Method and apparatus for the production of asphalt concrete

In the batchwise production of asphalt concrete only such aggregate is utilized, as comprises a coarse-grained portion, a fine-grained portion and a filler portion. The coarse-grained portion and the bituminous binder used are mixed in a mixer, and the filler portion is added to the mixer in such a way that it is has been taken up by and suspended in the binder film deposited on the coarse-grained portion before the free particles on the fine-grained portion are added. In such a way the thickness of the binder film and the capability thereof to take up the particles of the fine-grained portion are increased so that also said particles are coated with and suspended in the binder. An apparatus for carrying the production process into effect has a control device (30) to ensure that the binder film deposited on the particles of the coarse-grained portion has taken up the particles of the filler portion and has thereby obtained the desired thickness and viscosity before the free particles of the fine-grained portion are supplied.

This invention relates to a method and an apparatus for the batchwise 
production in a mixing plant of asphalt concrete from a bituminous binder 
tempered into liquid state and particulate aggregate comprising a filler 
portion, a fine-grained portion and a coarse-grained portion. In the 
preferred embodiment of the invention the filler portion has a principal 
particle size effective in applying the invention of below 0.1 mm, the 
fine-grained portion has a principal particle size effective in applying 
the invention of below 2 mm and the coarse-grained portion has a principal 
particle size effective in applying the invention of over 3 mm. 
A paving of asphalt concrete on for instance a road must be not only 
resistant to wear but also elastic so that it can withstand repeated small 
deformations of the load-carrying foundation without cracking of the 
asphalt concrete paving. A great deal of research has therefore been spent 
on establishing suitable proportions between the constituents of the 
asphalt concrete, suitable material choices and also suitable mixing 
methods, for it is important that all particles of the aggregate are 
coated with the bituminous binder or suspended therein in order that a 
satisfactory elasticity and cohesion shall be obtained. On the other hand, 
too high binder contents must be avoided because of the high cost of 
bitumen, because of the risk for bleeding, i.e. sweating of bitumen out of 
the asphalt concrete at high temperatures, and because of the risk for 
tracking in road pavings. 
One way of effectively utilizing the binding power of the bituminous binder 
and thus of reducing the required amount of binder in an asphalt concrete 
is described in Swedish Pat. No. 7016307-6 (Publication No. 347,989). 
According to the method described therein one first mixes the 
coarse-grained portion and the bituminous binder to coat the particles of 
the coarse-grained portion with a bituminous binder film before the 
remainder of the aggregate is added. This previously known method has 
brought considerable cost and binder savings while maintaining and in 
certain cases also improving the characteristics of the asphalt concrete 
produced. 
It has now proved possible to further improved the characteristics of the 
asphalt concrete to a still higher extent and utilize the binding power of 
the bituminous binder even more efficiently if the thickness and viscosity 
of a bituminous binder film deposited on the particles of the 
coarse-grained portion are increased by introducing the particles of the 
filler portion into the bituminous binder film in such a way that the free 
particles of the fine-grained portion when supplied to the mixing plant 
meet the coarse-grained portion already coated, on the particles of which 
the binder film has already taken up the particles of the filler portion. 
By this production method it is thus possible first to coat the 
coarse-grained portion with a relatively small amount of binder which, on 
the particles of the coarse-grained portion, forms a thin binder film 
which would per se be insufficient to catch and enclose the particles of 
the fine-grained portion but which after taking up the particles of the 
filler portion would have acquired a sufficient thickness and viscosity 
for this purpose. The invention is thus based on the realization that the 
particles of the filler fraction can be introduced into the binder film 
and suspended therein so that the filler portion actually will form an 
integrating part of the bituminous binder which is thereby given increased 
volume and viscosity and is thus able to take up and also to coat the 
particles of the fine-grained portion. Under suitable mixing conditions 
and temperatures it is thus possible, in applying the invention, to ensure 
that both the filler portion and the fine-grained portion are suspended in 
the bituminous binder film already deposited on the particles of the 
coarse-grained portion, which results in an optimum utilization of the 
bituminous binder supplied and also results in a very homogeneous 
consistency of the asphalt concrete produced. 
