Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to a method for carrying out road
maintenance works on road surfaces covered by a bituminous top layer and to
means for carrying out the method.
Generally, maintenance works on existing roads can be categorized
in two main classes, viz. a premixed asphal~ concrete which is spread out on
the roadway and then is compacted with a roller. The invention is concerned
with the other class of paving methods wherein a liquid asphalt binder and an
aggregate, preferably a crushed stone material, are applied in separate layers
to the roadway and then the aggregate is compacted into the binder.
A method falling into the latter class is known as a single pass
surface treatment or surface dressing, which may be applied to any prepared
surface and is frequently applied to existing bituminous paving in order to
provide a new and smooth surface and to make the old paving waterproof and
hard-wearing. This method usually entails the steps of sweeping the old base
in order to remove dirt and other foreign matter and then applying a coat of
a liquid asphalt binder to the base, covering the coat of asphalt binder with
a layer or course of crushed stone material, rolling the stone material down
into the binder until the desired surface finish is attained, and finally
brooming the road surface to ensure loose surplus stone being removed.
By the dosage of the binder it should be taken into account whether
the road generally carries heavy traffic or lightweight traffic only, since
i* is well known that on a heavily loaded road the binder should be applied at
a restricted rate compared to a light traffic road, because the heavy traffic
acts as a heavy roller which, if the binder is applied in a thick layer, will
cause th0 cover stone material to get forced through the binder into the sub-
strate and thus make the binder appear on the top surface, where it forms a
smooth, compact surface which is generally non-ideal as a road surface, espe-
cially from a skid resistance point of view. Therefore, the binder should be
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applied in such a thin layer that the stone layer gets bound, but is maintained
disintegrated on its top side. On a lightweight traffic road the binder should
bc applied in a thicker layer for binding the cover layer in a safe manner,
without loose and dangerous top surface material. Normally the amount of
binder lies within the interval of 0.6 - 3, preferably 1.5 - 2.1/m2.
In practice the binder is dosed according to a compromise between the
requirements of the heaviest loaded road portions, i.e. the normal wheel track
portions thereof, and the less loaded portions therebetween, and as well
known the results is generally dissatisfying in that loose stone material
appears on the said less loaded portions while the heavy loaded track portions
are liable to get black, compact and slippery.
It is the purpose of this invention to provide a method of maintain-
ing the road surface in such a manner that in the final result the various
portions of the road show a surface of a substantially optimal and uniform
character throughout the width of the road.
The invention is based on the idea that it is in fact possible to
detect the load distribution caused by the traffic over the width of the road.
This can be done photoelectrically or otherwise. The load distribution effects
the dosage of the binder in a differentiated manner throughout the transverse
profile of the road. When thereafter the aggregate material layer is supplied
the resulting character of the road will remain uniform and of good quality
during a long period of time.
The present invention may be generally defined as a process for
surface treatment of roads by separately applying to the road surface first a
liquid asphalt binder and then an aggregate, the amount of asphalt binder
applied being sufficient to bind the aggregate over the entire road surface,
said process being characterized in that the traffic load distribution is
measured across the width of the road to thereby establish which portions
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across the width of the road are swbjected to heavy loads and which portions
are subjected to light loads, and that the liquid asphalt binder is applied in a
larger amount upon those portions with light loads and in a lesser amount upon
thos~s portions with heavy loads.
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Preferably, the transverse distribution of the traffic load on the
road surface is determined and that the binder is applied on the surface in
such a way that one applies relatively less binder on heavier loaded portions
of the road surface than on the portions of the road surface less loaded.
There is a further parameter which affects the optimal binder supply
and the uniformity of the final road surface, viz. the rigidity of the old
road surface. If this surface is relatively soft it seems desirable to apply
a reduced amount of binder, whether generally or still differentiated across
the road, and for further perfection, therefore, it is possible to carry out
a real rigidity measurement by pointwise checking ~he penetration, in depth
and time, of a loaded test member into the road surface at selected points,
and from this test obtaining a readable result which is usable as a parameter
for the calculation of the required amount of binder. In this connection even
other parameters such as the ambien~ temperature should preferably be taken
into consideration, and a set of formulae or other calculation aids may be
applied for determining the optimal binder supply.
The roughness of the road surface may also be determined and the
influence of said roughness on the optimal amount of the binder is calculated.
According to another aspect of the present invention, a device for
measuring the load distribution comprises a base plate provided with a plural-
ity of mutually spaced contact members adapted to be activated by vehicles on
a road.
