Recycling in a Fragile Environment

Full-depth reclamation with foamed asphalt is the answer for Utah抯
ecologically sensitive Zion Canyon.

by Edward J. Kearney, P.E.

The use of foamed asphalt full-depth recycling to reclaim a scenic road in Utah抯 Zion National Park shows how roads in environmentally sensitive areas can be reconstructed economically, and with minimal impact on the ecology.

It also reminds us that the repair and reclamation of roads in our most pristine and vulnerable environments is becoming an increasingly important issue because the popularity of our national parks has grown tremendously over the years. And with the rising popularity has come increased traffic loads that far exceed the structural design of the original pavements.

According to the National Parks Conservation Association, 65% of the 5,000 miles of paved roads in U.S. National Parks are in poor to fair condition. Now stakeholders must ask,
揌ow should these failed roads be rehabilitated in a cost-effective but environmentally sustainable way??/p>

In the case of Zion Canyon Scenic Drive ?and many other National Park roads as well ?adding several layers of hot-mix asphalt would not economically or aesthetically solve the inadequate structural strength of the pavement. Instead, the Federal Lands Highway Division of the Federal Highway Administration has turned to full depth reclamation with foamed asphalt as the pavement rehabilitation method of choice.

In FDR, the entire thickness of an asphalt pavement is pulverized in-place and blended with a specified amount of the underlying granular road base materials. To this reclaimed material is added a combination of foamed asphalt and cement stabilizing agents. Just enough asphalt and cement are added to produce a strong yet flexible road base. This process has been used successfully throughout the world and in the U.S., including all Federal Lands Highway Divisions, on roads in Canyon de Chelly National Monument, Arizona; Helena National Forest, Montana; Delaware Water Gap National Recreation Area, New Jersey; and Rocky Mountain National Park, Colorado.

Use of foamed asphalt in Zion National Park minimized the disruption of tourist traffic, eliminated the noise, dust and commotion of pavement demolition and the 揳nt train?of haul trucks removing demolition materials and replacement with virgin base, and saved countless gallons of expensive fuel used by those trucks and by autos caught in construction-related congestion.

Fatigued and cracked pavement

The highlight of Zion National Park is Zion Canyon, a half-mile deep gorge formed by the Virgin River cutting through the red sandstone of the Colorado Plateau. The Zion Canyon Scenic Drive ?a 5-mile-long paved road within the canyon ?is one of the most splendid drives in the United States. However, the pavement had moderate to high severity fatigue and block cracking, with rut depths in excess of 0.5 inches. The roadway was in need of a structural rehabilitation.

At the same time, the number of yearly visitors to Zion has more than doubled in the last 20 years, to over 2.5 million. Traffic jams had become so bad that in 1991 cars were banned and a tandem shuttle bus service was begun. These buses are used to continually transport visitors into the main part of the Park and make several stops along the road. Other parks have instituted similar restrictions on the use of cars in their most popular areas.

In 2005, the FHWA Central Federal Lands in Denver funded a pavement rehabilitation project in the park. The 5 miles of the Scenic Drive, and about 1 mile of roadway from the south park entrance to the Scenic Drive, were to be rehabilitated.

The general contractor was Interstate Rock Products of Hurricane, Utah, and the reclamation/ stabilization portion of the work was subcontracted to Son-Haul of Fort Morgan, Colorado. The contract called for full-depth reclamation of the asphalt pavement and base soils to a depth of 8 inches. The reclaimed material was stabilized with foamed asphalt and Portland cement. After the stabilized base had cured, a 2.5-inch hot-mix asphalt overlay was placed. A single chip seal was to be applied in early 2006 so the roads will have the red color desired by the NPS.

Scott Wolfert, P.E., project manager of the FHWA抯 Central Federal Lands, and Robert Quadland of Parsons Brinckerhoff were the inspectors for the FHWA. All quality control testing was done by Travis Howlett of Earth Engineering Consultants of Fort Collins, Colorado. The initial mix design for the foamed asphalt stabilization process was also done be EEC. Sonny Weimer, president of Son-Haul, was the contractor抯 QC supervisor.

What is foamed asphalt?

Foamed or expanded asphalt (or bitumen) is a relatively new road base recycling process in the United States.

With foamed asphalt, a stabilized road base is created by carefully injecting a predetermined amount of cold water into hot Performance Graded (penetration-grade) asphalt in a pavement remixing unit, approximately 2 to 3% percent water by weight of asphalt.

