Foamed Asphalt Gains New Attention  in Cold In-Place Recycling

In a series of Iowa trials, foamed asphalt
 has made a promising debut in CIPR.

by Kelly D. Moore

Foamed asphalt has found a solid niche in the world of full-depth reclamation by reducing project times and costs, and now pavement recyclers are finding a role for it in cold-in-place recycling, too.

In the foamed asphalt process, hot asphalt cement is injected with a small, carefully metered amount of cold water in a mixing chamber. When the cold water comes into contact with the hot asphalt, it turns to steam, which is trapped inside thousands of tiny asphalt bubbles. This causes the asphalt to expand many times in volume and decrease in viscosity.

In its expanded state, when the foamed asphalt is mixed with recycled asphalt pavement it provides a partial coating of larger aggregates and a superior coating of fine particles — much better than is possible with conventional asphalt emulsions — creating a paste that holds the mix together.

There is no chemical reaction involved; only the physical properties of the asphalt are altered, and only for a brief time. The asphalt will return to its normal state in minutes.

After the foamed asphalt is mixed with reclaimed material and placed back on the roadway, the paved material looks much like a new asphalt pavement.

Foamed asphalt delivers two key benefits compared to conventional emulsions. One is a greatly reduced cure time. Since so little moisture is used, the foamed asphalt material sets up much faster and can be overlaid much sooner than with CIPR processes using emulsion, which may require several days to set up.

The other benefit is material cost. Foamed asphalt allows the use of liquid asphalt cement with no special additives, compared to the more expensive emulsions used in conventional cold recycling processes.

Will it work?

While foamed asphalt is well established in full-depth-reclamation work, its suitability for cold-in-place recycling was less certain when Koss Construction Company learned about the process at a workshop in Ames, Iowa in the Fall of 2000.

At issue was whether or not the cold-in-place recycling process would yield sufficient quantities of fines for the foamed asphalt process to work. Experts recommend that fines (material that passes a #200 sieve) make up at least 5% of the mix for foamed asphalt.

Fines are rarely an issue in full-depth reclamation, since some portion of base material is reclaimed with the pavement. Cold-in-place recycling, however, does not penetrate the base. The typical CIPR job involves recycling the top 3 or 4 inches of asphalt to eliminate transverse thermal cracking and form a uniform, stable platform for an asphalt overlay.

The most persuasive believer in the potential for the use of foamed asphalt in CIPR was Michael Heitzman, P.E., a bituminous materials engineer for the Iowa Department of Transportation and a major player in the Ames conference. Heitzman was also largely responsible for the letting of a number of pilot projects using foamed asphalt throughout the state in the months following the conference.

One of those projects — a 12.5-mile stretch of Highway 78 near Richland, Iowa in Keokuk County — was where Koss undertook its first foamed asphalt/CIPR project in July, 2001.

Limiting risk

Since there was no data from previous foamed asphalt/CIPR projects to draw on, all the bidders for the Keokuk County project had to deal with the risks of learning new technology and even the risk that the project would fail.

Koss decided to hedge its risk somewhat by putting a significant amount of the cost of the foamed asphalt equipment into the bid, then rolled the dice.

When the contract was awarded to Koss, we procured a foamed asphalt system from Soter Equipment of Canada. The system features a chamber with 16 individual expansion chambers, each with its own nozzle. It was mounted on the pugmill of the Koss cold-recycling machine.

A few days into the project, crew and management alike were getting comfortable with the process. The recycled material was laying down very well behind the paving equipment and it seemed to have a very rapid curing time. This allowed a longer work day, since we did not have to worry about raveling at the end of the day when the road was reopened to traffic.

Project specifications called for recycling pavement to a depth of 3 inches using a PG 52-34 asphalt binder. The asphalt was incorporated through the foaming process at a rate of 0.6 gallons per square yard. The project received a 3-inch overlay after completion of the recycling process. Manatts, of Brooklyn, Iowa, was the primary contractor.

There were several problems with the first project, but most of them were fairly simple to overcome. Hoses plugged, pumps stuck, tempers flared, but a system was soon worked out and then the operation ran smoothly.

