Recycling: it's an integral part of our world, at home, at work, at school--and on the road. Thousands of miles of streets and roads in the U.S. are deteriorating, and pavement engineers and contractors are taking up the mantle to repair them, many in the form of full-depth reclamation (FDR) with cement.
A Trace of benefits
"Reclaiming is recycling," said Dale Cronauer, vice president of Blount Construction Co., Marietta, Ga. "It's reusing what we already have, and what we've already bought, instead of throwing it away and starting over."
Cronauer's company specializes in pavement reclamation using a variety of additives including cement, and acted as subcontractor to APAC, Atlanta, in the recent full-depth reclamation of deteriorated pavement in the Natchez Trace Parkway just south of Jackson, Miss.
The Natchez Trace Parkway project originally was bid to use foamed asphalt, an additive that worked well for the first 1,500 ft, but work stopped when the crew encountered unexpected variances in soil and moisture conditions. For the next 1,000 ft, hydrated lime was applied in an attempt to dry up the soil, but durability and density requirements could not be satisfied. Finally, contractors successfully incorporated approximately 8,400 tons of cement to complete 30 miles of pavement.
Built in the 1930s, the Natchez Trace Parkway stretches 444 miles through parts of Alabama, Mississippi and Tennessee. The scenic roadway commemorates a trail that ran through the Natchez Trace, used most heavily by Native Americans from the late 1780s to the early 1820s, and is governed today by the National Park Service (NPS). Because the area is a popular spot for hikers, campers, bikers and horseback riders, it was an inconvenient setting for a major construction project.
"We did the project under very close scrutiny of the park," said Cronauer, as well as representatives of the Federal Highway Administration (FHWA). NPS and FHWA rules required that equipment be moved off the road and parked remotely overnight, and crews had to take extra care to avoid damage to surrounding foliage and soils. Despite the restrictions, Cronauer said, the project was completed on schedule and with minimum disruption to traffic flows.
The project was a first for the FHWA, and a testing ground for the Mississippi Department of Transportation (MDOT). Al Crawley, P.E., Mississippi consultant to the American Concrete Pavement Association's (ACPA) Southeast Chapter, viewed the successful Natchez Trace Parkway reclamation as a "good prototype" for the state's return to using cement in reclamation. From the 1950s to 1980, he said, state roads had a soil-cement base course, but strengths (and cement contents) were specified so high that cracking was inevitable, often propagating up through the asphalt overlays.
Since 1980 MDOT had shied away from cement-based reclamation efforts, but recent efforts by ACPA and the Portland Cement Association (PCA) helped convince the state and federal agencies to give it another chance. On this two-lane, low-traffic road, said Crawley, the FHWA's strong drive to economize by using much lower cement content (about 3.5%) is a perfect fit: "It will help them avoid the cracking we've seen in the past . . . but for what they're doing it's more than ample" in terms of strength and durability.
As is the case with the Natchez Trace Parkway, many county and state roads serve lighter traffic, so pavements with lower specified strengths can still carry adequate loads, said Crawley. The result: FDR is gaining popularity for roads in the region: "We've got a couple jobs coming up with the county in the next six months based on the success of the Natchez Trace project," said Crawley. "Having a 30-mile proj-ect helps us get past the credibility problem."
Sweet results, Alabama
FDR with cement also provided an assist in fall 2002 to a stretch of Franklin County Highway 84, just north of Cedar Lake in Franklin County, Ala. The state's first project to use FDR with cement, the roadway used 440 tons of cement to reclaim 1.7 miles of pavement.
"The county saw an immediate cost savings in using full-depth reclamation with cement" compared to its usual method, said Gregory Halsted, soil-cement/roller-compacted concrete pavements engineer with PCA. The county already owned a reclaimer and enough trucks to do the work, and was embarking on some other reclamation jobs using stone.
But two issues plagued the county: roads reclaimed with crushed stone only a few years ago already needed to be re-shaped, and numerous trips to the only accessible quarry in the state were taking a toll on the budget and schedule. FDR with cement was an option that would save money and create a more durable roadway.
Testing determined that a mix design with 5% cement blended with the existing roadway material would create a base with a minimum unconfined compressive strength of 350 lb per sq in., more than meeting county requirements. While it was the county's first time to use FDR, "it was a fairly straightforward application," said Halsted. "Once they got the hang of it, they were able to work with incredible speed." At locations throughout the project, the crew used a nuclear density gauge to ensure a minimum density of 96%. The reclaimed pavement eventually was topped with an application of double bituminous surface treatment.
"It was a learning experience for them, and they learned very quickly," said Halsted. "Aside from the economic value was the speed with which they could finish the project." The county now wants to go back to reclaim some of the roads that were treated in recent years with stone, some of which are already deteriorating again. "Using cement, the county can rehabilitate roadways that are seeing greater traffic and be confident that the solution is much more permanent."
As with the Natchez Trace Parkway in Mississippi, the Franklin County project serves as a model application in Alabama, boosting interest across the state. In April 2003, pavements in a Trussville subdivision in Jefferson County were reclaimed with the proc-ess, and the Alabama DOT currently is finalizing plans for its first FDR with cement project.
A sharper green
With more focus than ever on green construction and recycling, Cronauer added, it's no wonder reclamation is moving into the spotlight: "Reclamation, no matter what project it is, is by far cheaper than rebuilding something, and is every bit as good as buying new material," said Cronauer. "It utilizes the existing material so there's less disruption to the environment-which in turn provides more cost savings-and a time savings, which is another part of the money-saving equation."
The deal on FDR
Full-depth reclamation (FDR) with cement uses materials from failed asphalt pavements, adding cement to create a new stabilized base that can be re-paved to add decades of service life and effectively "recycle" pavements.
In a typical FDR process, the site is investigated and materials analyzed to determine the mix design and pavement thickness. Reclamation begins with a pulverizer to rip up the old asphalt and base course, and a grader follows to even the surface to achieve the desired cross section and grade. (At this stage, crews may choose to re-open the roadway for temporary traffic.) Cement is then spread over the pulverized material, and a pulverizer hooked to a water truck takes a pass, mixing the water, cement and pulverized material to a depth of 6-10 in. Finally, the base is compacted, cured and finished with a riding surface like chip seal or hot-mix asphalt.
For many pavements, the reclamation process can be completed in as little as a day, restoring traffic flows with minimal disruption and creating strong, durable pavements. Compared with constructing new pavements, FDR requires significantly less material, uses far fewer trucks, creates no construction waste and consumes a fraction of the diesel fuel. For cash-strapped states and counties, FDR often makes good environmental and economic sense.
