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 Asphalt-Rubber, which has been
around since the 1960's, was first defined in 1988 by ASTM, a
universally recognized technical body, as "A blend of asphalt
cement, reclaimed tire rubber, and certain additives in which
the rubber component is at least 15%* by weight of the total
blend and has reacted in the hot asphalt sufficiently to cause
swelling of the rubber particles."
* Standard practice in user agencies is 18-20% percent
rubber.
Asphalt-Rubber was first introduced
in Arizona as a patented process, with different patents being
held by two entities. One by inventor Charles McDonald, who licensed
Altos Rubber which then issued a sublicense to Sahuaro Petroleum
Company. Later Arizona Refining Company, a subsidiary of Unocal,
developed a parallel process based on similar high percentages
of rubber but with different types of material (high natural
rubber) and asphalts.
The first applications, after the initial "band
aid" pothole patches used on streets by McDonald, the Engineering
Supervisor for the City of Phoenix, were spray applied "Chip
Seals." The chip seals, with the highly vicious rubber and
asphalt binder, necessitated the development of equipment specially
designed to handle the material. The company developing the first
equipment was Bearcat of Wickenburg, Arizona. Today there are
several companies offering Asphalt-Rubber distributors and blenders.
Among them, CEI Enterprises of Albuquerque, New Mexico, Hot Mix
Industries of Texas and Valley Slurry Seal (VSS) Macropaver Division
of Hickman, California. However, many contracting companies build
their own equipment.
The Arizona DOT became involved with
the material in the mid sixties, working with McDonald and the
City of Phoenix. Gene Morris, P.E. the ADOT Asst. District Engineer
for the Phoenix district at that time began to try different
applications of the materials. Morris was later named Research
Engineer and despite the usual early "disasters," pressed
ADOT forward with research and test sections. Their work through
the years has led to successful routine usage of the process
in seal coats, interlayer, membranes, gap-graded Asphalt-Rubber
concrete and open-graded Asphalt-Rubber concrete surface courses.
Today, the Arizona DOT has over 3,500 miles of its 7,000-mile
state highway and interstate system paved with Asphalt-Rubber.
Interest in the new process material
intrigued highway materials researchers and led to the First
Seminar on Asphalt-Rubber in Scottsdale, Arizona in 1980. The
meeting was sponsored by the FHWA, the Arizona Department of
Transportation; Genstar Conservation Systems, Inc.; Arizona Refining
Company, Sahuaro Petroleum and Engineers Testing Laboratory.
The FHWA had commissioned a demonstration project in 1976 in
which 43 Asphalt-Rubber demonstration projects were constructed
in various states. The FHWA program included 16 chip seals, 23
interlayers, 3 bridge deck seals and 1 embankment stabilization.
At the seminar, a $3,500,000 research,
development and implementation needs program was developed to
build on the seven technical presentations and the 66 major research
papers referenced at the conference. Even at that date, the research
was not limited to the U.S., fourteen reports came from Canada,
two from Australia, one from Great Britain and one from France.
Today, over one thousand major reports chronicle the development
and success of Asphalt-Rubber.
John Epps, Ph.D, P.E., and Bob Galloway
both of Texas A&M University wrote the conference summary.
The conclusions of the summary contained some positive statements:
- 1. "Asphalt-Rubber is a viable material that provides
an attractive economic alternative for construction, rehabilitation
and maintenance of our nation's roadway network. Economic advantages
stem from increased life and reduced thicknesses of pavement
section. In addition the material is attractive from an energy
and resource recovery
point of view."
- 2. "Asphalt-Rubber is a new binder system and does not
respond to analysis by conventional tests associated with asphalt
technology. Furthermore, the binder and the binder-aggregate
system are both chemically and mechanically complex and far from
being fully understood."
- 3. "A nationally recognized group of users, producers
and researchers needs to help formulate research and development
programs, monitor research, sponsor conferences, workshops, etc.
It is recommended that the Transportation Research Board create
a Task Force on Asphalt-Rubber Binder Systems and that ASTM Subcommittee
on D04.37 actively pursue the standardization of test methods,
quality control techniques and definitions. User-producer workshops
should be continued to provide an open forum for discussion of
the problems associated with the new and emerging binder systems."
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- 4. "Additional research, development and implementation
efforts need to be undertaken to improve the economy and predictability
of performance of the asphalt-rubber binder for specific applications.
These efforts have been identified in this conference and appear
in the proceedings."
