Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Background and description of prior art
Asphalt pavements consist of a mixture of asphalt cement (or asphalt binder)
and
aggregate. The aggregate consists of coarse aggregate such as crushed stone,
and fine
aggregate such as sand and fillers. The asphalt binder is most commonly
obtained as the
heavy fraction of petroleum refining and makes for approximately 4 to 7% of
the total
weight of the mix. The binder can be further modified through the addition of
additives in
order to improve the performance of the pavement, for example the rutting
resistance,
moisture sensitivity or serviceable temperature range.
Until the recent years there were two prevalent methods of obtaining asphalt
concrete,
namely the hot mix process and the cold mix process. In the hot mix process
the
aggregate and the binder are heated at elevated temperature, and then mixed
until uniform
coating of the aggregate is obtained. The hot mix facilities can be batch
plants or drum-
mix plants, where the aggregate and the asphalt cement are continuously fed in
the
process, while hot mix is withdrawn at the same rate.
A common method of obtaining cold mix asphalt is to emulsify the asphalt
binder prior to
mixing with the cold aggregate. This does not require a significant energy
intake, and
hence it is less expensive and more environmentally friendly than hot mix
asphalt. The
major drawback is that the quality of the asphalt concrete produced in this
process is
significantly inferior to hot mix asphalt, resulting in short life cycles of
the pavement.
The reasons for the poor performance are related to the presence of water in
the mix.
Among other things, this causes incomplete coverage of the aggregate with
binder and
poor adhesion of the two components. The lack of adequate adhesion causes
further
deterioration of the pavement by what is known as stripping, ravelling or
pitting.
A more recent approach to asphalt paving is the warm mix asphalt. In general
terms, this
process operates in a similar manner with the hot mix technology. One defining
difference is that the components are heated to temperatures lower than those
involved in
hot mix plants. This temperature depends on the grade of the asphalt, but
decreases of up
to 50 C have been reported. Some of the advantages of the warm mix asphalt
technologies are lower fuel consumption, less emissions of greenhouse gas and
other
pollutants, as well as shorter times before the road can be open to traffic.
There are
several types of warm mix technologies available and considering the means
used to
obtain the reduction in temperature they can use either some kind of additive,
or a process
for foaming the asphalt, or a combination of both.
Asphaltan B is a mixture of compounds based on lignite (Montan) wax and other
higher
molecular weight hydrocarbons. Sasobit is a Fisher-Tropsh wax which is added
in
small amounts to the asphalt binder or directly to the mixing drum. The
principle behind
using waxes is the lubricating effect that they have when the temperature
exceeds the
melting temperature of the additive. Waxes however change the properties of
the binder
significantly, generally shifting up the service temperature range. This will
carry with it a
financial burden, since softer binders, which are needed to obtain a certain
grade, are
usually more expensive than harder binders. There is also concern that the
presence of
wax in the asphalt binder negatively affects the low temperature performance
of the
pavement due to crystallization. Other chemical additives are used to improve
the
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workability and compaction of asphalt mixtures and they are available as
emulsions or as
stand-alone packages (EvothermTM family). As a general rule the use of
additives
significantly increases the unit cost of asphalt mix produced and may
negatively affect
the performance of the pavement.
US patent application 2008/0259714 describes a method and the apparatus to
produce
warm mix asphalt by foaming the asphalt binder prior to mixing it with the
aggregate.
The foaming process reduces the viscosity of the binder, allowing mixing and
compacting
at reduced temperatures. The foaming of the asphalt is achieved by introducing
from
about 0.6 to about 2% water in the hot binder. This poses a concern from at
least two
perspectives. Firstly, water is introduced intentionally into the mix, and
this carries with it
the risks mentioned before, namely poor adhesion between the binder and the
aggregate,
and consequently it can be detrimental to the mix stability. Secondly, the
water is
vaporized and this is an energy intensive process, energy which is lost.
Other foaming processes make use of synthetic zeolites, such as Advera or
Aspha-
min . The zeolites contain approximately 20% water, which is released when the
inorganic additive comes in contact with the heated binder. The released water
vapours
cause the foaming of the binder reducing the viscosity and thus improving
workability.
The cost of the additive and the moisture introduced in the mix make this
option less than
satisfactory for producing warm mix asphalt.
Another technology using the foaming of the binder is known as LEA (Low
Emission
Asphalt). In brief, the hot coarse aggregate is mixed with the hot asphalt
binder
containing a chemical additive, the cold finer aggregate (sand) being
subsequently added
to the mix. The sand is wet, and the moisture present causes the foaming
action of the
binder. Significant energy savings are reported for this process, but again
the moisture
introduced in the mix points away from the generally accepted idea that
moisture present
in the mix is detrimental to mix properties and pavement performance.
Canadian patent 2238368 describes a process to prepare warm mix asphalt, in
which a
binder with a penetration of less than 200 dmm as measured by ASTM D5 at 25 C,
most
preferably below 10 dmm, is added to mixture of aggregate and another softer
binder
component of penetration higher than 200 dmm as measured by ASTM D5 at 25 C,
most
preferably higher than 800 dmm. This process can be operated at temperatures
below
140 C. However, the use of two binder components would require additional
storage
facilities at the plant and some operational challenges. Also, most user
agencies have
specific requirements regarding the asphalt binders used on their projects.
