Darrell Pettis, the county engineer in rural Le Sueur County, Minn., often faces a common challenge: an aging road network, increasing loads, and the skyrocketing cost of everything from fuel to materials. But Pettis' response has been unique. In September 2008 he oversaw a full-depth pavement reclamation (FDR) project incorporating both engineered asphalt emulsion and fly ash — a departure from the use of cold in-place recycling.
Standard emulsions “break” (or change from a liquid to a solid) only through evaporation. So faster breaking is desired because in the liquid state the material is unstable: If a section of the asphalt does not break, one heavy truck could easily cause a rut in the pavement. Standard emulsions take two to four weeks to break and are affected by temperature and humidity. Poor conditions can delay the FDR because the overlay acts as a cap that retards breaking, and if it is not allowed to break fully, the project fails.
Engineered emulsions break in about two hours. Compared to standard emulsions, engineered emulsions (in which a chemical additive in the asphalt mixture causes the asphalt to break much faster than standard emulsions) increase the speed of construction and prevent damage. The additive itself is a proprietary secret of the emulsion manufacturer, in this case SemMaterials Inc.
This project was designed to increase the strength of 11 lane miles of a two-lane county road that had been paved in 1992. But by 2008, a section of the road was failing due to unstable subgrade caused by improper compaction resulting from not enough moisture — which, ironically, was caused by good weather.
Good weather is a mixed blessing: It is easier for projects to be completed on time, but if the base and subgrade are too dry, the materials cannot achieve optimum compaction.
Pettis chose FDR in part because the equipment used for such projects (in this case, a Wirtgen WR 2500 S reclaimer) is light and doesn't sink into weak soils. The use of asphalt emulsion to the FDR project in addition to a 6-inch asphalt o overlay increased the road's load limit to 10 tons from 7 tons. “We could have just overlaid without the FDR,” says Pettis, “but in eight years we'd need to do it again because an overlay doesn't address the problems underneath.”
The cost of the FDR process was about 25% of the overall cost of the project, Pettis says, adding that the 6-inch asphalt overlay accounted for about 75% of the project's cost.
The FDR project also offered other benefits since none of the milled asphalt was sent to the hot-mix plant located 50 miles away. “It's about 20 cents/ton per mile — and it was 10 cents/ton per mile 10 years ago,” Pettis explains.
What made the project innovative was the step after mixing the asphalt surface and base.
Without the use of fly ash in the project, Pettis would have had to increase the percentage of emulsion. Since fly ash costs about one-tenth of that of emulsion by weight, its use reduced the overall cost. The project included more than 553 tons of fly ash worth $33,214 and more than 969 tons of emulsion worth $484,710.