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A reclaimer train combined previously milled roadway materials, fly ash, and engineered asphalt emulsion with the existing surface by remixing the materials with 2% fly ash and 3.5% asphalt emulsion. Photo: Richard Kronick

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.