Credit: Photo: James R. Rosenmerkel
During construction on County Trunk Highway S in northwestern Racine County, Wis., construction crews travel on recycled base stabilized with fly ash.
Credit: Photo: James R. Rosenmerkel
Road crews place hot-mix asphalt over the stabilized, recycled subgrade.
Not long ago, farmers who hauled their crops to market built new roads using nearby materials. So it was with a road now called County Trunk Highway (CTH) S in northwestern Racine County, Wis. The north-south highway connects the village of Wind Lake with CTH K to the south, a distance of about 3 miles.
This area of the county is known for sod farms, which are noted for wet, low areas and poor drainage. When a road was needed to move sod, peat from the sod flats was graded to form a road embankment. Gravel and other granular materials were added to provide some strength, ditches were graded along the core, and there was the road.
According to Racine County engineer Cecil Mehring, “The county took over the road in the late 1940s. It was a gravel road for years.”
Jeff Katz, county project manager, said it remained a gravel road until the early 1960s, when it was surfaced with asphalt road mix. The area originally was a swamp, drained to make way for the sod farms. Over the years, asphalt material was added as the surface deformed and as traffic loads reached beyond the pavement's capacity. The old road surface was badly deformed and alligator cracking predominated. Katz termed it “the worst mile of road in the county.”DESIGN ANALYSIS
As CTH S needed reconstruction, county engineers secured soil borings and a set of reconstruction alternatives. Soil borings confirmed that much of the sub-grade was indeed peat along with additional fill, and the pavement consisted of a thin layer of base course (3 to 4 inches) covered with asphalt pavement layers ranging from 5 to 9 inches. The geotechnical engineer, Kenosha, Wis.-based KTE (now Terracon) suggested three alternative reconstruction options:Reconditioning the subgrade with aerationStabilizing with chemical agent (cement, cement kiln dust, or fly ash)Removing and replacing the unstable materials.
After analysis and learning of recent successful stabilizing projects using fly ash, the county chose to reconstruct 1 mile of CTH S using a full-depth reclamation process, stabilizing with Class “C” fly ash. According to Katz, since it was necessary to strengthen the subgrade, they considered stabilizing with either foamed asphalt or fly ash; there was consensus that the fly ash would be less likely to rut under traffic. He also said there would be additional traffic from local development nearby, and the rehabilitation needed to perform well. It is common for the California bearing ratio (CBR)—a measure of a subgrade's supporting strength—on FDR projects like this will increase by as much as 10 times.CONSTRUCTION DETAILS
The original cross section consisted of a 22-foot pavement with 5-foot shoulders; the proposal called for a 24-foot pavement with 4-foot shoulders. Existing pavement was pulverized to a depth of 10 inches, compacted, and graded. In a number of low areas, new gravel was added over the pulverized material to improve riding profile, and the entire roadway was stabilized. Class “C” fly ash provided by Herndon, Va.-based Lafarge North America from the We Energies Oak Creek (Wis.) Power Plant was distributed at an average rate of 100 pounds per square yard and blended to a depth of 9 inches.
Moisture content was closely monitored but provided best results at 6% to 8% total moisture. Crews took care to ensure initial compaction by a vibratory pad-foot roller occurred immediately after blending was completed. Tri-County Pavers, DeForest, Wis., acted as the stabilizing subcontractor to local asphalt paving contractor Payne & Dolan Inc.
Roger Therkelsen, project manager for Payne & Dolan, said that the FDR project worked particularly well, especially in terms of blending the newly added gravel with the fly ash application. “Stabilizing production was good and the work was completed in less than three days,” he said. The specifications required that a one-day delay occur between acceptance of the fly-ash stabilized base and hot-mix asphalt (HMA) paving. Stabilizing was finished on Friday, left over the weekend, and the lower course of Type E-3 HMA was constructed on Tuesday; Mon-day was rained out.
“The evening after the stabilizing was completed, it rained very hard,” said Mehring. “The new surface was not affected; paving followed immediately. It's not likely that other types of reconstruction would have survived that well.” Rain on a properly stabilized, compacted, graded, and smooth rolled base is actually an asset, since the additional moisture prolongs the hydration process. Some jurisdictions actually require the contractor to spray water on a newly stabilized FDR base for several days prior to paving.