Note: Descriptions are shown in the official language in which they were submitted.
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~l~;Z795
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 laid 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 assume 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 dis-
advantageous, 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.
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~L52795
This i~ achieved by means o the process according to the present
invention, which is characterized in that the asphalt compound is laid out imme-diately 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 emul-
sion 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 compound then has a temperature of about 150 degrees C. After this com-
pound 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 coatis 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.
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The reason for waiting until the water llas evaporated is that accord-
ing to prevailing wisdom up to now, full adhesive effect is not achieved imme-
diately, and this is necessary to prevent the new layer from slipping during the
subsequent rolling and under traffic loads.
Tests have now shown that 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 150 degrees
C 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 adhe-
2a sive into the pores and capillaries both upwards and downwards, producing espe-
cially 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, with-
out resulting in any negative effects.
~L~52795
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 adhe-
sive effect, which is obtained by the increase in volume of the tack coat as the
~; water is vaporized.
The advantages of the process 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 saving 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.
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