In the summer of 2005, Champaign County, Illinois, embarked on an ambitious experimental pavement reconstruction project on County Road 9 that would test the viability of foamed asphalt technology for low-volume, heavy-load rural roads. Home to both rustic farm roads and the sprawling University of Illinois campus, the county needed a cost-effective alternative to traditional hot mix asphalt (HMA) overlays that could still withstand agricultural truck traffic. The result was a 3.75-mile foamed asphalt section that engineers and contractors continue to study as a model for asphalt safety comprehensive guide to hazard management in pavement preservation and base stabilization practices.
Understanding Foamed Asphalt Technology
Foamed asphalt, also called expanded asphalt, is a stabilization method that transforms ordinary hot penetration-grade asphalt into a high-surface-area binding agent through the controlled injection of water. When cold water meets hot asphalt (typically 160-180 degrees Celsius) inside the mixing chamber of a pavement remixing unit, the water vaporizes instantly, causing the asphalt to expand to many times its original volume. The resulting foam has a low viscosity and a large surface area that coats aggregate particles efficiently, creating a stable, waterproof base course using existing in-place materials.
How the Foaming Process Works
The foaming process follows a precise sequence of steps that must be carefully controlled to achieve consistent results:
- Hot penetration-grade asphalt binder is heated to the appropriate temperature in the machine’s tank.
- A carefully metered amount of cold water (approximately 2-3 percent by weight of the asphalt) is injected into the hot asphalt stream.
- The water instantly flashes to steam, creating millions of tiny bubbles that expand the asphalt volume by 15 to 20 times.
- The foamed asphalt is immediately injected into the mixing chamber where it coats the aggregate and reclaimed asphalt pavement (RAP) particles.
- The treated material is spread, graded, and compacted before the foam collapses.
- Traffic can be permitted almost immediately after compaction, with no cure period required.
Key Advantages Over Emulsion Stabilization
Compared to asphalt emulsion stabilization, foamed asphalt offers several distinct benefits that made it attractive for Champaign County’s application:
- Substantial cost savings through elimination of the emulsion manufacturing process.
- Complete elimination of the cure or break period required for emulsions, enabling immediate traffic reopening.
- No water needs to evaporate from the finished base, allowing the road to carry heavy trucks almost immediately after compaction.
- Better coating of fine aggregate particles due to the foam’s low viscosity and high surface area.
- Reduced carbon footprint by reusing existing pavement materials in place.
The Champaign County Road 9 Project
The County Road 9 project was designed as a two-segment experimental comparison. The first segment involved milling the existing bituminous pavement and rubblization of two miles of Portland cement concrete (PCC) pavement, followed by a hot mix asphalt overlay. The second segment was the foamed asphalt section that became the primary focus of the study.
The condition of the foamed section before treatment was a deteriorating oil-and-chip surfacing that exhibited hot-weather rutting and lateral spreading of material. This type of distress is common on rural roads that were originally built with minimal structural support and have since been subjected to increasing heavy truck traffic from agricultural operations.
Project Specifications and Execution
The project was executed by Dunn Co. of Decatur, Illinois, using a Wirtgen WR 2500 S road reclaimer. The following table summarizes the key specifications of the foamed asphalt section:
| Parameter | Value |
|---|---|
| Total foamed area | 49,200 square yards |
| Road width | 22 feet |
| Foamed section length | 3.75 miles |
| Treatment depth | 7 inches |
| Liquid asphalt content | 2.5 percent (PG 64-22) |
| Fly ash content | 1 percent (Class C) |
| RAP base thickness | 4 inches |
| Mixing passes per lane | 3 passes |
| Equipment | Wirtgen WR 2500 S |
Gosia Adamczyk, P.E., senior resident engineer for Champaign County, explained that the millings from the first segment of the project were transported to the second segment, where they were used as a base for the foamed asphalt treatment. Class C fly ash was spread on top of the RAP and chip seal layers in 1,000-foot segments, applied in three passes ahead of the WR 2500 S. The reclaimer then mixed the 4-inch lift of material along with the existing surface to a total treatment depth of 7 inches while simultaneously injecting water, liquid asphalt, and the fly ash.
Mix Design and Quality Control
The mix design for the foamed asphalt section was developed entirely by Dunn Co. based on their previous experience with foamed asphalt projects. According to Adamczyk, the county reviewed specifications from previous foamed jobs that Dunn had executed and used them as guidelines for writing the special provisions for County Road 9. The binder selected was a PG 64-22 performance-graded asphalt, which is a standard grade for the region and provides good resistance to both high-temperature rutting and low-temperature cracking.
The addition of 1 percent Class C fly ash served a dual purpose. It provided the minus-200 mesh material needed for proper mix density and contributed pozzolanic activity that enhanced the long-term strength gain of the stabilized base. This combination of foamed asphalt and fly ash created a composite material that was both immediately traffic-ready and capable of gradual strength development over time.
