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.
Once the full depth of the existing pavement, along with a predetermined portion of the underlying subbase, has been pulverized and blended with an appropriate stabilization additive, the road is re-shaped and compacted to produce a strong, durable base suitable for a subsequent overlay. Cutting Edge uses a 10-ton steel drum vibratory roller for the initial knockdown compaction, followed by a 10-ton pneumatic-tire roller and then a steel drum static roller to smooth out the surface in preparation for the new overlay. The compaction process called for continuous monitoring by a trained and certified technician utilizing a nuclear density gauge, with the reclaimed base course ultimately being tested for compaction, smoothness and grade accuracy. It generally takes a week for the full-depth treatment to cure, allowing water from the emulsion stabilizer to evaporate. The end result is a new base that is stronger, more uniform and more moisture-resistant than the original base, resulting in a long, low-maintenance life.
Following the required cure time for the reclaimed base course, Old Oakdale Road received a 4-inch ID-2 binder course of hot mix asphalt and a 1 1/2-inch ID-2 wearing course.
When asphalt pavements fail due to fatigue cracking, rutting, shoving or deterioration of foundation support, Cannon says the FDR process allows him to add virgin aggregates or reclaimed asphalt pavement if it’s determined that the pulverized material does not meet the aggregate requirements for a sound stabilized base.
For customers like South Fayette Township, FDR with stabilization additives provides the following benefits:
- Stabilization increases the stiffness and strength of the base material. A stiffer base reduces reflections due to traffic loads, which results in lower strains in the surface. This delays the onset of surface distress, such as fatigue cracking, and extends the life of the road.
- Strong uniform support provided by stabilization results in reduced stress applied to the subgrade. A thinner stabilized section can reduce subgrade stresses more than a thicker layer of untreated aggregate base. Subgrade failures, potholes and road roughness are reduced.
- Stabilized pavements form a moisture-resistant base that keeps water out and maintains higher levels of strength, even when saturated.
- A stabilized base also reduces the potential for pumping of subgrade fines.
The FDR stabilization process also eliminates the cost of trucking away and disposing of the old asphalt pavement, as well as the cost of a new aggregate subbase, which is generally required in a full-depth pavement rehabilitation project.
“Since the existing asphalt pavement structure can be used to create a new subbase, it saves a lot of money, time and disruption for the community,” Cannon says. “The reclamation process provides significant savings, generally in the neighborhood of 40 percent compared to the conventional HMA mill and fill approach.”