As mentioned in the foregoing, it is essential in applying the present 
invention that the free particles of the fine-grained portion when 
supplied to the mixer meet coated particles of the coarse-grained portion, 
the bituminous binder film of which has already been caused to take up 
filler particles and is thus given greater thickness and higher viscosity. 
In applying the invention it is therefore possible to pour the entire 
filler portion into the mixer and mix it homogeneously with the mixture of 
coarse-grained portion and bituminous binder before the fine-grained 
portion is added. This supplying sequence gives the best and most reliable 
result. Within the scope of the invention, however, it is also possible to 
supply the filler fraction and the fine-grained portions substantially at 
the same time if these two fractions are added at such points of the mixer 
that at least the major part of the filler portion has been taken up by 
the bituminous binder film on the particles of the coarse-grained portion 
before said particles meet the free particles of the fine-grained portion 
supplied. It is also possible, within the scope of the invention, to 
introduce the entire or part of the filler batch into the mixer together 
with the coarse-grained portion. 
Exploiting the present invention it is also possible to control the 
production of the asphalt concrete in a more exact and even better way 
than hitherto if the filler portion supplied and/or the temperature of 
said filler portion and/or the temperature of the coarse-grained portion 
supplied is regulated to thereby regulate the viscosity of the binder film 
deposited on the particles of the coarse-grained portion so that it will 
be the most suitable viscosity in order that the binder film shall be able 
to take up the particles of the fine-grained portion and enclose them in 
suspended state. It is also possible to regulate the viscosity of the 
asphalt binder when it is supplied by adding different asphalt binder 
types or by regulating the mixing ratio of different asphalt binder types 
or by regulating the temperature of the binder. To this end, there may be 
provided in the asphalt plant supplying and dosing means for preheated and 
not preheated filler portion and for preheated and not preheated mineral 
aggregate as well as control means for controlling said supplying and 
dosing means in dependence on the desired viscosity of the bituminous 
binder film on the particles of the coarse-grained portion. This 
embodiment of the invention may also be exploited to provide in situ 
heating of the filler portion supplied, the excess heat of the particles 
of the coarse-grained portion and/or the preheated bituminous binder being 
utilized for heating the filler portion. 
It should be mentioned in this context that experiments have shown that 
adding the fine-grained portion to the previously bitumen-coated 
coarse-grained portion before the filler portion is added results in a 
reduced thickness of the bituminous binder film on the particles of the 
coarse-grained portion since the binder film is not sufficiently thick and 
has not sufficiently high viscosity to take up and wholly enclose the free 
particles of the fine-grained portion and since the bituminous binder will 
thereby be distributed over a larger free particle surface (i.e. the total 
surface area of the particles of the coarse-grained portion and the 
fine-grained portion). In conventional production processes said 
insufficient coating has been counteracted by an increase of the amount of 
bituminous binder supplied, which actually involves increased costs and in 
many cases also to a lower extent optimum characteristics of the asphalt 
concrete produced. Those skilled in the art will therefore realize the 
importance of the thickness increase and viscosity increase of the binder 
film on the particles of the coarse-grained portion attained by utilizing 
the present invention. Thus the invention makes possible a volume 
increase, performed in the mixer, of the bituminous binder so that the 
binder, without necessitating an increase of the amount of bituminous 
binder supplied, will be enabled to take up and suspend in it the 
particles of the fine-grained portion. The binder which is hereby given 
the form of a suspension of solid particles in a matrix of bitumen, would 
seem to act, in applying the invention, as a viscous liquid. 
