Champaign County, IL, is home to both rustic rural roads, and the sprawling University of Illinois home campus. And both of those reasons contributed to its studying foamed asphalt for reconstructing farm roads in summer 2005.
On County Road 9, the county put in place alternative base stabilization methods prior to hot mix asphalt (HMA) overlay, and a Wirtgen WR 2500 S owned by Dunn Co. of Decatur, IL, played a critical role.
"Both segments are experimental," says Gosia Adamczyk, P.E., a U of I engineering graduate, and senior resident engineer, Champaign County. "The project consists of two segments: the first, milling of the existing bituminous pavement and rubblization of two miles of PCC pavement, and hot mix asphalt overlay; and the second, the foamed asphalt section."
Surfacing improved with RAP
The condition of the foamed section was a deteriorating oil-and-chip surfacing, with hot-weather rutting and spreading of material. 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.
"We milled off the existing pavement from the PCC under the west section, brought it here, placed it, rolled it, and now the foamed asphalt process is mixing this 4-inch lift of material, as well as the surface, to a depth of 7 inches while injecting water, asphalt and 1-percent fly ash," Adamczyk says.
Foamed asphalt is a cost-effective way of stabilizing road bases that's gaining interest from coast-to-coast. Foamed or "expanded" asphalt is created by carefully injecting a predetermined amount of water into hot penetration-grade asphalt in the mixing chamber of a pavement remixing unit, and offers a cost-effective alternate for road base stabilization.
The expanded asphalt has a resulting high surface area available for bonding with the aggregate, leading to a stable road base using the existing in-place materials. The benefit is substantial cost savings over use of asphalt emulsions for base stabilization, and complete elimination of the cure or "break" period. The foamed base then is graded and compacted, and can permit traffic — including heavy trucks — almost immediately.
Fly ash was being spread on top of the RAP/chip seal layers in 1,000-foot segments, in three passes, ahead of the WR 2500 S. Design of the foamed asphalt section was developed by Dunn Co. "They came up with the mix design and the right procedure," Adamczyk says. "We reviewed specs from previous foamed jobs that Dunn has executed, and used them as a guideline for us to write our special provisions."
Even though the RAP had been down just a week, already it had begun knitting together in the summer sun to create a continuous driving surface for local residents. "It looks very good," Adamczyk says. "The heat wave we've just gone through really helped ‘work' that material as they were rolling it. Everyone was amazed how ‘glued' together it stayed; it almost looked like a finished asphalt surface."
The project foam-stabilized 49,200 square yards of roadway, and the reclaimer was foam-stabilizing the 22-foot-wide road in three passes. The foamed section was 3.75 miles, and the mix design called for 2.5 percent liquid asphalt in addition to the 1 percent fly ash. The asphalt was a PG 64-22 performance graded material. The foamed base was to be covered by a chip seal, and possibly with a 2-inch lift of HMA in a couple of years.
First look at foamed asphalt
County Road 9 was Champaign County's first look at foamed asphalt.
"This is new for us," Adamczyk says. "We are looking for alternatives that are less expensive than your straight hot mix design, but more stable than oil-and-chip. We have a lot of roads that don't have a very high traffic volume, but still are subject to heavy loads, and we need a less-expensive alternate pavement design."