| ASPHALT IN-PLACE RECYCLING
CALIFORNIA PUTS
FOAMED ASPHALT TO THE TEST
Conventional reconstruction of the California Delta’s
rural roads
is cost prohibitive, so Caltrans is testing foamed asphalt as a
solution.
by Tom
Kuennen, Contributing Editor
 In the
famous Delta region of California’s San Joaquin Valley, where the
California Delta sprawls across 738,000 acres of prime agricultural land
south of the state capital, farm roads are built on an excavated alluvium
that just won’t stay in place.
The roads are subject to baking hot weather in summer and drenching
cold Pacific rain storms in winter, and agricultural trucks that often are
well overloaded. Road driving surfaces heave, shift, and collapse.
In maintaining these roads, the owning agency, the California
Department of Transportation, has a perplexing problem: The cost of
conventional reconstruction of these roads — months-long closure,
excavation, trucks out with old pavement and base, trucks in with new base
and pavement materials, and a parade of construction equipment — made
reconstruction prohibitively expensive, considering their remote location
and the few vehicles per day they served.
But Caltrans may have found the answer with in-place base recycling
using foamed or expanded asphalt. Contractor Western Stabilization applied
this process as a state pilot project late last year. Western
Stabilization performed the job with the powerful Wirtgen WR 3000.
With its 10-foot-wide drum, the WR 3000 is big brother to the more
common WR 2500 S with an 8-foot-wide drum. However, the equipment platform
is the same, and the only difference between the two models is the width
of the drum and the mechanics and hydraulics that serve it.
Agricultural lowlands
The California Delta is a network of a thousand miles of rivers,
tributaries, and channels located south of Sacramento. Much of the land is
composed of large islands that sit below sea level, protected by levees,
isolated by streams and canals fed by five major rivers, including the
Sacramento and the San Joaquin rivers.
Water is such a part of the delta that folks refer to the roads and
highways as “asphalt sloughs”.
One such “asphalt slough” is California 220, a blacktop road which
last year was nearing complete failure. In late 2002, a 3.1-mile length of
the road in Solano County became a testing ground for foamed asphalt.
The road suffered from longitudinal cracks, with cross-slope anywhere
from 3 to 5%. Existing asphalt had been placed in depths from 6 to 18
inches. And while traffic is very light, heavy agricultural loads do their
damage. Corings indicated not much base below the pavement.
The road was being stabilized 6-inches deep, except where roadway “whoop-dee-doos”
— base failure-related “roller-coaster” waves in the pavement —
were present. Those were being stabilized to a depth of 10 inches, which
would allow more material for a motor grader operator to smooth out voids
and take out humps.
Cold foaming prolongs life
“If we had done conventional reconstruction on this road, and
dig-outs, we’d end up with a very, very limited life,” said Caltrans
North Region materials engineer Joseph F. Peterson, P.E. “By cold
foaming to a depth of 6 inches, we believe that we’re going to add 10
years of life.”
 “Foamed”
or “expanded” asphalt is a road base recycling process in which
pulverized pavement is mixed with an asphalt froth to create a stabilized
road base. The foamed asphalt is formed by carefully injecting a
predetermined amount of cold water into hot penetration-grade asphalt in
the mixing chamber of a pavement remixing unit.
There, air bubbles in the expanded liquid asphalt froth act as the
carriers of liquid asphalt to fines in a reclaimed asphalt pavement
aggregate mix. While expanded asphalt doesn’t completely coat all
aggregate surfaces, it does form a mortar or glue which bonds the
particles together.
In less than 15 seconds, the froth subsides and the dispersion of
asphalt is achieved, eliminating time waiting for the “break” required
when expensive asphalt emulsions are used. The technology also sidesteps
use of costly cutback solvents. The liquid asphalt cement is pure, with
nothing added to it to change its properties. That makes it more
economical to use than emulsions, which are a processed oil.
Caltrans’ foamed asphalt mix design called for 2.25% liquid asphalt,
and 1% cement. The cement was required because of the lack of base
material and minus 200 material, as investigations indicated. Also, water
for compaction was being added to the mix at a rate of 1.5 to 2.5%,
depending on weather conditions. The water also was serving to activate
the cement.
Road treated in two passes
The road was being treated in two passes. The initial foamed surface
was compacted with a sheepsfoot roller, then rough-graded, compacted with
a smooth roller, and fine graded, all without stakes required. A
rubberized chip seal was to be placed as a driving surface.
As always with foamed asphalt stabilization, a mix design using actual
materials from the job site must be developed prior to construction. The
tool used for this job was a Wirtgen portable WLB 10 foamed asphalt lab.
Western Stabilization owns a WLB 10, and so does Caltrans.
“Right now we have a lot of roads that are highly distressed and,
given our high traffic loads, we need a methodology to rebuild our roads
quickly and with minimum impact,” said Caltrans’ Peterson. “Cold
foam seems to have the ability to build a road from the bottom-up, in a
timely manner, and has very little impact to traffic.”
And that was borne out on California 220. “This cold-foamed asphalt
process will take us four working days to rebuild from bottom-up at a
depth of 6 inches,” Peterson said. “We’ll put a chip seal on top and
that will take another two days, so in effect we’ve rebuilt the roadway
in six days. If we were to do this conventionally, we’d be out here for
30 to 60 days.”
Still, the route 220 job is a pilot project as Caltrans studies the
efficacy of foamed asphalt in different climates.
“We’re evaluating the cold foam under different conditions,”
Peterson said in late 2002. “We’ve used it in a valley environment in
which there are wild temperature swings, and we just completed 20 lane
miles in snow country near Truckee, so we can see how it behaves under
freeze-thaw conditions. And now we’re trying it here in the delta as a
maintenance strategy, so we can see if we can rebuild the roadways very
economically and quickly.”
Contractor spreading word
The California 220 project also served to introduce local government
agency personnel — and Caltrans engineers from other parts of the state
— to the foamed asphalt process.
“We’ve had visitors from different cities and counties, and from
the University of California-Berkeley,” said Paul Jones, foamed asphalt
business development manager, Western Stabilization. “I’m spreading
the word on foamed asphalt and bringing individuals to view the job
on-site, rather than hear about it in the office. It can open a lot of
doors for us.”
“We’re able to work without road closures, and provide a road that
will meet all criteria for less money, less time and with less
environmental impact than conventional construction,” Jones said. “That’s
because we’re not trucking aggregate in and out, but using the existing
material in-place.”
While Western pursues the big fish, it hasn’t forgotten the smaller
local governments, which form a major part of its foamed asphalt
stabilization work, said Dick Stuart, general manager.
“Smaller jurisdictions may be able to put a project together within
its maintenance budget, as opposed to new construction,” he said. “They
also tend to be squeezed the hardest financially, so they’re looking for
the most economic way to improve their roads, and foamed asphalt
stabilization has proven to be a very economical solution.”
More information about contractor Western Stabilization is available at
www.wstabilization.com. More
information about foamed asphalt is available at www.wirtgenamerica.com.
Adapted with permission from Wirtgen Technology,
Spring, 2003.
Reprinted from Better Roads Magazine
July 2003 |