As is well known, filler occurs in three different ways in a conventional 
asphalt plant in which the aggregate is heated and dried, viz. (i) the 
filler amount accompanying the coarse-grained and fine-grained portions of 
the heated and dried aggregate, (ii) the filler separated with the aid of 
cyclone separator or like means in the heating installation of the asphalt 
plant, and (iii) foreign filler which is added besides the two 
first-mentioned filler amounts. The foreign filler as a rule has the same 
temperature as ambient air while the cyclone-separated filler as a rule 
can have a temperature of up to 100.degree. C. in the normal operation of 
the plant. At the start-up of the asphalt plant the cyclone-separated 
filler which is kept stored in silos, may also have ambient temperature. 
Exploiting the present invention, there is understood by "filler portion" 
only foreign filler and filler separated in the asphalt plant, while the 
filler remaining in the coarse-grained portion and fine-grained portion 
and possibly the filler added to the fine-grained portion are considered 
to be parts of the respective portion. 
It has been found that a varying and low filler temperature may give 
undesired variations in the capability of the bitumen film deposited on 
the particles of the coarse-grained portion of taking up and encapsulating 
further particles from the fine-grained or filler portions. It is 
therefore essential that the temperatures in the asphalt plant are 
carefully controlled so that also when large amounts of filler and 
fine-grained portions are supplied, there is maintained a viscosity of the 
bituminous binder film on the particles of the coarse-grained portion 
sufficiently low to permit the particles supplied to penetrate into and be 
taken up by said bituminous binder film, but still sufficiently high to 
retain them in suspension. Many existing, conventional asphalt plants have 
an open-pan mixer with two spaced parallel agitator shafts which rotate in 
opposite directions and also convey the mass in opposite directions so 
that the mass during the mixing process is moved downwards in two streams 
at the opposite long sides of the pan, said streams meeting at the bottom 
midway between the shafts. In such asphalt plants it is customary for the 
fine-grained portion to be supplied at one long side and the filler 
portion at the other long side of the pan. If the filler portion in such 
an asphalt plant is admitted at the same time as the fine-grained portion 
the materials meet at the bottom of the pan before the filler has been 
taken up the binder films on the particles of the coarse-grained portion. 
This actually implies that the filler when the streams of material meet at 
the bottom of the mixing pan, has temporarily saturated the binder films 
on the particles of the coarse-grained portion and that the particles of 
the fine-grained portion cannot therefore be taken up by the binder films 
but go on being loose particles. This phenomenon has a disturbing effect 
on the characteristics of the future asphalt concrete mix, and the mix 
will be dry and hard. In conventional plants it has therefore been 
necessary to increase the amount of bituminous binder supplied in order to 
counteract this problem. As mentioned in the foregoing the present 
invention resides in the realization that one must regulate the thickness 
and viscosity of the bituminous binder film deposited on the particles of 
the coarse-grained portion to permit utilizing the binder supplied in an 
optimum manner and cause it to catch and take up the supplied fine-grained 
portion. The invention therefore also relates to an apparatus for the 
batchwise production of asphalt concrete, said apparatus being 
characterised by the features defined by claim 6. In such an apparatus 
according to the invention use is thus made of a control device, for 
instance a programmed computer, for controlling the dosing means and the 
supply of materials to the mixing plant so that the free particles of the 
fine-grained portion added, when supplied to the mixer, will meet the 
coarse-grained portion already coated, on the particles of which the 
bituminous binder film has already taken up the particles of the filler 
portion. The control device is preferably also arranged to regulate the 
amount of filler portion supplied and/or the temperature of the filler 
portion supplied, in dependence on the viscosity of the bituminous binder 
film deposited on the particles of the coarse-grained portion. 
To prevent that the binder films on the particles of the coarse-grained 
portions are temporarily saturated with the particles of the filler 
portion, when the fine-grained portion gets into touch with the 
binder-coated particles of the coarse-grained portion, the control device 
may be adapted to provide an addition of the entire filler portion to the 
mixer before the addition of the fine-grained portion is started. To the 
same end, the mixer and the supply means thereof, which are connected with 
the dosing means, may be so designed that the filler portion and the 
fine-grained portion are supplied at such points of the mixer that at 
least the major part of filler portion has been taken up by the binder 
film on the particles of the coarse-grained portion before said particles 
meet the free particles of the fine-grained portion supplied. 
