Deteriorating roads and tight budgets are a constant problem for cities and counties. As a result, engineers and public works officials are turning to a process called full-depth reclamation (FDR) with cement. This process rebuilds worn out asphalt pavements by recycling the existing roadway. The old asphalt and base materials are pulverized, mixed with cement and water, and compacted to produce a strong, durable base for either an asphalt or concrete surface. FDR uses the old asphalt and base material for the new roadway base. There's no need to haul in aggregate or haul out old material for disposal. Truck traffic is greatly reduced, and there is little or no waste.
This is exactly what Glen Allen, Virginia-based Slurry Pavers, Inc., is doing on a 4-mile project in Dover, DE, and it is a process that David Stowell, head of business development for Slurry Pavers, has been educating state governments about for several years.
“Using this process takes about half the time of conventional processes and is one-third the cost,” says Stowell. With budgets getting tighter and the public's growing demand to protect the environment, FDR is a cost-effective and green alternative for maintaining and rehabilitating the nation's roads.
FDR with cement makes the reconstruction of old roads a largely self-sustaining process. The original “investment” in virgin road materials becomes a one-time cost, which is reclaimed through cement stabilization and addition of a new, thin surface course. The basic procedure is simple. The complete recycling process can be finished in one day, and traffic can be maintained throughout construction.
Preconstruction: According to the Portland Cement Association (PCA), the site should be investigated to determine the cause of failure and samples from the failed road should be taken and evaluated. Using these material samples, an analysis is done to establish the aggregate-soil mix design for the rehabilitated road. Finally, the thickness design for the rehabilitated road is determined.
At the Dover site, Slurry Pavers is using 6 percent portland cement in 6 inches of material.
Pulverization: Construction begins with pulverizing the existing asphalt pavement using equipment that resembles a large rototiller. Slurry Pavers uses a Wirtgen WR2500 road reclaimer and soil stabilizer. Pulverizing, or rubbelizing, the base material prepares it to accept the portland cement. Stowell points out that two passes with the WR2500 are necessary to adequately pulverize the material. According to PCA, the particle distribution should be such that 100 percent passes the 3-inch sieve, 95 percent passes the 2-inch sieve, and at least 55 percent passes a No. 4 sieve.
“(Pulverizing) the material is important because you have to size the material properly,” explains Stowell. “If you have large chunks of asphalt, then you can't grade it. Going over it twice alleviates a lot of those issues.”
The depth of pulverization ranges from 6 inches to 10 inches, although the depth of pulverization can reach 18 inches when necessary. On secondary roads this will typically include all of the surface and base, plus some part of the subgrade. Delaware DOT specs require a 6-inch depth for pulverization on this project.
Shaping and Grading: The pulverized material is shaped to the desired cross-section and grade. This can involve additional earthwork in order to widen the roadway. At the Dover site, Slurry Pavers is widening the road 1 foot on each side. The final base elevation requirements may necessitate a small amount of material removal or addition.
Cement, Water, The Mix: A measured amount of cement is spread either in dry or slurry form on the surface of the shaped roadway. Water is added to bring the aggregate-soil-cement mixture to optimum moisture content. This mixture is combined and blended with the pulverizing machine. More than one pass of the mixer may be required to achieve a uniform blend of materials.
At the Dover site, a spreader truck adds dry portland cement to the rubbelized road. A water truck precedes the spreader truck in order to hydrate the bottom of the base material. Later, the water truck will move through the site again in order to hydrate the top layer of the subgrade.
According to Stowell, hydrating throughout the application of the portland cement is the key to a strong subgrade material.
“It is important to hydrate the material all the way through. In years past, they would hydrate it from the top down. If you're going down 6 inches, only the first 3 to 4 inches are getting water. You've lost the value of the portland cement at the lower end,” says Stowell.
“Technology has really allowed these machines to quantify each gallon of water to each yard of material that we pulverize. We can specifically hydrate 6 inches of material. Now you have a homogeneous material that is completely hydrated, which is a tremendous benefit. You get use of all the portland cement and a better subgrade material.”
Compaction And Curing: The mixture is then compacted to the required density and should take place no more than two hours after initial mixing of the cement. The new subgrade is allowed to cure, and the goal of curing is to keep the base continuously moist so the cement can hydrate.
Slurry Pavers prefers to wet cure the subgrade, but at the Dover site they are applying a bituminous primer over the road to seal in the moisture. Both methods are acceptable, according to PCA.
Pavement Surface: Finally the new pavement surface consisting of a chip seal, hot-mix asphalt or concrete is constructed to complete the FDR process. At the Dover site, a 3-inch asphalt overlay will be the final course.
“One of the things that's really great about this process is that it's green, and it's very cost effective relative to a conventional fix,” says Stowell. “They are paying $85 a ton for asphalt here. That would have cost you probably $40 a square yard on this road to fix it (conventionally). But when you can take $40 a square yard and cut it to $15 a square yard, then you've saved the government and the taxpayer. Now you have a road that's going to last you 25 to 30 years, and you've recycled 100 percent of the existing material.
“We certainly need to take responsibility for the environment and what we're doing, but if we do something let's make it a real change – real and quantifiable. When you take trucks off the road, when you stop taking virgin material out of a quarry and you're reusing the existing material that you already paid for – that is doing something. You've just made a green process a no-brainer. Why would you do anything else?”
Additional material provided by the Portland Cement Association.
Editor's note: The Portland Cement Association offers a broad range of resources on soil-cement applications for pavements. Visit their website at www.cement.org/pavements.