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.
Pettis hired Dave Rettner, a consulting engineer at American Engineering Testing in St. Paul, Minn., to perform the mix design. Rettner tried various combinations of fly ash and emulsion by creating small samples commonly referred to as “pucks” and then testing them in the laboratory for three factors: tensile strength, amount of air voids, and stability projected for a 20-year period.
“For stability, it's a standard bell curve: As you add emulsion to the mix, stability goes up; but at a certain point, you're adding too much emulsion so the material is mushy, and the stability goes down,” Pettis explains. Similarly, as emulsion is added to the mix, the amount of air voids is reduced, and the density increases. Air voids often trap water, which may damage the pavement through expansion and contraction.
Contractor Tom Johnson, president of Midstate Reclamation and Trucking, provided the equipment that controlled the amounts of both materials.
The fly ash and asphalt emulsion worked hand in hand: The emulsion was engineered specifically for the project to achieve a balance between strength and the flexibility needed to withstand Minnesota's spring freeze-thaw cycles.
An asphalt emulsion contains water to reduce its viscosity, allowing it to be used at about 120° F. That's safer for workers and requires less energy than straight asphalt. Before the liquid asphalt can do its job — gluing materials together — the water must be removed. That's where fly ash entered the picture. It is hydrophilic, actually sucking the water out of the emulsion. Furthermore, fly ash hardens when mixed with water. “By controlling the proportions of these materials and how they're used, the resulting pavement has a strength of about 250 to 300 psi,” Pettis says, adding that he expects the pavement to last 18 to 22 years.
— Kronick is a freelance writer based in Minneapolis.
Standard 2-inch overlay on 11 lane miles:
– Cost to haul away old asphalt: 20 cents/ton per mile
– Cost of asphalt overlay: approximately $480/ton
– Expected lifespan of pavement: 8 to 10 years
– Total cost: $658,000, or about $120,000/mile
Full-depth reclamation using fly ash and engineered asphalt emulsion on 11 lane miles:
– Cost of fly ash: $33,214
– Cost of emulsion: $484,710
– Expected lifespan of pavement: 18 to 22 years
– Total cost: $603,000, or about $110,000/mile
Hit the road
Fly ash makes its way into asphalt reclamation projects.
A study prepared by CTC & Associates LLC and released in 2008 by the Wisconsin DOT revealed that although several states have performed full-depth reclamation (FDR) projects with fly ash, few — if any — had used it in conjunction with engineered asphalt emulsions like the project in Le Sueur County, Minn.
The report includes the results of a North American survey of state and provincial transportation departments. The five-question survey answered by 23 states and three provinces indicated that just three states — Iowa, Nebraska, and Texas — routinely use fly ash as part of full-depth asphalt reclamation projects.
For example, the Nebraska Department of Roads uses fly ash or emulsion in all FDR projects, but emulsions are used only when the roadway sits on sandy soils, not in combination with the fly ash. Fly ash is currently used on a limited basis in Wisconsin DOT full-depth asphalt reclamation projects.
A byproduct of coal-fired plants, fly ash is often added to soils to stabilize weak bases and subgrades. Less than 5% of the 22 million tons recycled annually is used to strengthen bases and subbases, according to the Federal Highway Administration. About 60% of recycled fly ash is directed toward concrete production.