When the fine-grained portion is produced in a mixing plant, the particles 
thereof are separated by means of a screen having a mesh width of 
approximately 3-4 mm. This will give a fine-grained portion substantially 
having a particle size of 2 mm and less. The choice of screen is dependent 
on the material flow over the screens and the type of screening device. 
The flow may be 80-600 tons/h. A presence, if any, of a minor portion of 
large particles in the fine-grained portion does not take away the effect 
of the particles less than 2 mm which are always present in the 
fine-grained portion and effective in applying the invention. 
One object of the invention is to produce an asphalt mix the 
characteristics of which preferably agree with those of a mastic asphalt 
mix as to composition and elastic and self-healing properties. The method 
and apparatus according to the invention for supplying the different 
materials to the mixer have been adapted with the aid of a new measuring 
method for measuring the characteristics of the asphalt mix compared with 
those of the mastic asphalt. This measuring method which is described in 
more detail in the following, measures on the one hand the temperature of 
decomposition (ToD) of an asphalt concrete test specimen, and on the other 
hand the deformation of the same test specimen before it reaches the 
temperature at which it cracks. The method expresses the properties of the 
mix in the following way: 
(1) For mixes conventionally mixed ToD is approximately 
200.degree.-210.degree. C. without any deformation of the test specimen. 
The fracture is relatively dry. 
(2) For mixes having a conventional filler content but mixed in accordance 
with the invention, ToD is 150.degree.-180.degree. C. The fracture is 
relatively rich in bituminous binder. 
(3) For mixes having a high filler content and mixed in accordance with the 
invention, ToD is 225.degree.-250.degree. C. When ToD is measured on mixes 
which have been mixed in accordance with the invention and have a high 
filler content, a deformation of the test specimen begins at approximately 
180.degree. C. because the binder, which is a suspension, passes into 
liquid state. The deformation increases with increasing oven temperature. 
This can be compared with a mastic asphalt which is in a liquid state at 
190.degree.-200.degree. C. Exploiting the present invention it would 
therefore seem possible, whenever mixing is done, to ensure that the 
binder which is in the state of a suspension has a sufficiently large 
volume in the mix in order that the measurable properties of the mix may 
be expressed in viscous state.

The apparatus according to the invention shown by way of example in FIG. 1 
has a heating drum 10 which is heated by means of a burner 11 and supplied 
with aggregate (mineral aggregate) through an inlet 12. From the heating 
drum an outlet 13 leads to the lower end of a bucket elevator 14 the 
outlet 15 of which opens above a set of screens 16. The screens divide the 
mineral aggregate into different portions which are collected in bins 17, 
18 and 19. The exhaust gases from the heating drum 10 are passed to a 
cyclone 31 and the outlet thereof for separated dust (which is used as 
filler) is connected to a hot filler bin 32. There is also a bin 20 for 
separately supplied filler material. Each of the various bins is provided 
at its lower end with a controllable feed opening which opens above a 
weighing device 21 having an openable emptying socket 22. Below said 
socket is placed the open pan mixer 23 of the apparatus, which is of the 
above-mentioned conventional type having two shafts rotating in opposite 
directions and provided with agitator blades. Besides there is mounted 
above the mixer a diffuser pipe 24 which is connected with a supply 
container 25 for bituminous binder heated into liquid state. For the 
heating of the bituminous binder and the control of the temperature 
thereof a heating coil 26 is provided in the supply container 25. A feed 
pipe 27 from a mineral aggregate bin 28 with controllable discharge 
opening opens into the outlet pipe 13 from the heating drum 10. The 
mineral aggregate bin 28 contains non-heated mineral aggregate. 