Hot liquid asphalt rapidly expands into millions of bubbles (foam) when it comes into contact with cold water, similar to the spattering which takes place when drops of water stray into hot cooking oil on a stove top. When injected into the hot liquid asphalt, the water evaporates abruptly, thus causing explosive foaming of the asphalt in the saturated steam.

The water is the carrier of the atomized asphalt, and within a few seconds, the asphalt can thus be expanded to 15 to 30 times its original volume. Precisely added water allows control of the rate and amount of asphalt foaming or expansion. The expanded asphalt has a lower viscosity and a resulting high surface area available for bonding with aggregate fines.

The intensity and effectiveness of the foaming process can be further improved by control of pressure and temperature, which is possible when in-place foamed asphalt road stabilization is accomplished in a remixing unit designed for that purpose.

The foamed asphalt is immediately mixed with the reclaimed asphalt pavement and the base material.

While expanded asphalt doesn抰 completely coat all aggregate surfaces, it forms a mortar or glue which bonds the particles together. The expanded asphalt has an affinity for finer particles, those of 75 microns or less. This effective coating of finer particles increases the available surface area of the expanded asphalt for bonding with the coarser particles of material and spot welds the material matrix together.

Typically, the recycling or mixing machine is coupled with an asphalt supply tanker which is propelled by the recycler and a water cart which is pulled by the recycler.

When hot (350 degrees F) liquid asphalt cement is injected with a small amount of water, the volume of most types of liquid asphalt expands greatly as it forms black foam. Two foam properties are critical to successful road base stabilization, the expansion ratio and the foam抯 half-life.

The expansion ratio is the maximum volume of foam relative to the original asphalt volume and is an indicator of how well the foam will disperse and coat the RAP and soil particles in the reclamation process. The half-life is the amount of time it takes for the foam to collapse to 50% of its maximum expanded volume and is an indicator of the foam抯 stability. As the asphalt temperature or the amount of injected water increase, the expansion ratio increases, but the half-life decreases. Typically, an expansion ratio of 15 times and a half-life of 12 seconds are best for the base stabilization process.

However, each foamed asphalt mix design is different and must be undertaken based on materials that will be encountered on-site. Based on lab tests of material extracted from the existing roadway, the initial mix design called for 3 +/- 0.3% foamed asphalt plus 1% Portland cement, and compaction of at least 97% modified Proctor maximum dry density.

Bus traffic drives structural design

The 20-year pavement structural design was relatively high for a Federal Lands project (800,000 to 1,000,000 ESALs). About 70% of this loading was projected to come from the intensive shuttle bus traffic. Design engineers limited the maximum grade rise to 2.5 inches to meet minimum geometric and safety standards. To meet these criteria, an 8-inch foamed asphalt FDR followed by a 2.5-inch HMA overlay was specified. This design is projected to accommodate about 600,000 ESALs or 14 to 15 years of service life.

On this project, a prototype SH-1000 cement/asphalt/water buggy ? designed by Sonny Weimer, president of Son-Haul Inc., and built in their Fort Morgan, Colorado, facility ?was used with a Wirtgen WR 2500 S reclaimer. Individual chambers of the buggy hold 6,000 gallons of asphalt, 25 tons of cement, and 1,500 gallons of water. A burner unit in the SH-1000 kept the asphalt at the 320-degree F minimum, the asphalt temperature at which foam expansion was maximized.

The Wirtgen WR 2500 S reclaimer pushed the buggy as it fed precise proportions of asphalt and water into the cutting chamber and spread cement dust-free on the pavement in front of the reclaimer.

Initial compaction was provided right behind the WR 2500 S by several passes of a Hamm Model 3412 vibratory padfoot roller. A motor grader then bladed the recycled material to produce a smooth riding surface with the proper cross-slope. The specified 97% density was achieved by additional rolling with a steel vibratory roller. The recycled base was then lightly watered and rolled with a rubber-tire roller. The action of the rubber tires on the wet foam stabilized base created a slush that brought fines to the surface. These fines made the surface smooth, tight, and ready for traffic.

While traffic can ride on the foam-recycled base once compaction is complete, a surface seal is (usually) eventually required. On this project, a 2.5-inch HMA pavement added structure and sealed the stabilized base. A single chip seal with a local red aggregate was to be placed in 2006 to provide a final aesthetic touch to assure that the road fits into its surroundings and will 搇ie lightly on the land?