When the measuring device, which may also be referred to as guage
means, is kept operative during a representative interval of time and the
measuring results are caused to be memorized, according to any suitable prin-
ciples, it is possible to thereafter read out the relevant data pertaining to
maximum and/or average load on the single portions of the width of the road at
the particular place, and when a measuring place is chosen which is representa-
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tive of a considerable length of the road, the binder can then be applied inthe desired crosswise differentiated manner throughout that length of the
road. Of course, the measuring equipment may be so adjusted that the data
read-out is usable in a direct manner for the differentiated setting of the
capacity of binder nozzles in a ~inderapplicator. Preferably the measuring
system is so adapted that it provides for registration solely of traffic
above a predetermined minimum weight.
The binder may be applied to the road surface by means of a moving
cross boom provided with a plurality of binder outlet nozzles of individually
adjustable capacity, whereby the binder is applicable simul*aneously all over
the wid~h of the road or a partial width thereof.
The invention is described in more detail in the following~ reference
being made to the accopanying drawing, in which:-
Figure 1 is a top view showing a preferred embodiment of a trafficload measuring device according to the invention;
Figure 2 is a side view of the device of Figure l; and
Figure 3 is a graph showing traffic load distribution and binder
dosage curves.
In order to make it possible to determine the correct maintenance
programme for a certain road surface to be renovated by a so-called surface
dressing operation it has been found very important to know precisely the actu-
al traffic load picture of the roadway in question. To this end may be used a
measuring band structure as shown in Figures 1 and 2 and which is mounted
stretched out across the road surface to be examined concerning the traffic
load distribution thereon. The band structure shown comprises a rubber mount-
ing base plate portion 2 provided with a plurality of mutually spaced contact
members 6 adapted to be activated by the wheels of the vehicles on the road.
Tn order only to register the heavy traffic load influence on the road the
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contact members themselves are protected from being actuated by the lighter
traffic load by means of hard rubber elements 5 at both sides of the contact
members 6. The latter are connected to a multiplex connection box 8 a~ one
end of the band structure by means of flat cables 7 which are protected by a
rubber cover plate 3.
The whole measuring band structure is held in correct position
across the road surface or preferably only across one half thereof by means
of a wire system 4 along the opposite edges of the band structure. The wires
4 are anchored outside the road e.g. as shown by means of particular tighten-
ing and anchoring means 9 and 10. The measuring band structure, of course,is connected with suitable registering means being able to store the traffic
load picture of the road being measured by the activation of the contact mem-
bers 6 during the measuring period of time.
Another factor that is preferably known in order to plan the main-
tenance operation of a given road surface according to the present method is
the strength condition of the existing road. For measuring this, a device may
be used which comprises a stand having three supporting legs, which rest di-
rectly on the road surface to be measured while a measuring leg is provided
with a steel ball adapted to be forced into the road surface by the weight of
a load body which may be supported solely upon the steel ball. The depth of
penetration of the steel ball will be indicative of the strength of the sur-
face. The device includes measuring equipment for registering this penetra-
tion depth in respect of the period of time of the strength test.
In accordance with the results of the measurements of the actual
crosswise traffic load distribution on the road and the strength test of the
road surface it is possible to determine the crosswise differentiated dosage
of the coat of asphalt binder to be used in the surface dressing operation to
ensure optimal effect of the maintenance operation.
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Examp]e
Determination of the optimal amount of binder for dressing a road
surface.
The hardness was determined in a number of points along the road
and during the measurements the temperatures were registered simultaneously.
The hardness is disclosed in table I below representing the quasi-
static penetration of a steel ba]l in the substrate (the penetration is ex-
pressed in mm) after 5 and 10 minutes wi~h a load of 40 kp. The quasistatic
penetration was converted to dynamic penetration by means of van der Pools
diagramme. The "roughness" of the road surface was determined by a conven-
tional sandpatch method and is shown in diameter expressed in mm ~amount of
sand used = 5000 mm3).
Vsand Dsand Temp. C _ ~k(t=10 min)
mm3 mm mm mm
, _ . _ . . . .~ __ _ _ _
136 26.3 1,782 2,070
5000 135 25.0 1,500 1,720
132 25.3 1,665 1,856
_ 115 27.6 1,740 1,948
The transverse distribution of the traffic load exceeding l ton was
determined.
The distribution in percent is disclosed in Figure 3.
Through meteorological measurements the temperature of the road sur-
face was determined hour by hour during one year. In this case temperatures
below 20C have not been considered as these temperatures do not influer.ce the
2a final result to any appreciable extent.
The amount of aggregate that can be retained on the road surface
was determined experimentally in a simple dressing with the given amount of
aggregate. By doing so it is possible to calculate the average void volume
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in the aggregate layer.
The optimal dosage of the binder was determined by the values thus
obtained. The result i.s disclosed in Figure 3.