Temperature sensors 29 are arranged in the various bins 17-20, 28 and 32, 
the supply container 25 and the outlet pipe 13 upstream of its connection 
with the feed pipe 27. The temperature sensors 29 are connected with a 
computer 30 serving as a control device. The computer 30 is also connected 
with the discharge doors of the bins 17-20, 32 and with the discharge door 
of the supply bin 28, with the weighing device 21 and its discharge door 
22, with a control valve in the diffuser pipe 24 and with a control device 
for the heating coil 26. 
With the use of the apparatus according to the invention the streams of 
material to the mixer and the relative proportions of said streams are 
controlled with the aid of the computer 30 so that the particles of the 
coarse-grained portion will be coated with a bituminous binder film which 
already in suspension contains the filler particles and has a suitable 
thickness and viscosity in order that the particles of the fine-grained 
portion, when said fine-grained portion is supplied, shall be taken up and 
retained by said binder film. The computer thus processes the measured 
values sensed and adjusts them to preprogrammed mixing proportions. If 
need be, the mixing proportions are modified in order that the binder 
films on the particles of the coarse-grained portion shall have the most 
suitable thickness and viscosity when the fine-grained portion is supplied 
and encounters said particles. The temperature and viscosity control may 
take place by certain reproportioning and/or by supply of non-heated 
mineral aggregate from the aggregate bin 28. But one aggregate bin 28 is 
shown, but the apparatus may have several such aggregate bins for dosing 
cold mineral aggregate fractions in adjusted proportions. 
The computer 30 can thus be programmed to provide a certain reproportioning 
by a change of the filler content, by a change of the content of 
fine-grained portion and/or by a change of the asphalt content, and also 
to provide a change of the temperature of the mineral aggregate (supply of 
non-heated mineral aggregate for compensation of temporary excess heating 
in the heating drum 10) and to provide a change of the filler temperature 
(supply of a large or small amount of non-heated foreign filler or a large 
or small amount of cyclone-separated filler). All of these control 
possibilities with the aid of the computer thus aim at providing any 
desired viscosity of the bituminous binder film deposited on the particles 
of the coarse-grained portion before the fine-grained portion gets into 
touch with said coated particles. 
In the following examples use is made of a newly developed measuring method 
for judging the elasticity and self-healing properties of an asphalt 
concrete. In this measuring method the so-called temperature of 
decomposition (ToD) is determined. Measuring is performed on Marschall 
test specimens (diameter 10 cm, height 6 cm) which are placed in an oven 
tempered to 100.degree. C. After 30 minutes tempering of the test 
specimens the temperature in the oven is increased in steps of 25.degree. 
C. until the test specimens decompose. The decomposition of the test 
specimens occurs suddenly, and the prevailing oven temperature at the time 
of decomposition is recorded. In experiments made the temperature of 
decomposition was in the range of 100.degree.-250.degree. C. Rising values 
of the temperature of decomposition show that the asphalt concrete becomes 
harder and displays decreasing elastic and selfhealing properties. 
ToD-values of 150.degree.-180.degree. C. are at present considered as the 
most suitable. Exploiting the present invention, it is possible to produce 
asphalt concrete mixes of predetermined ToD-values since it has been found 
that the filler content and the filler temperature as well as the fines 
temperature have a very strong effect on the temperature of decomposition 
in the production method according to the present invention. 
EXAMPLE 1 
In this Example use was made of a bituminous binder having a penetration of 
180/200 mm/10 for the production of an asphalt concrete of quality MAB 16, 
i.e. medium hard asphalt concrete with a mineral aggregate size of 0-16 
mm. The mineral aggregate was divided into a coarse-grained portion, the 
particles of which mainly had a particle size of over 2 mm, a fine-grained 
portion, the particles of which mainly had a particle size of less than 2 
mm, and a filler portion, the particles of which mainly had a particle 
size of less than 0.1 mm. 
The production was carried out in a laboratory asphalt concrete mixer which 
was first supplied with the coarse-grained portion and the bituminous 
binder which was heated to 150.degree. C. Then the filler portion was 
added, which was thoroughly admixed before the fine-grained portion was 
added. Also the filler and fine-grained portions had been heated to 
150.degree. C. 
