Coast-to-Coast, Foamed Asphalt
Offers New Option for
Foamed asphalt offers new options for contractors and state and
local road agencies to undertake economical base stabilization.
Low-cost foamed asphalt -- the product of careful injection of
a predetermined amount of cold water into hot penetration-grade
asphalt into a series of individual expansion chambers -- replaces
costly asphalt emulsions in base stabilization.
Foamed asphalt does not require a "break" period before it can
be mixed, unlike asphalt emulsions.
And foamed asphalt technology is completely compatible with
in-place recycling or crushing of degraded asphalt or gravel road
wearing courses. After grading and compaction, this surface can
support traffic immediately, but more often is soon overlaid with
a fresh, virgin wearing course. This in situ base recycling and
stabilization is the most common application of foamed asphalt
throughout the world and North America.
Foamed (or expanded) asphalt technology stabilizes and improves
the performance of existing road materials, producing high quality
base courses and cold mixes at the lowest possible cost.
expanded asphalt has a resulting high surface area available for
bonding with the aggregate, leading to a stable road base using 100
percent of the existing in-place materials.
The recycled lift is more resistant to penetration of
Foamed asphalt-stabilized bases are usually less expensive than
a bituminous emulsion or a combination of emulsion and
Additional water is not added to the recycled material, as is
necessary when emulsion is used.
The rapid strength gain from use of foamed emulsion means that
traffic may be introduced onto the recycled road as soon as
compaction is complete.
What is foamed asphalt?
"expanded" asphalt (or bitumen) is a road base recycling process
relatively new to the United States. The process, popular in many
other countries, offers a cost-effective alternate for road base
stabilization to other techniques.
With foamed asphalt, a stabilized road base is created by
carefully injecting a predetermined amount of cold water into hot
penetration-grade asphalt in a pavement remixing unit, approximately
2 to 3 percent water by weight of asphalt.
Hot liquid asphalt rapidly expands into millions of bubbles
(foam) when it comes into contact with cold water, similar to the
spattering which takes place when drops of water stray into hot
cooking oil on a stove top. When injected into the hot liquid
asphalt, the water evaporates abruptly, thus causing explosive
foaming of the asphalt in the saturated steam.
The water is the carrier of the atomized asphalt, and within a
few seconds only, the asphalt can thus be expanded to 20 to 30 times
its original volume. Precisely added water allows control of the
rate and amount of asphalt foaming or expansion. The expanded
asphalt has a resulting high surface area available for bonding with
The intensity and effectiveness of the foaming process can be
further improved by a by control of the basic physical conditions,
such as pressure and temperature, which is possible when in-place
foamed asphalt road stabilization is accomplished with the Wirtgen WR
The foamed asphalt then leaves the individual expansion chambers
through nozzles and is immediately mixed with the reclaimed asphalt
pavement (RAP) or mineral aggregate to be treated inside the WR 2500
S mixer hood.
While expanded asphalt doesn't completely coat all aggregate
surfaces, it does form a mortar or glue which bonds the particles
together. The expanded asphalt has an affinity for finer particles,
those of 75 microns or less. This effective coating of finer
particles increases the available surface area of the expanded
asphalt for bonding with the coarser particles of material and "spot
welds" the material matrix together.
Typically, the recycling or mixing machine is coupled with an
asphalt supply tanker, in front, which is propelled by the recycler.
The asphalt is heated to a temperature of 350 deg F. A water cart,
in rear, is pulled by the recycler.
Advance testing a must
The principle of
foamed asphalt is simple, but because in situ materials vary widely
in composition, placement and quality, the best foamed asphalt
results will be obtained only through advance
Considerations in the life cycle of the expanded
asphalt, measured in its half-life, and the time the asphalt remains
expanded vs. its ability to coat aggregate, require carefully
controlled pre-application testing in the lab. Portable test stands
such as the Wirtgen WLB 10 are useful in pretesting materials and
developing mix designs.
Because the stability of a mixture of materials essentially is
dependent on the composition of the minerals, addition of cement,
lime, or certain aggregate fractions -- even including fines -- may
be necessary to improve the final mechanical properties of the
stabilized course. The WLB 10 mobile lab can create these actual
mixes and help determine properties of the foamed asphalt mix, and
thus the optimum asphalt foam.
Preliminary work using Wirtgen's WLB 10 foamed
asphalt laboratory system permits the exact definition of the
quality of the mixes to be produced for a proposed job, making
reports of attainable material properties -- such as the
load-bearing capacity -- possible before the start of the work, so
there are no surprises.
It goes without saying
that base recycling using foamed asphalt and reclaimed asphalt
pavement (RAP) offers substantial environmental benefits. Most
importantly it saves valuable aggregate resources. While in most
areas of North America there are plenty of aggregates resources in
situ, the industry is threatened by depletion of aggregate that is
permitted for extraction.
Existing aggregate sites once on the outskirts of town now are
surrounded by new neighbors who are not only opposed to existing
mining operations and truck traffic, but any kind of expansion of
the quarry or pit in the future.