Time and perseverance by agencies like ADOT,
TXDOT, the FHWA, Caltransmnand the Rubber Pavements Association
has fulfilled the expectation of the seventy-eight attendees
of the 1980 Asphalt-Rubber User-Producer workshop. The agencies
have moved forward with research in the field and their labs;
RPA provides seminars and workshops on an international basis
and has commissioned much of the research identified 23 years
ago. ASTM has adopted a definition and standards and research
is continuing at well-known Universities throughout the US and
in other countries. Four of the attendees of this historic conference
now serve on the 19 member RPA Technical Advisory Board; Doug
Bernard, P.E. FHWA ret., Dr. Jon Epps, Prof. Emeritus University
of Nevada Reno, Dr. Rudy Jimenez, Prof. Emeritus, University
of Arizona and Gene Morris, P.E., ADOT ret.
In the early years of the material, no
one really thought much about the tire recycling aspect, as the
discarded tire problem was not fully recognized by the nation
until the late 80s and early 90s. Even today, most highway engineers
will tell you that performance of the material outweighs its
recycling benefits in their decision process. However, once
the appeal and success of the environmentally enhanced asphalt
material started spreading beyond the three original states,
it also caught the attention of the oil companies as a means
of making healthy profits on modified asphalt binders. It appears
the oil companies decided they wanted a bigger piece of the market
share than being an asphalt supplier afforded them. Soon a proliferation
of modified binders hit the market, with their owners claiming
they too contained recycled scrap tire rubber. The problem is
that the majority, if not all the new "rubberized"
asphalt binders are blended at the oil terminals, and have specifications
that require a very fine gradation of rubber. The rubber content
of these materials range from a low of 3 or 4% to a high of 10%.
Verification on the actually rubber content relies on a "certification"
as opposed to the visual "inspection" process available
with the Asphalt-Rubber materials. Some products require proof
of the absence of the rubber via solubility tests.
According to Gene Morris, the low percentage
of fine grind rubber was tested and rejected by ADOT as failing
to achieve an equivalent level of properties and field performance
as the mixtures containing the higher percentage of coarser rubber
particles. Morris has postulated that a minimum percentage and
size of the swollen rubber particles is necessary to form a continuous
elastomeric phase in the reacted material. This continuous phase
is necessary to prevent reflective cracking, provide the significant
increase in resistance to aging, and to develop the modifications
in behavior that result in material improvements in both the
high and low temperature ranges of pavement performance. Also
these properties developed with the ASTM defined Asphalt-Rubber
permit a significant increase in the binder content of asphalt
concrete plant mixes. This increase substantially improves the
pavement performance with respect to resistance to cracking,
aging, pavement life and maintenance costs. A real world example
of the performance improvement was demonstrated in the first
thin overlay of a Portland Cement Concrete pavement. In 1988
the I-19 south of Tucson, Az, was overlaid with a 1" thick
open-graded Asphalt-Rubber mixture. In 2001 an in-depth inspection
of this 2-mile section revealed no reflective cracking, high
skid resistance, and zero surface maintenance costs over the
13-year period of use. (The I-19 is the major produce-trucking
route from Nogales, Mexico to the U.S.)
According to Morris, Vice Chairman of
the Technical Advisory Board for RPA, resistance to reflective
cracking is thought to be a function of the elastic deformation
of the binder, particularly in the post failure zone, and the
reduction of the coefficient of thermal expansion. Morris claims
neither of these improvements can be obtained with low percentages
of fine grind rubber. He is adamant in his claims that extensive
research, field evaluations and long-term performance have shown
that a minimum of 15 percent of ground rubber, properly reacted
with asphalt, is necessary to achieve this type of performance.
When one examines the countless research performed by leaders
in the field of asphalt technology, it would be difficult to
dispute Morris's claims. In commenting on the "new generation"
of terminal blended modified binders who claim to be "equivalent"
to Asphalt-Rubber, Morris challenges the producers to reveal
the research and long-term performance to support their claims.
The claims, as stated in their advertising,
often defy logic and engineering principles, such as more binder
produces a better performing mix. A good example is an ad, which
appears in a west coast industry magazine. In the ad the benefits
of the material are touted in four bullet points. Three of the
points are reasonable but the one that catches the eyes of an
"old asphalt hand" like Morris is "Cost Savings.Allows
for Lower Asphalt Binder Content in Mix." This combined
with a misleading statement "A Refinery Produced Asphalt
Rubber Binder" should give one an insight to their marketing
strategy of aligning their products with the real ASTM defined
Asphalt-Rubber that has years of research and proven performance.
It is interesting to note that the modified binder proponents
are calling for a new definition of Asphalt-Rubber, presumably
with something less than a minimum 15% rubber content to legitimize
their misuse of the term Asphalt-Rubber.
Is their strategy working? Unfortunately,
in some cases it is. The RubberPavements Association, often gets
calls from agencies complaining that they tried Asphalt-Rubber
and it failed. When asked about the rubber content of the mix,
they are sometimes shocked to learn they did not use Asphalt-Rubber
at all, but had placed a terminal blended product thatnot only
contained less than 15%, in most cases, less than 10% rubber.
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