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Pavement Preservation Compendium II
Protecting Roads in the Desert
Chip Sealing over Fabric Retards Reflective Surface Cracks
by Lita Davis
The county of San Diego, California, like many other public agencies, is always looking for cost-effective ways to maintain roads. Innovation and creativity are necessary because the funding often does not increase from year to year, although the maintenance needs and costs continue to escalate. The county of San Diego maintains approximately 2,000 centerline-miles of public roads in the unincorporated area of San Diego. The county includes coastal areas, inland valleys, mountains, and desert valleys.
The county Department of Public Works (DPW) follows a preventive maintenance system that applies surface treatments to extend the life of structurally sound roadways. The surface treatments in the DPW "toolbox" are chip seal, fog seal, slurry seal, cape seal, thin lift overlay, and chip seal over fabric.
Chip Seal over Fabric
In Borrego Springs, the desert area of San Diego County, the adverse climate and rainfall conditions generate many large surface cracks in the asphalt roadways. Elevations at Borrego Springs range from mean sea level to 1,830 meters (6,000 feet), with ambient temperatures from freezing in the winter to 57˚C (135˚F) in the summer. Rainfall is short in duration, but forceful, and is associated with flash floods.
Crack sealing was a common maintenance method for desert roads, but the cost of addressing the large quantities of surface cracks did not leave sufficient funds to apply the final surface treatments to the road. In 1987, DPW developed test sections on Yaqui Pass Road to evaluate the performance of several surface treatments. The goal was to find a treatment to retard reflective surface cracks under desert conditions.
The following surface treatments were applied and evaluated:
All of the treatments sealed the road surface well, but only chip seal over fabric eliminated reflective surface cracks. Moreover, a 30-year life-cycle cost analysis showed that the annual cost was one-half that of chip sealing with crack sealing.
Chip sealing over fabric, therefore, has become the standard surface treatment for heavily cracked roads in the desert area of San Diego County. Material specifications and application procedures are as follows.
The requirements for the pavement-reinforcing fabric follow the California Department of Transportation's standard specifications: fabric manufactured from polyester, polypropylene, or polypropylene-nylon material. The fabric is nonwoven and is heat-treated on one side.
The roads are prepared by cleaning the surface, removing pavement markers, and placing protective covers on public improvements such as valve cans (which provide access to underground utilities), survey monument covers, and storm-drain inlets. Liquid paving asphalt (AR8000) is the binder for the fabric, applied between 290˚F and 350˚F at a rate of 0.25 to 0.30 gallon per square yard.
After placement, the fabric is lightly sanded and then seated with pneumatic rollers into the underlying paving asphalt, until the pavement texture is replicated on the fabric surface. On low-speed roads (35 mph or less), the sanded fabric is exposed to traffic for 5 to 10 days before the chip seal is applied. On high-speed roads (40 mph or more), the fabric and chip seal are placed on the same day.
Chip Seal Placement
When the fabric is properly saturated, the chip seal is applied at the same rate as on an asphalt surface. If the fabric is not saturated, the chip seal emulsion must be increased to allow for absorption by the fabric and to leave enough emulsion to bind the chips. If the fabric is oversaturated, the emulsion must be reduced.
The 1987 test section on Yaqui Pass Road is still functioning. The fabric spans the surface cracks, so that crack sealing or crack filling have not been necessary.
For more information contact Lita Davis, Project Manager and Resident Engineer, County of San Diego, Department of Public Works, 5469 Kearny Villa Road, Suite 201, San Diego, CA 92123 (telephone: 858-874-4067; e-mail: Lita.Davis@sdcounty.ca.gov).
The author is Project Manager and Resident Engineer, Department of Public Works, County of San Diego, California.
From TR News, September-October 2003, pp. 4-15. Copyright, Transportation Research Board (TRB), National Research Council, Washington, D.C. Reprinted with permission of TRB.