One of the major obstacles that had to be overcome was keeping the asphalt cement hot during the day while the recycling was being done. Only part of each load could be stored in the nurse tanker which feeds the oil to the recycling equipment. Koss solved the problem by buying a 6,500-gallon heated Etnyre tanker trailer to store the asphalt while the delivery truck returned to the terminal for another load.

A little figuring and a lot of luck helped us end up with empty asphalt tanks at the end of each work day.

Another concern in the beginning was the safety of the employees. With the asphalt delivered at temperatures between 300 and 350 degrees F, caution had to be used in unloading the trucks. Proper safety equipment had to be worn, including face shields and long sleeves to avoid potential burns.

More projects

Since that initial experience, Koss has completed three more CIPR projects in Iowa using foamed asphalt.

In May and June of 2002 Koss completed an 8.74-mile project on Highway 44 in Harrison and Shelby Counties. The road was recycled at a depth of 3 inches using a PG 52-34 asphalt binder, and then topped with a 4-inch overlay. Henningsen Construction Company of Atlantic, Iowa was the prime contractor on the project.

In June 2002, Koss completed a 4.2-mile project on Highway 6 near Council Bluffs, Iowa, recycling the old pavement to a depth of 3 inches using PG 52-34 asphalt binder. This project was a bit more challenging due to the steep hills in the area. The grades were so steep that climbing lanes were provided for the truck traffic. The climbing lanes also had to be recycled as the CIPR process was completed. The recycling equipment had to be towed up the hills with the assistance of a large front-end loader. The project received a 2-inch overlay after completion of the CIPR process. Western Engineering of Harlan, Iowa was the prime contractor on this project.

The latest foamed asphalt CIPR project done by Koss was in Poweshiek County, Iowa in June of 2003. We recycled 7.15 miles of County Road F46 near Ewart to a depth of 4 inches, again using PG 52-34 for the asphalt binder. The project received a 2-inch overlay after completion of the CIPR process. Manatts was the prime contractor on this project.

Other contractors have also completed foamed asphalt CIPR projects in Iowa.

With four projects completed as of the start of the 2004 road construction season, Koss considers the use of foamed asphalt in cold-in-place recycling to be a tried and proven process and looks forward to many more such projects in the future.

Kelly D. Moore is Asphalt Division Manager for
 Koss Construction Company, Topeka, Kansas.

Why Iowa tried foamed asphalt in CIPR

Even before Koss Construction won its first bid involving the use of foamed asphalt for a cold-in-place recycling job in Iowa, the Iowa Department of Transportation had experimented with the procedure and found the results promising.

“We tried it when a CIPR sub-contractor proposed using it on a project that was being done toward the end of the construction season,” recalls Iowa DOT bituminous materials engineer Michael Heitzman. “There were some concerns about the weather getting too cold for a conventional CIR emulsion to cure.”

Iowa was aware of the use of foamed asphalt in full-depth reclamation, but had no previous experience with it in a cold-in-place recycling application. They gave the project the go-ahead on a trial basis. The results from that initial project were promising, and Iowa has used the technology on a number of projects since, with good results.

Heitzman and his Iowa DOT colleagues conduct lab tests using foamed asphalt with a range of possible milling gradations and like what they see.

Heitzman thinks of CIPR with foamed asphalt as another tool in the pavement management toolbox. Its most distinct advantage is rapid curing time, which can reduce total project time by several days. Iowa finds that emulsion-style CIPR requires five to seven days to cure, while foamed asphalt CIPR generally cures in two to four days.

Other purported benefits to foamed asphalt are more complex to calculate, says Heitzman. Asphalt binder for foamed asphalt costs less than emulsion on an equal residual binder basis, since emulsion is about one-third water before curing. “But we can add more residual binder using foamed asphalt in the CIPR mix than using emulsions,” points out Heitzman. How much cost benefit actually accrues through the use of foamed asphalt is not clear, though Heitzman thinks the placement of more binder will result in better long-term performance.

Similarly, while foamed asphalt makes CIPR possible in colder weather than emulsion-based CIPR, the curing time increases as the ambient temperatures decline, says Heitzman.

The DOT’s specification for CIPR mixes will leave it up to the contractor to select between emulsion or foamed asphalt for cold-in-place recycling projects in the future. Meanwhile, the department is currently studying past CIPR projects in the state to see how they have performed and what variables are most important in long term performance.

Reprinted from Better Roads Magazine
July 2004

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