The major
problem facing highway, road, and street agencies in the mid 1960’s was
that the systems that had been improved after World War II were near
the end of their design life. The pavements needed rehabilitation
and/or structural upgrading.
Overlays of the pavements, whether rigid or flexible, were
the obvious answer. Unfortunately, when overlayed, all the cracks
and/or joints in the existing pavement reflected through in a
relatively short time. A solution was extremely important.
It had been recognized since the beginning of asphalt
pavements that if an elastic component could be introduced into the
asphalt matrix performance of the pavements could be greatly improved.
But all efforts to do this resulted in cost increases that were
unacceptable.
Then a materials engineer for the City of Phoenix came up
with the answer. Charles McDonald found that ground, vulcanized rubber
from old tires when combined with asphalt at a high temperature for a
specific time would react to form an elastomeric material.
The use of this material, now called Asphalt-Rubber, was
economical since it involved a waste product. In time it was
technically defined by the American Society for Testing Materials
(ASTM).
Phoenix immediately began field testing this material by
placing small test squares of it over areas of pavement distress. Test
sections were also placed on state highways to evaluate cold weather
applications.
From these simple tests it was readily apparent that
material held great promise. But there were a lot of questions that had
to be answered first. Some of the questions were:
- What was the reaction process between the asphalt and
the vulcanized rubber? How could it be optimized? How could it be
evaluated? What material properties of the asphalt and the rubber were
critical to the end product?
- How could the materials be tested? Standard asphalt
tests were empirical and not applicable.
- How were the material’s properties affected by
temperature change or aging?
- What type of paving systems could be utilized with
these materials?
- How could it respond to wheel loads?
- What equipment modifications or new equipment would
be needed?
Getting answers was a monumental task and involved many
factions of the industry
- a joint effort of civil engineers, chemists,
engineering professors, materials suppliers, and contractors.
Government agencies included the City of Phoenix, Arizona Dept. of
Transportation (ADOT), Federal Highway Administration, and Army Corps
of Engineers. Primary material suppliers were Arizona Refining and
Sahuaro Petroleum.
ADOT conducted or sponsored numerous lab research projects
and built many field test and full scale projects between 1968 and
1988. This effort resulted in over 75 formal papers and many
presentations.
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Here is what we found.
- Asphalt-Rubber chip seals (SAM) control fatigue
cracking and have an effective life of 15 years.
- Asphalt-Rubber interlayers (SAMI) controls all
cracking,
substantially reduces stresses in overlays and provides membrane
control of expansive clay.
- Open-Graded surface courses are effective overlays of
rigid
pavement at a thickness of an inch, and of asphalt pavement at a
thickness of a half-inch. They also provide high skid resistance,
reduced water splash, cut noise by at least half, and have a high
resistance to aging.
- Gap-Graded hot mix applications are structurally
equal to
standard asphalt concrete of over 2-to-1, are effective on asphalt
concrete, have a high resistance to rutting, and ideal for urban
arterials and residential streets.
The cost savings to the agencies have been substantial.
Two sections of the
concrete pavement on I-40 near Flagstaff needed rehabilitation.
One five mile section was reconstructed using standard
design. It
took two years to construct and needed a maintenance overlay in 10
years.
The other 10-mile section used a 2½- inch
gap-graded Asphalt-Rubber
mix followed by a ½-inch open-graded Asphalt- Rubber surface
course. It
took four months to construct and was still in excellent condition with
no reflective cracking after 10 years.
The state estimates that it saved over $30 million on this
single project.
Another example is the one-inch Asphalt-Rubber open-graded
overlay
placed on I-19 south of Tucson in 1988. It cost $2.45 per square yard.
The alternative solution was to grind and groove the pavement at an
estimated cost of $5.00 per square yard and a projected life of eight
years.
After 13 years, the Asphalt-Rubber overlay has been
maintenance free
and is still is excellent condition. Besides its ride quality, the
overlay has reduced noise levels by more than 50 percent, which has
been very important to nearby residents.
To date, nearly all of Arizona’s interstate highway system
has been
rehabilitated with one of the Asphalt-Rubber applications.
One other application that should be mentioned is
maintenance crack
sealing. Prior to the development of Asphalt-Rubber there were no
economical crack sealing materials nor equipment to routinely seal
pavement cracks.
Now, virtually every state and county have programs for
routinely
sealing pavement cracks. The development of Asphalt-Rubber also led to
a new industry with a worldwide market and an additional industry to
grind old tires. Over 85 percent of the rubber from the tires in
Arizona is recycled into the roadways.
The Arizona project has developed a new paving material
that has
paid major dividends to government agencies, found a positive use for a
major waste product, and created new industries.
Gene Morris, P.E., is
still active in Asphalt-Rubber research and technology transfer after a
long career in
the public and private sector that included serving the Arizona
Department of Transportation as Director of the Arizona Transportation
Research Center. He also served as an asst. district engineer, senior
resident engineer, quality control engineer, and research engineer. His
agency career spanned from 1962 to 1983. In the private sector, Morris
has been affiliated with Western Technologies, Inc., International
Surfacing Systems, and was co-owner of Pavement Technologists, Inc.
Morris was the ADOT engineer who introduced Charlie McDonald’s work to
state agencies throughout the U.S. He has authored over 50 technical
publications and conducted field reviews on the performance of the
state’s 2,500 miles of Asphalt- Rubber pavements. Morris is an active
participant in RPA workshops and conferences and serves as the vice
chairman for its Technical Advisory Board. |