresh, hot asphalt compound is applied immediately after the sweeping and the spreading of the tack coat, so that the asphalt compound drops on the wet tack coat film.

The tack coat is spread in amounts on the order of 0.2 - 0.4 kg per square meter and has a water content of about 30-50 per cent. This should be compared with the asphalt compound, which has a temperature of about 150ºC and which is laid in amounts of about 60-120 kg per square meter. When the hot asphalt is laid on the tack coat, the water in the tack coat or tack coat emulsion will boil violently and be vaporized, thus breaking the emulsion. This conversion results in a sharp increase in volume in the tack coat and appears as foam. The foaming and increased pressure forces the adhesive into the pores and capillaries both upwards and downwards, producing especially good adhesion at full effect immediately. A deep acting adhesion is obtained which is clearly superior to the adhesion obtained by conventional tack coating. Any remaining water in the pores etc. does not affect the adhesive result, which is best demonstrated by the fact that water-coated rollers can be used to avoid having the newly laid asphalt compound stick to the rollers, without resulting in any negative effects. Series of tests carried out on a pilot scale to determine whether the method would work, have shown that significantly better results as regards the mechanical adhesion are obtained than with the conventional technique, and, as was indicated above, this depends on, among other things, the deep-acting adhesive effect, which is obtained by the increase in volume of the tack coat as the water is vaporized.

The advantaces of the Drocess according to the invention are thus that the need for an extra work team for the tack coating work is eliminated, as well as the need for a separate tack coating unit.

Furthermore, the waiting time for breaking and drying of the tack coat is eliminated, resulting in a considerable time savings in the road servicing work. At the same time, an improved mechanical adhesion is obtained between the layers.

The process according to the invention is most suited to tack coats in the form of an aqueous emulsion, but will work for all asphalt binders which are used at present if the amounts of tack coat are adapted to the process.

Claims (3)

1. Process for tack coating between layers of road surfacing material, such as asphalt concrete, where an existing layer is swept clean and spread with a thin film of asphalt binder or tack coat, whereafter asphalt compound is laid out on the tack coat, characterized in that the asphalt compound is laid out immediately after the existing asphalt concrete layer has been swept clean and tack coated, and in that the tack coat is dried and broken by the applied fresh, hot asphalt compound.

2. Process according to claim 1, characterized in that an aqueous emulsion of an asphalt is used as a tack coat.

3. Process according to claims 1-2, characterized in that the asphalt compound with a temperature of about 150ºC is applied onto the tack coat, thereby vaporizing the water in the tack coat in order to achieve such an increase in volume in the tack coat that it penetrates into pores and capillaries in the two layers, thereby achieving a deep adhesion.