Mixing two
components would make it very difficult to predict the properties of the
resulting asphalt
mix. A variation of the above process is described in US patent 6846354, in
which the
hard bituminous binder is foamed by adding 2 to 7% water. Beyond suffering
from the
same shortcomings as the Canadian patent referenced herein, water is purposely
introduced in the mix and this can adversely affect the properties of the mix.
A review of the available warm mix asphalt technologies reveals some important
reasons
for which they are less than ideal alternatives to hot mix asphalt. The cost
of additives can
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be prohibitive, the properties of the asphalt binder are changed, or the
moisture
introduced in the asphalt mix raises concerns regarding the durability of the
pavement. It
would be desirable if a method would be developed that would allow the
production of
warm mix asphalt while at the same time circumventing the above mentioned
shortcomings. One way would be to use a foaming agent other than water to
produce
foamed asphalt.
US patent 2,876,126 discloses a process for mixing fine aggregate with asphalt
by
foaming the asphalt prior to mixing with the fine aggregate in order to obtain
a flowable
composition. The foaming action is obtained by incorporating a gas, a gassing
agent or a
solvent into the asphalt mass. Optionally, the process can be carried out at
elevated
pressures when solvent is used to create the foaming action. The invention
requires the
bitumen to have a proportion of 8 to 15% by weight of the total composition,
fine
aggregates (maximum size 7 mm) and it is used for making asphalt mastics.
US patent 4,256,734 discloses the use of foamed asphalt for road surfacing.
The foaming
action can be obtained for example by injecting a gas or a liquid into the hot
binder. The
boiling point of the liquid must be lower than the temperature of the binder
to ensure it
evaporates. Optionally, when a liquid is used as foaming agent the pressure
can be
increased in order to keep the liquid from evaporating before leaving the
mixing
chamber. The invention deals exclusively with road surfacing, where the binder
and the
aggregate are not mixed prior to laying on the road.
Explanation of technical terms
As used herein, the terms asphalt cement or asphalt binder or binder or
asphalt or bitumen
refer to a material that is used in conjunction with aggregate to produce
asphalt concrete
and they are used interchangeably.
As used herein, the terms aggregate or aggregate material refers to crushed
stone and
other particulate materials that are used in the construction of asphalt
concrete such as
sand, gravel, natural and synthetic fibres and others.
As used herein, the term asphalt concrete refers to bituminous paving mixes.
As used herein, the terms gas and gaseous component refer to a substance or
mixture of
substances that exist in gas phase under conditions of normal temperature and
pressure.
As used herein, a gassing agent is a substance or a mixture of substances that
release a
gaseous component when added to the hot bitumen. The released gas is the
result of
either thermal decomposition of the gassing agent, or of a chemical reaction.
As used herein, a dry asphalt foaming method refers to a method of foaming the
asphalt
in which foaming is caused by a component that under conditions of normal
temperature
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and pressure is not in liquid form. Notwithstanding the above, and although
not
recommended because of the reasons discussed above, water vapours could still
be used
in conjunction with the said non-liquid components without departing from the
spirit and
teachings of this invention.
Summary of the invention
The present invention describes a process to produce asphalt-aggregate mixes
used for
paving applications. In one embodiment of this invention the asphalt binder is
mixed with
a gas or a mixture of gases at super atmospheric pressure. The asphalt and the
gas
component are introduced in a mixing chamber where they are allowed to stay in
contact
such that the gas will dissolve and/or disperse in the binder. The efficiency
of the mixing
process can be improved with the aid of one or more mechanical mixers or some
other
method available to the practitioner, such as, for example, recirculation of
the mixture of
asphalt cement and gases. The mixing chamber may consist of one independent
enclosure
or several enclosures. The chamber has one or more inlets for the liquid
asphalt, one or
more inlets for the gaseous component, and one or more outlets for the
asphalt/gas mix.
Alternatively, the liquid asphalt and the gas component may be pre-mixed prior
to
entering the mixing chamber, such that they share some or all of the inlets.
The asphalt-
gas mixing chamber can alternatively be replaced by in-line mixing such as
directly into
the line that transports the liquid asphalt to the aggregate-asphalt mixing
compartment.
When the mixture of asphalt and gas exits the mixing chamber at pressures
below those
present in the mixing chamber, the gas expands and causes the foaming of the
liquid
binder. In this foamed state the asphalt binder may be further processed in
the asphalt
mixing plant.
In another embodiment of this invention, the bitumen is mixed with gassing
agents in
order to obtain foamed asphalt. Without limiting the scope of the invention,
examples of
possible gassing agents are inorganic carbonates, azo derivatives and the
combination of
isocyanates with a suitable co-reagent.
Brief description of the drawings
Figure 1 shows a schematic comparison of asphalt mix production routes using
the
conventional hot mix process and the process involving dry foaming of the
binder prior to
mixing with aggregate.
Figure 2 shows an example of a method to foam the binder. This particular
example uses
air specified amounts (ratio) of air and binder which are mixed together with
the air of
mechanical mixers. The asphalt/air mix is inspected and should the desired
properties not
be met, some of the mix can be recirculated to obtain better dispersion or air
in asphalt.
