A quality base under any pavement is essential to its success. Whitetopping is a great way to revive old asphalt pavement (see page 17) but not when the asphalt is failing and a stronger base course is needed under the new surface pavement. One technique with proven success is full-depth reclamation (FDR) with cement.
In this process, the old asphalt roadway is pulverized, mixed with portland cement, and compacted to create a stabilized base. This is typically a soil-cement or cement-treated base mix but includes asphalt. This approach eliminates the need to haul away the old asphalt pavement and dump it in landfills and greatly reduces the need for additional road base aggregate. Recycling this material just makes good sense, both from an environmental and economic viewpoint.
“Another advantage of using FDR is that it can be done under traffic,” says Wayne Adaska, director of pavements at the PCA. “There’s no need to close the road completely down. It is also quicker and less expensive than removal and replacement.”
FDR starts with a lab evaluation of the materials to determine the mix design that will be used in the field. The cement content will be based on achieving an unconfined compressive strength of 300 to 400 psi after seven days.
On the jobsite, the asphalt pavement is pulverized using a reclaimer, a piece of equipment that resembles a large rototiller. The depth of the cut is 6 to 12 inches, which on secondary roads usually includes the pavement and aggregate base course and even some of the underlying subgrade, or soil.
The predetermined amount of portland cement is spread across the material, usually dry. Then the pulverized material, cement, and water is mixed using the reclaimer. Compaction follows within an hour of mixing and usually happens immediately. Vibratory rollers compact the cement-treated base to at least 96% of standard Proctor density. Curing follows to keep the material moist so the cement can hydrate. An asphaltic primer is sprayed on to seal in moisture or water can be sprinkled on the surface using water trucks.
Now the base is ready for pavement—either concrete, asphalt, or simply a chip seal in some light-duty pavements. The FDR base requires a surface layer because it is not durable on its own, especially to resist freeze/thaw action and surface abrasion from traffic.
This article is based on the Portland Cement Association brochure, "Full-Depth Reclamation: Recycling Roads Saves Money and Natural Resources, SR995," which is available for free download.