In the meantime, due to environmental opposition, aggregate
operations are finding it hard to get approval for new extraction
sites, even way out in the country, far away from the cities where
most of the aggregate is consumed.
Little wonder that base recycling using any process is so
desirable, because it reuses existing aggregates resources that
already have been acquired, permitted, processed and hauled. The
existing investment in processed aggregate (in the road) is
optimized, because the material is simply lying there in the roadbed
waiting to be reused. The fact that this material is often asphalt
coated allows 100 percent recycling of that expensive
Foamed asphalt in Maine
In situ foamed
asphalt base recycling using the Wirtgen WR 2500 S was executed from
coast-to-coast in 2001, including Maine, Ontario, Louisiana and
A late summer 2001 demonstration of base recycling using foamed
asphalt and the Wirtgen WR 2500 S was to give participating New
England states a look at how foamed asphalt could perform in their
harsh Northeast climate.
The Maine Department of Transportation research project involved
the full-depth reclamation using foamed asphalt as a stabilizing
agent on Maine State Route 8 near Belgrade Lakes.
"We're evaluating the effectiveness of foamed asphalt," said John
R. Devin, project manager, Maine DOT Regional Program. "We want to
look at new techniques and inexpensive ways to improve roads without
full reconstruction. We wanted to try foamed asphalt, because it
really hasn't been used in this part of the country."
Prior to this work a Wirtgen WLB 10 testing lab was purchased
jointly by the Maine DOT with funding from the Recycled Materials
Resource Center of the University of New Hampshire, and Worcester
Polytechnic Institute. It will be used in Maine and in other
participating New England states.
The Maine S.R. 8 project was providing apples-to-apples
comparison with different reconstruction techniques, Devin said. A
final report will document construction practices and evaluate the
performance of the WLB 10. Also, section performance will be
monitored for at least five years.
"Better utilization of existing materials, cutting down on
hauling costs, and using cold mix to speed up our work all fits very
well with our philosophy on improving our low-volume roads without
completely rebuilding them," said John E. Dority, Chief Engineer,
Maine DOT, who visited the project. "We hope foamed asphalt bases
will give us the strength we need to get a service life of 12 to 15
For the foamed asphalt portion, about 2.8 miles of Route 8 were
foam asphalt-recycled. Tests of existing material were undertaken
with the WLB 10 Foamed asphalt Laboratory to determine the optimum
amount of foamed asphalt to be added, and other mix elements.
Large boulders in the road's gravel base were found at varying
depths from the surface, ranging from 6 to 8 inches deep. Because an
average foam recycled layer of 8 inches is required to meet the
design criteria, an additional 2 inches of crusher dust was placed
on the road surface prior to recycling.
The construction process began with cleaning out and re-cutting
of roadside drainage ditches, followed by recycling of the existing
pavement in-place, and placement of the 2 inches of crusher dust
from local sources onto the reclaimed existing pavement. The entire
section then was pulverized again, shaped and compacted to
accommodate existing traffic.
Then the portland cement was spread and the entire travel lane
width was foam recycled in three passes with the WR 2500 S,
compacted and finished. This was followed by a 3.2-inch asphalt
wearing course. As a further test of the structural capabilities of
the foamed asphalt treated-base, a half-mile long section of road
received only 1.6 inches of wearing surface.
'Green' road building in Ontario
2001, base recycling with foamed asphalt played the central role in
an environmentally focused rehabilitation of the Trans-Canada
Highway (Highway 17) east of Wawa, Ont., in a provincial park above
spectacular Lake Superior.
Because of the location of this 14.3-mile project within
Ontario's Lake Superior Provincial Park, the province planned a
completely "green" reconstruction for this late-summer 2001
The foamed (or as some prefer, expanded) asphalt base recycling
by contractor Roto-Mill Services Ltd., Brampton, Ont., reused the
complete, existing roadway as a base course, albeit strengthened
significantly through the foamed asphalt process
Rock removed from widened road cuts was crushed and used as
aggregate in asphalt overlays from an on-site portable asphalt
plant, sited in a manner that would not draw attention to it,
The recycled road itself had been recycled 20 years earlier, so
the project constituted a recycling of a recycled
"We're building a stabilized, expanded asphalt base," said Ted
Arscott, president, Roto-Mill. "Just about everything used on the
job is being recycled. We're recycling the old asphalt and the
underlying base, plus all the stone from the rock cuts, which
becomes aggregate for the new asphalt pavement."
Conventionally, that rock would have become part of the
cut-and-fill along the roadway, or wasted.
A test section of
full-depth reclamation without foamed asphalt also was
While some contractors choose to pulverize the existing road and
apply expanded asphalt in one pass using the Wirtgen WR 2500 S,
Roto-Mill prefers to pre-pulverize the existing road before foaming;
then return with the WR 2500 S and apply expanded asphalt.
"If you have a road with very shallow asphalt (2 to 3 inches), of
very consistent depth, and you only want to do limited correction of
the road's slope or alignment, you can do it in the one-pass
method," Arscott said.
Significantly, this was the first application of expanded asphalt
base recycling by the Province of Ontario, although Ontario
municipalities have been using the process for five years.