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ABSTRACT: The Arizona Department of Transportation
(ADOT) has used crumb
rubber from ground tires since the late
1960s to primarily reduce reflective
cracking. In 1988, ADOT started to use
crumb rubber mixed with hot asphalt,
commonly referred to as Asphalt Rubber (AR) as a binder in hot
mix asphalt (HMA). Typically, these mixes are either open-graded
or gap-graded and from 12.5 mm (half inch) to 25mm (one inch)
or 25 mm (one inch) to 50 mm (two inches) in thickness respectively.
Open-graded mixes generally contain 9 to 10 percent AR binder,
whereas the gap-graded contains generally 7.5 to 8.5 percent
AR binder. To date field
performance has been very good. As an extra benefit, the ground
tire rubber from over eight and one half million tires in Arizona
has been recycled since 1988, in the making of HMA with AR.
Pertinent Excerpts
3. Current OGFC/CRM Composition
and Mix Design
In 1988, a 25 mm (one inch) layer of an open-graded
asphalt rubber asphalt concrete friction course commonly referred
to as AR-ACFC was placed on several miles of Interstate 19, south
of Tucson. The gradation of this mix is shown on Figure 1. This
AR-ACFC mix, containing 10.0 percent asphalt rubber by weight
of the mix as the binder (note: diluent is no longer used), was
placed on top of a plain jointed con-crete pavement. Table 1
shows the mix design equation used to determine the AR-ACFC binder
content. All AR mixes for ADOT projects are designed in the Materials
Central Laboratory. Since 1988, no cracks reflected through until
1996, when only a few transverse cracks appeared over the con-crete
joints.
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In 1999 District Maintenance reviewed this project and concluded
that as before no maintenance was needed and amazingly to date
twelve years later no maintenance has been performed on this
section. From this first project, dozens of projects have been
successfully built with asphalt rubber as the binder.
The AR contains 20 percent ground tire crumb
rubber by weight of the asphalt content. These projects were
built with the expressed purpose of controlling reflective cracks
with a very thin layer of very elastic material. To date, all
projects have performed as expected. As a further extension of
this work, a structural overlay called a gap graded AR-AC (Figure
1) was designed and built in 1990 on Interstate 40 near Flagstaff,
using AR as
the asphaltic concrete binder (5 & 7). This project also
contained numerous Strategic Highway Research Program (SHRP)
test sections as well as ADOT test sections. The purpose of the
project was to overlay a severely cracked and failed concrete
pavement. As of the most recent objective crack mapping review
in May 1999, the asphalt rubber sections built as the top portion
(overlay top 50 mm (two inches) AR-AC, 12.5 mm (half inch) AR-ACFC)
have the least percentage of reflective cracks. Indeed the percent
cracking of the AR section is less than one third of the 100
mm (four inch) conventional overlay and less than one half the
200 mm (eight inch) overlay.
4. OGFC/CRM Construction
Construction of an AR pavement involves first
mixing and fully reacting the crumb rubber as required by specification.
Typically 20 percent ground tire rubber that meets the gradation
shown in Table 2 is added to a hot base asphalt heated to a temperature
of about 190°C (375°F) and mixed for at least one hour.
After reaction the AR mixture is kept at a temperature of about
175°C (350°F) until it is introduced into the mixing
plant. Samples of the rubber, base asphalt, and AR mixture are
taken and tested accordingly. The AR-ACFC which typically has
one percent lime added to the mix is placed with a conventional
laydown machine and immediately rolled with a steel wheel roller.
In the past on rare occasions a small amount of sand, 1 kg/mm
(two pounds per square yard) was specified in case it was needed
as a release agent. Presently lime water is used on rare occasions
(high temperatures) in place of sand to reduce pickup from tires.
Generally one bag of lime is added to a water truck and sprayed
on the pavement.
continued on page 12
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