|
Where Rubber Meets...
continued from page 11
4.1. General Usage
AR-ACFC is generally used as the final
wearing surface for both concrete and HMA pavements. For concrete
pavements the joints are cleaned and resealed with AR. Spall
areas are cleaned and filled with HMA to level the surface. A
25 mm (one inch) AR-ACFC is placed to improve the smoothness,
reduce reflective cracking, improve skid resistance, and reduce
noise. If the concrete is in poor condition and the road-way
geometrics allow a leveling and
strengthening course of AR-AC is placed 50 mm (two inches) thick
before the AR-ACFC
is placed. For HMA pavements a standard deflection based design
is conducted to correct structural defi-ciencies.The AR-ACFC
is used as the final wearing surface. It is placed 12.5 mm (one
half inch) thick and is used to improve smoothness, reduce cracking,
provide adequate skid resistance, and reduce noise. On some badly
cracked pavements a gap-graded AR-AC, generally 37.5 mm (1.5
inches) to 50 mm (2
inches) thick, is placed to address cracking. An AR-ACFC may
be placed depending upon the traffic volume and type of highway.
4.2 Cost
Cost comparisons would indicate that AR binder
can be up to twice as expensive as asphalt binder. After incorporation
into the HMA, the finished AR product is generally from 25 to
75 percent more expensive for the gap-graded AR mix than the
typical dense-graded HMA and 80 to 160 percent more expensive
than the typical open-graded friction course. These higher costs
need to be examined in light of actual usage. On the I-19 project,
only a 25 mm (one inch) AR-ACFC was placed at a cost of about
$2.45 per square meter. The comparable repair strategy is to
grind the concrete which costs about $5.00 dollars per square
meter, thus the AR mix was actually less expensive to construct.
The AR-ACFC continues to provide a smooth riding, virtually crack
free, good skid resistant, quiet and virtually maintenance free
surface for twelve years.
|
Similarly, the AR-AC, AR-ACFC project on the I-40 Flagstaff
cost about ten dollars per square meter including the cost of
the cracking and seating. The adjacent reconstruction project
was built at a cost of about $25 per square meter for the paving
alone. When all other costs including detours are included the
cost for reconstruction is about $45 per square meter. In addition,
the 16 km (ten mile) AR overlay project was built in four months,
whereas the adjacent 8 km (five mile) reconstruction project
took two years to build. Also, the reconstruction project was
overlaid with AR after ten years of service due to excessive
cracking and rough ride. The AR overlay project built in 1990
as of today, ten years after construction, still has no cracking.
It should be noted
that the Flagstaff projects are located at about 2134 m (7,000)
feet elevation. Typical rainfall is about 625 mm (25 inches)
per year with an average annual snowfall of about 2250 mm (90
inches) per year. The coldest temperature recorded since construction
on this project has been 32C (-25°F). The use of AR
on this I-40 project alone conservatively saved at least $18
million dollars and about four years of construction traffic
disruption.
With regard to the price of AR, Table 3 shows
usage and bid prices since 1985. In 1985, one Arizona company
became the owner of all asphalt rubber patent rights which had
previously been owned by two Arizona companies (8). As can be
seen, since 1985, Asphalt-Rubber prices have been going down.
At present, seven companies supply AR in Arizona. ADOT monitors
the
price of all the products it buys and has used asphalt rubber
only when its usage appeared to be well suited to the problem
and cost effective. In 1992 the patents on AR ended. Since then,
the AR price has dropped significantly with increased competition.
Table 4 shows the cost of AR HMA mixes compared to dense-graded
HMA made with neat asphalt binders. Asphalt-Rubber has proven
to be so cost effective that over 2500 miles of AR mixes have
been placed since 1988.
4.3 Performance
Pavement performance has been routinely monitored by ADOT's
pavement management system since 1972. Over that time a general
trend of cracking, rutting, rideability, maintenance cost, and
skid resistance have been observed.
AR has reduced the amount of reflective cracking as expected
and
designed for. A value of ten percent cracking is considered as
fatigue
cracking, therefore virtually no fatigue cracking has been seen
in the AR
rubber projects.
|
