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Stabilization

While the original specifications called for the use of an emulsified asphalt (E-5 or CMS-2) at an application rate of 1.5 percent (1.04 gallons per square yard) to take full advantage of the 7.2 percent residual asphalt content in the average 5.4-inch existing roadway material, Cutting Edge proposed conducting a parallel mix design utilizing 2-percent Type I Portland cement in addition to the 1.5-percent emulsion. This was explored to address a number of concerns facing the project, including heavy bus traffic from the school, heavy construction traffic working on the school renovation, the presence of slag in the subbase, the curing time of the stabilizer and the potential for excessive raveling as a result of heavy traffic during a prolonged cure time.

“We submit core samples of the road to a lab facility in order to develop the stabilizing mix design required for the project,” says David Cannon, president and owner of Cutting Edge Reclamation.

Coming up with the right design, based on the material to be blended and the gradation of the material blended, is critical to achieving the compaction and stability of the new base, which generally receives a hot mix asphalt overlay. The comparative mix designs indicated that the FDR process would yield successful results with either the emulsion-only or the emulsion/cement additives. The emulsion/cement additive, however, reduced the curing time from seven to three days, reducing the potential for raveling and increasing the stability of the compacted base by as much as 25 percent. Application of the Type I Portland cement was performed by pneumatic spreader trucks.

Controlling the process

Material gradation is controlled by the forward speed of the machine and the opening in the rear of the cutting system. The longer the material is confined to the cutting system, the finer the material will be when it’s spread out on the roadbed. When an additive like Portland cement is added, the base is re-cut to blend the cement with the other pulverized materials. The reclaimer/stabilizer Cutting Edge uses has an 8-foot-wide cutting system that pumps the stabilizer emulsion additive from a distributor truck. While the type of emulsion is determined from core samples taken prior to the start of the project, the reclaimer/stabilizer operator monitors the application rate of the additive, as well as the compaction density of the final pulverized and blended material after it has been compacted.

“With a formal mix design in hand, we’ll do a 100-foot control strip at the beginning of project and take six density readings. We’ll average the readings and use that as our target density to mirror the formal mix design for the project,” Cannon says.