Equipment and Contractor Expertise
Dunn Co. brought substantial experience in both foamed asphalt and emulsion stabilization to the project. The company operates seven Wirtgen machines, creating one of the largest cold recycling fleets in the region. Jim Schwarz, vice president of Dunn Co., noted that the WR 2500 S played a central role in demonstrating foamed asphalt capabilities to county engineers throughout Illinois.
The capabilities that it gives with the one-pass mixing, and the way the mandrel mixes, is by far the best machine for mixing oil like this. It is a very reliable machine.
Jim Schwarz, Vice President, Dunn Co.
Dunn Company Equipment Fleet
Beyond the WR 2500 S used on County Road 9, Dunn Co. maintains a comprehensive fleet of cold milling and recycling equipment that allows the company to tailor its approach to each project’s specific needs:
- Wirtgen WR 2500 S road reclaimer for foamed asphalt and emulsion stabilization.
- Wirtgen W 2200 cold milling machine with integrated paving capability.
- Two Wirtgen W 2000 cold milling machines for large-scale milling operations.
- Two Wirtgen W 1900 cold milling machines for medium-width applications.
- Wirtgen W 1000 and W 600 smaller milling machines for utility and detail work.
- Wirtgen 2200 CR combination cold in-place recycler and milling machine.
Selecting the Right Stabilization Method
Not every road is a candidate for foamed asphalt stabilization. Dunn Co. evaluates each project by analyzing core samples and assessing the existing pavement structure before recommending a method. As Schwarz explained, some roads are better candidates for emulsion stabilization, others for foamed asphalt, and some may require fly ash or cement stabilization instead.
For County Road 9, core analysis revealed that the existing materials had the right gradation and stability characteristics for foamed asphalt. The presence of natural fines in the aggregate base meant that foamed asphalt could effectively coat and bind the particles without the need for additional emulsifiers. Engineers considering asphalt bitumen tar materials for stabilization projects can use similar core analysis methods to determine the most appropriate binder system for their specific conditions.
Performance Observations and Future Applications
One of the most striking observations from the County Road 9 project was the immediate performance of the foamed asphalt base. Despite having been placed just one week before observation, the RAP base material had already begun knitting together in the summer sun, creating a continuous driving surface for local residents. Adamczyk noted that a heat wave during the rolling operation helped work the material effectively.
Everyone was amazed how glued together it stayed. It almost looked like a finished asphalt surface.
Gosia Adamczyk, P.E., Senior Resident Engineer, Champaign County
The foamed base was designed to be covered by a chip seal in the short term, with the possibility of a 2-inch HMA overlay in subsequent years. This staged approach allowed the county to spread its pavement preservation budget across multiple fiscal years while still achieving a structurally adequate road surface. For contractors and agencies evaluating asphalt plants and pavement construction equipment a complete range of options, this kind of incremental approach offers flexibility in managing both pavement quality and budget constraints.
Long-Term Monitoring Plans
Champaign County engineers committed to an extended monitoring program for the experimental sections. The planned observation schedule included tracking performance through multiple seasons and under various operational conditions:
- Seasonal freeze-thaw cycling effects on the foamed asphalt base integrity.
- Performance under snow plow operations during winter months.
- Rutting and shoving resistance during the summer heat.
- Fatigue cracking development over repeated heavy truck loading cycles.
- Drainage characteristics and moisture susceptibility of the stabilized base.
Adamczyk emphasized that the county was specifically interested in understanding what types of problems might emerge over time and what to expect in future foamed asphalt applications. The willingness to experiment with new technology while maintaining rigorous observation protocols reflects a growing trend among county engineering departments to move beyond traditional specification-based approaches toward performance-based pavement management.
Implications for Rural Road Networks
The lessons from County Road 9 have significant implications for the thousands of miles of rural roads across the United States that carry moderate traffic volumes but heavy agricultural, logging, or energy-sector loads. These roads often fall into a difficult middle ground, too heavily trafficked for simple chip seals but too low-volume to justify the cost of full-depth HMA reconstruction. Foamed asphalt stabilization offers a third option that can deliver HMA-like structural performance at a fraction of the cost.
For county engineers considering this approach, the Champaign County project demonstrated several critical success factors: experienced contractors with the right equipment, thorough mix design based on site-specific materials, and realistic expectations about the staged surface treatment plan. The project also showed that asphalt pavements types can be strategically combined within a single corridor, using different stabilization methods for different sections based on existing pavement conditions and traffic demands.
The contractor community has responded enthusiastically to foamed asphalt projects across Illinois. Schwarz noted that with every job Dunn Co. completes, many county engineers come to watch the process and inspect the finished results. This word-of-mouth education has been instrumental in building confidence in foamed asphalt as a reliable stabilization technology that can extend limited road maintenance budgets while delivering durable, long-lasting pavement structures.