|
Asphalt is a product that is used in many everyday
applications (highways, streets, roads, driveways, parking lots, etc.)
but most people do not think about it until there is a problem with it
such as cracks, potholes, raveling, etc.
There are a number of inherent problems (as there are with most
materials - since they are not perfect) with asphalt as a construction
material. Asphalt wears out, cracks, ages, dries out, potholes, etc. It
will deteriorate even quicker than normal if sound engineering and
construction practices are not followed. If you try to make it softer so
that it won't crack, it is more susceptible to rutting. If you make it
harder to resist rutting it is more susceptible to cracking. Thus
all kinds of modifiers, designs, additives, etc. have come into play to
give asphalt the ability to resist oxidation, aging, raveling, rutting,
stripping, cracking, etc. - driving the cost up.
Over the last 40 years, the quality of what we generically call asphalt
has declined to the point that we now have to add modifiers to get it to
work satisfactorily. This is due to several factors - the increased use
of Processing Oils in the tire industry to disperse carbon black and
other fillers, advancing refinery technology allowing a greater amount
of lighter ends to be extracted from the bottom of the barrel of crude
thus reducing the quality of liquid asphalt, and a dramatic increase in
the number of automobiles and trucks on the road and miles driven.
One of the modifiers used to enhance the performance of
liquid asphalt is crumb rubber (ground - up recycled tires). Rubber-modified
asphalt dramatically out-performs conventional asphalt and lasts
significantly longer. These superior results have been achieved while using
less material than is required with conventional asphalt. Rubber-modified
asphalt will reduce reflective cracking and rutting, improve skid
resistance, reduce maintenance costs, increase pavement life, is more
resistant to weathering/aging, and reduces road noise. IN addition to
increasing the performance of asphalt, crumb rubber modification has the
side benefit of reducing the environmental hazard that old tires present.
Crumb rubber was first added to asphalt over 20 years ago, but there have
been and continue to be problems associated with the process and end
product.
One method used to modify asphalt with crumb rubber is the so-called "wet"
or "Arizona Method". The equipment required for this process is very
expensive, and there are significant operational constraints and
limitations. The process requires cooking the liquid asphalt and crumb
rubber at high temperatures (375 to 400 deg. F) for almost an hour. The
processing Oils start to migrate out of the GTR (Ground Tire Rubber) into
the asphalt in about 10 to 15 minutes. If cooking (or digestion) continues
from 15 to 45 minutes the low molecular weight from the asphalt, and to
increase the tackiness of the mix. This mix has to be cooled to prevent it
from sticking to the rolling equipment - costing valuable production time.
Vestenamer® polyoctenamer reacts chemically with both the
crumb rubber and the asphalt to produce a uniform, low-track, rubber-like
composite. This chemical bond is due to the double-bond structure of the
Vestenamer® product, which permits cross-linking of the sulfur associated
with the asphaltenes and maltenes in the asphalt, as well as with the sulfur
at the surface of the GTR to create a macropolymer network. Vestenamer®
polyoctenamer also chemically bonds to the aggregate surface, which reduces
stripping.
|
 |
In traditional methods - the process of "digestion",
the polymer structure is totally destroyed by high-shear, high-temperature
tearing of the polymer bonds. This dissociates the carbon black from the
rubber oligomers. Then there is a sediment problem in the storage tank to
deal with.
In our process, we are just "extracting"
out the asphalt soluble materials from the GTR, namely, the rubber
processing oils and the low molecular weight oligomers. this lwaves an
exoskeleton of high molecular weight rubber polymers and carbon black, which
swell as the hot asphalt fills in the spaces or voids left in the
crumb rubber after the solutes are extracted. This swelling of the GTR
changes it's density. The resultant swelled GRT exoskeleton's density makes
it more resistant to settling out ot suspension, and more importantly makes
it more easily redispersed with agitation or recirculating pumps.
None of the dispersion issues are of any importance if the dry process is
used since nothing has a chance to settle out of the hot mix asphalt. The
Vestenamer® quickly dissolves in the grinding of the
aggregate in the presence of the hot asphalt and reacts with the low
molecular weight soluble rubber oligomers that dissolve into the asphalt.
This reaction and the swelling of the GTR takes about 30 minutes. So, by the
time the hot mix is produced, to the time it is moved to the silo, then to
the truck, then to the jobsite, then to the paver, and finally onto the road
surface, it takes at least 30 to 60 minutes until the roller compacts it and
cross-links it all together.
Other advantages of using Vestenamer®:
-
Easier, faster, and more uniform mixing
-
Faster Paving
-
Densities achieved with less rolling, and only requires
static rollers
-
Superior surface finish
-
Application at low road-surface temperatures
-
Reduced cracking and rutting
-
Longer serviceable pavement life
-
Eliminate terminal bleeding
-
Lower cost per mile (blend with a low grade of asphalt
cement, thinner mats)
Rubber oligomers start to come out and add a considerable amount of tack
to the asphalt. The the asphalt concrete becomes difficult to roll due to
continual pick up on the rollers. These pick up spots on the surface of the
mat are areas where freeze/thaw cycling causes considerable damage. Also,
this digestion process will reduce the viscosity of the material below
acceptable limits. Asphalt modified with this method must be used within 24
hours of production or it becomes unusable. This method also has the
constraint of very limited production capacity.
In the early 1990's the United States Congress passed a bill requiring
States to utilize crumb rubber in 25% of their Federal road projects or face
the possibility of losing Federal funding. As a result of this legislation,
the states began testing crumb rubber in asphalt. Most of the tests were
failures because of lack of experience and knowledge on how to handle the
materials. Until, recently, the only successful method for modifying asphalt
with crumb rubber has been the "wet" method.
What has been needed is a process that would provide all of the benefits of
a rubber modified pavement without the significant costs and handling
problems and limitations of existing methods.
We now have the process to produce superior
rubberized asphalt at a fraction of the "wet" method, and eliminates the
production, handling, and application problems of other procedures.
With the use of Vestenamer® asphalt modifier and GTR, you
can have a superior asphalt rubber road surface at a lower cost per mile.
Using our process can modify your Base AC Binder to a higher performance
grade Binder. For example, a Pg-64-22 can go up to Pg-70-22, 76-22 or
higher. |