In experiments A-D batches of 500 g bituminous binder, 6712 g 
coarse-grained portion and 2198 g fine-grained portion were used, while 
the filler amounts in experiments A and B were 590 g and in experiments C 
and D 820 g. This gave a composition of 5% binder, 5.9% filler, 21.98% 
fine-grained portion and 67.12% of coarse-grained portion in the test 
specimens of experiments A and B, and the composition 4.89% binder, 8.02% 
filler, 21.49% fine-grained portion and 65.60% coarse-grained portion in 
the test specimens of experiments C and D. The only variable thus was the 
supplied amount of filler. 
Measuring the temperature of decomposition it was established that the 
increased filler content in test specimens C and D gave an increase of the 
temperature of decomposition, i.e. the asphalt concrete had become harder. 
The measured values of the various test specimens were A=175.degree. C., 
B=173.degree. C., C=190.degree. C. and D=185.degree. C. The results are 
shown in the form of diagrams in FIG. 2, which have also been confirmed by 
a long test series. 
EXAMPLE 2 
In this Example the effects of the filler and mineral aggregate 
temperatures on the temperature of the composition were examined. For the 
experiment use was made of the same material composition as for the test 
specimen A in Example 1. 
Four test specimens E-H were made. As for the test specimens E and F the 
filler temperature only was varied, while as for test specimens G and H 
the temperature of the mineral aggregate portion only was varied. 
Otherwise the other materials had a temperature of 150.degree. C. at the 
mixing thereof. The filler temperature was 60.degree. C. and 150.degree. 
C., respectively, in experiments E and F, while the temperature of the 
coarse-grained and fine-grained portions was 125.degree. C. in experiment 
G and 160.degree. C. in experiment H. 
The temperatures of decomposition of the four test specimens were 
E=168.degree. C., F=195.degree. C., G=155.degree. C. and H=173.degree. C. 
The result is shown in the form of a diagram in FIG. 3, which is also 
confirmed by a long test series. 
As will appear from this experiment the temperature of decomposition can be 
influenced by a change of the temperature of the materials supplied. An 
increase of the mineral aggregate temperature results in an increase of 
the temperature of decomposition. 
EXAMPLE 3 
In this Example a series of experiments was carried out to establish how 
the bulk density of the asphalt concrete mix produced by the method 
according to the invention was influenced by the filler content, the 
mixing temperature and the penetration of the bituminous binder. The 
asphalt concrete produced was proportioned so as to have a medium hard 
quality MAB16 as in Examples 1 and 2. 
In the series of experiments shown in FIG. 4 use was made of a mineral 
aggregate having a lower true density than in the series of experiments 
shown in FIGS. 5, 6 and 7. Otherwise, conditions were the same and only 
the variable indicated in the various FIGS. 4-7 was varied. 
As will appear from FIGS. 4 and 5 an increase of the filler content gave an 
increase of the bulk density, which shows that the binder films on the 
particles of the coarse-grained portion became thicker (and also obtained 
a higher viscosity) at the supply of more filler. An increase of the 
mixing temperature from 120.degree. C. to 140.degree. C. according to FIG. 
6 gave a decrease of the bulk density when all other conditions were the 
same. When the type of bituminous binder was varied (FIG. 7) an increase 
of the bulk density was obtained on changing over to a harder bituminous 
binder, i.e. at a successive lowering of the penetration of the bituminous 
binder. 
The Examples accounted for above show that an asphalt concrete produced in 
accordance with the invention approximately agrees with a poured asphalt 
in regard of viscous properties and that it is therefore possible to carry 
out various series of experiments to establish empirically how the asphalt 
concrete is influenced by a variation of various parameters so that it is 
possible on the basis of the results of these experiments to program the 
computer comprised in the apparatus according to the invention so that the 
computer modifies the filler content and temperature and other variables 
during the production process, whereby the contemplated asphalt concrete 
quality is obtained.