Coast-to-Coast, Foamed Asphalt
Offers New Option for Base Stabilization

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.
The 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.

Other benefits include:

  • The recycled lift is more resistant to penetration of water.

  • Foamed asphalt-stabilized bases are usually less expensive than a bituminous emulsion or a combination of emulsion and cement.

  • 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?
"Foamed" or "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 aggregate fines.

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 2500 S.

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 testing.
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.

Environmental benefits
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 ingredient.

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 California.

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 years."

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
In summer 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 project.

  • 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, and

  • The recycled road itself had been recycled 20 years earlier, so the project constituted a recycling of a recycled road.

"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 constructed.

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.

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