Usually reserved for rural locations with plenty of wide-open space, full-depth reclamation (FDR) with portland cement has come to the city—specifically, to downtown Reno, Nev.
The Regional Transportation Commission, which maintains almost 500 miles of roadways in Nevada's Washoe County, saved $1.4 million in construction and material costs by using this environmentally friendly pavement-reconstruction method to restore six lane miles of traffic-bearing pavements and three lane miles of parking pavements.
In selecting the appropriate rehabilitation strategy, the commission had to take into account curb elevations, the presence of cobblestones in the subgrade, insufficient or contaminated base, and shallow utilities. High traffic volumes and a limited budget (depending on sales tax revenues, its annual budget ranges from $9 million to $12 million) added to the challenge.
The commission retained Lumos & Associates, a multidisciplinary consulting firm in Reno, to manage engineering and construction.
After evaluating the impact of four rehabilitation options—full removal and replacement, partial removal and replacement incorporating geotextiles, thick hot-mix asphalt layers, and FDR using portland cement—on structural requirements, schedule, costs, and traffic flow, the firm recommended FDR.
U.S. Bureau of Labor Statistics shows that cement is now less expensive than asphalt. In addition to reducing the amount, and thus the cost, of asphalt necessary for a top course, FDR also saves on trucking costs, landfill fees, and time and labor. That's why, according to Greg Halsted, pavements engineer for the Portland Cement Association, the process is often 25% to 50% less expensive than full removal and reconstruction.
“If we'd gone with a full remove-and-replace option, we would've spent $6.6 million, 20% more than the $5.2 million cost,” says former Lumos & Associates project designer Greg Lyman. “If you figure cost per square foot on just the roadway structural sections, the savings are on the order of 50%.” Much of the savings came from eliminating the need for an entire lift of asphalt by providing a stiffer, stronger base course.
Recycling was also a politically correct option.
“Recycling the pavement instead of using more mined aggregates is good public relations for our agency,” says Michele Dennis, PE, Regional Transportation Commission project engineer.
To design the optimum cement-stabilized base, engineers dug test pits to obtain representative samples of surface, base, and subgrade materials. They pulverized the samples in the lab and added portland cement to prepare specimens.
Three sets of test specimens were prepared at 2%, 4%, and 6% cement, and tested for strength at 2, 7, and 28 days. The designers were looking for early strength because the roadways had to be paved and open to traffic as quickly as possible. Exact cement contents ranging from 3.5 to 4.5 by dry unit weight of the pulverized material were selected for each of the four streets to meet the commission's strength and durability requirements.
Sierra Nevada Construction of Sparks, Nev., used a reclaimer to pulverize the existing pavement and base to -3 inch sieve size with 95% to 100% at -2 inch. (See side-bar, Zip it Up for information on another way to recycle aggregate.) Then it graded 5 to 6 inches of material off the top so the finished elevation with the hot-mix asphalt course would match the existing curbs and gutters.
At this point, two inspectors checked the pulverized material to make sure it met specifications.
Next, water was added until testing indicated the material had reached its optimum moisture content.
Then a cement-spreading machine passed over the area, metering out the cement. The reclaimer made a second pass, thoroughly mixing the cement with the watered-pulverized materials. The material was then graded to the appropriate plan lines, grades, and cross sections, and then compacted.
The commission required that all the roadways be repaved within seven days of surface removal to accommodate traffic coming into downtown Reno for community events. Because the cement in these roadways was designed to achieve the highest balance between strength and durability, engineers were concerned that shrinkage cracking might occur and reflect back up through the newly placed asphalt surface.
To minimize this possibility, Sierra Nevada “microcracked” the roadways before applying the asphalt lift. Vibratory rollers passed over the cement-stabilized base after a short curing stage to create a fine network of cracks. This fine cracking prevents wider, more severe cracks from occurring.
Each roadway section was pulverized, blended with cement, shaped, and compacted in one day, and then moisture-cured for 48 hours. The contractor performed the process at the two-day mark on each roadway, and all sections were paved within 12 hours of microcracking. This procedure put the roadways back under traffic in less than four days.
Because Reno has weather extremes with warm summers and cold winters, the commission usually requires asphalt thickness of 8 inches. But the cement-stabilized pulverized asphalt produced such a stable base that asphalt thickness was reduced by 1 to 3 inches, decreasing asphalt quantities by as much as 30% and eliminating the time that would have been required for an additional hot-mix lift.
“Previously, this method would have been cost-prohibitive due to limited equipment availability and cement cost,” says the Regional Transportation Commission's Dennis. “But now other factors offset that, so it's a viable method for future reconstruction.”
— Carol Carder is a Denver-based freelance writer.
See side-bar, Zip it Up for information on another way to recycle aggregate.)