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Not all asphalt base recycling is best done
in-place. When available space and schedule permit, a portable recycling
plant may be a cost-effective option for building durable road bases
from existing materials. That’s what the State
of Louisiana found when it used a portable recycling plant to prepare
stabilized base for a substantial, but less time-critical project. The
project, creating a base for a portland cement concrete pavement,
employed a portable plant to make foamed asphalt-stabilized base at a
central, fixed location, without EPA permits.
In this field test, researchers from the
Louisiana Transportation Research Center determined that foamed asphalt
base could be used in lieu of lime-stabilized base with 8 inches of
crushed limestone.
The use of foamed asphalt treated reclaimed
asphalt pavement mixtures is very promising, and can be used as an
alternative to the traditional limestone base under portland cement
concrete, wrote Louay N. Mohammad, Ph.D., Murad Y. Abu-Farsakh, Ph.D.,
P.E., Zhong Wu, Ph.D., and Chris Abadie, P.E., in their paper, Louisiana
Experience with Foamed Recycled Asphalt Pavement Base Materials.
At the January 2003 Transportation Research
Board meeting in Washington, the authors described Louisiana’s use of
existing recyclable materials as a key to more efficient and economical
highway construction. “The use of foamed asphalt technique to stabilize
[RAP] is one strategy for an efficient use of salvage construction
materials,” they wrote.
The Louisiana Transportation Research Center
experimented with alternative uses of RAP during the recent
reconstruction of U.S. 190 near Baton Rouge. The continuously reinforced
concrete pavement design called for a lime-treated subbase and 8 inches
of stone base.
“RAP by itself is not a qualified base material
as its strength is not comparable to other base materials traditionally
specified, especially stone,” the authors wrote. Test sections were
placed to determine how the actual strength of the foamed RAP would
compare to stone.
The Louisiana researchers generally followed the
Wirtgen Cold Recycling Manual. In this study, the optimum water
injection rate was found to be 2.75% with the PG 58-28 binder at 320
degrees F. A WLB 10 laboratory unit was used to foam the hot liquid
asphalt and mix the RAP materials with foamed asphalt.
A performance-graded, Superpave-style asphalt
cement, PG 58-28, was used. The asphalt temperature for laboratory mix
was 320 degrees F, as was the minimum temperature required of transport
on delivery at the field site.
Both materials had 100% passing the 1.5-inch
sieve, and had less than 3% fines. Two foamed asphalt-treated RAP
mixtures were studied, one of 100% RAP, the other 75% RAP and 25%
crushed concrete mixture from the existing roadway.
After testing, two field-test sections of foamed
asphalt RAP base were constructed on U.S. 190. A KMA 150 mixer from
Wirtgen — predecessor to today’s KMA 200 — was used to mix the RAP with
foamed asphalt and portland cement. A scalping/screening unit was placed
ahead of the pug mill to insure the proper gradation of the RAP (100%
passing 1 inch required).
The researchers concluded:
Foamed asphalt-treated RAP materials showed
higher in-situ stiffness values and structure numbers than those of a
limestone base layer, according to the field test results, although this
evaluation was made at the construction stage.
There is no significant difference in the
measured field strength/stiffness between the foamed asphalt base with
100% RAP materials and the base with 75% RAP/25% crushed concrete.
The addition of 1.5% portland cement in the
foamed asphalt mixture design resulted in a contribution to strengths
among soaked samples. Also, as the curing period increased, the layer
stiffness values and structure numbers of the foamed asphalt bases are
also significantly increased. |
