As an industry, we are constantly tasked with doing more with less. Pave more roads with less money. Recycle more materials in less time. Do more work with less people.
Luckily, as an industry, we are no stranger to finding innovative ways to do all of the above and a new kind of pavement preservation method has emerged to help.
Paver-Placed, Stabilized Full Depth Reclamation, or SFDR, is a roadway rehabilitation process for poor to very poor condition roads (PCI’s typically below 50). The process involves pulverizing the heavily distressed existing pavement and blending it with some of the underlying granular materials. During the blending process, new foamed liquid asphalt and portland cement are added as strengthening agents. This blended and stabilized material is then paved back onto the road in one thick lift, thereby reducing construction time and the inconvenience to residents and motorists.
indus, a Northeast paving company specializing in pavement preservation, heard about SFDR last year and knew they had to learn more about it to be able to start offering it to their customers.
“We are constantly looking for ways to help our road owner agency clients improve and optimize their pavement management programs, and with this year’s introduction of the SFDR treatment, we now offer a cost-effective solution at every point up and down the asphalt deterioration curve,” Dan Patenaude, P.E., and strategic advisor at indus says. “Prior to 2017, our solutions were limited to the top half of the curve with preservation treatments such as crack sealing, fog sealing & rejuvenation, microsurfacing and cape seals.”
The company first learned of the paver-placed SFDR process while attending the Asphalt Recycling & Reclaiming Association (ARRA) semi-annual meeting in Charleston, SC last October. Presentations by Wirtgen America, the South Carolina Department of Transportation (SCDOT) and their paver-placed SFDR contractor (King Asphalt) inspired the company to learn more.
“The week following that conference, we sent a small contingent including one of our owners, our QC Manager, and two recycling Superintendents down to visit a paver-placed SFDR project King Asphalt was in the process of completing for SCDOT,” Patenaude says. “They returned from that job confident in our crew’s ability to perform the same type operation.”
SFDR vs. FDR
Many companies who specialize in pavement rehabilitation are familiar with Full-Depth Reclamation (FDR). Traditional FDR is not paver-placed, and instead the pulverized and blended materials are typically windrowed behind the reclaimer where they then need to be bladed and shaped with a motor grader before compaction.
Paver-placement on the other hand, eliminates the need for the grader which helps to reduce mix segregation and minimize the impact of construction on motorists and abutting property owners.
“Most FDR in this region is not stabilized and agencies rely on thicker lifts of hot mix asphalt (HMA) on top of the recycled layer to meet the traffic load requirements for a particular roadway,” Patenaude says. “The structural layer coefficient of these unstabilized materials is about 0.14 per inch of thickness depending primarily on the quality of the subbase granulars and the proportioning of pulverized HMA with those granulars. The structural coefficient for the stabilized FDR is more than twice that at around 0.30 per inch.”
indus was familiar with the FDR and other preservation methods and knew they could provide SFDR with some additional training and support.
“With Wirtgen’s outstanding support, we added cold in-place recycling (CIR) to our service offerings in late 2017,” Patenaude says. “And with the 2021 upgrade of our Wirtgen recycler to their newest model, we are now able to offer the SFDR treatment which provides a competitively priced and less disruptive solution for communities’ poorest condition roads.”
The newest Wirtgen W 380 CRi cold recycler has 100 additional horsepower, and redesigned front and rear door and side plates which allow for cuts up to 12-in. deep.
“Currently, we are only considering recycling projects up to 8-in. deep, and if we eventually decide to go deeper, we will need to upgrade at least one of our rollers to insure adequate compaction,” Patenaude says.
Since much of the SFDR process is substantially similar to the CIR process the company has been doing for five years, not much additional training was required. Patenaude says the only major differences with SFDR from CIR are:
The mix design sampling is more extensive and complex because it includes a properly proportioned amount of the underlying granulars (CIR only recycles the pavement and none of the unbound material below it), and
The compaction effort is more intensive given the greater lift thicknesses (CIR is typically done 3-in. to 5-in. deep, whereas the SFDR layers usually range from 6-in. to 12-in. deep).
Putting SFDR to the Test
This summer, indus put SFDR to the test for the first time on Woodcock Road in the Town of Dartmouth, MA. The roadway is about 4,000 feet long, a little less than one mile.
"This was a very old county road that hasn't been paved since 2007," Paul Pacheco, Dartmouth DPW Superintendent of Services & Infrastructure says. "The route was heavily decayed as it is a major cut off with major trucking activity. It received an enormous amount of traffic and weight and we needed a strong repair to bring the road back to good condition and withstand future activity."
The Town of Dartmouth has been working with indus for the past 10 years, finding innovative ways to save money on their road projects and turned to them again for Woodcock Road.
"We have been partnering with indus the last few years completing CIR in our area and achieving 60-70% cost savings," Pacheco says. "They have been trying new technologies and they invest the money in the equipment and procedures they know will help us get the best road we can for the money that will last the longest."
Patenaude agrees.
“Our municipalities here in the northeast are relatively small compared with towns and counties out west, and their budgets are small, as well, so most of our recycling projects are in the 1 to 2 miles long range,” Patenaude adds. “The Town of Dartmouth chose Paver-Placed SFDR for this project over conventional, unstabilized FDR primarily because of the reduced inconvenience the paver placement process provided to their residents. Other benefits that helped convince them to give this new process a try were that it cost slightly less than their conventional unstabilized FDR option, is likely to be longer lasting because of the strengthened base layer, and was more environmentally friendly with lower greenhouse gas emissions.”
indus pre-milled about 4-ft. wide and 8-in. deep on both outside edges of the road to remove materials and make room for the expanded, recycled material plus the new HMA wear course without raising the elevation of the road. Following the pre-mill, they drop-spread about 8 pounds of portland cement stabilizer additive per square yard per the mix design.
Next, the SFDR recycling train followed. This set up consists of a water tanker connected in tandem with a liquid asphalt tanker and the recycler which conveys the blended and stabilized material back to the hopper of the paver. Behind the paver is a dual-drum roller, a vibratory pneumatic roller and a technician with a nuclear density testing gauge.
“Once densities are achieved, we then apply an asphaltic fog seal before reopening to traffic within a few hours of starting the recycling on a section of road,” Patenaude says. “The fog seal protects the recycled layer from water intrusion until the HMA wearing course is applied and protects it from traffic picking the fines out of the mat.”
The recycling portion performed by indus was completed in two days (one day for each lane keeping the opposite lane open for alternating one-way traffic), and one day for the HMA paving of the 1-½-in. wear course.
The mix design (performed by American Engineering Testing in St. Paul, MN) called for the addition of 2.2% foamed asphalt PG binder plus 1% portland cement by weight of the blended RAP and subbase granular materials. For this project, that worked out to 15,429 gallons of PG binder and 32.65 tons of portland cement. After pre-milling to remove material, indus cut about 8-in. deep and then paved the stabilized materials back at an average thickness of 7-in.
Just less than two weeks after the completion of recycling, the Town of Dartmouth’s HMA paving contractor (P.J. Keating, a CRH company) installed a 1.5-in. thick 9.5mm Superpave mix as the wearing course. 988.09 tons of the Superpave HMA mix was applied.
To make the most of their SFDR efforts, indus hired an independent testing company (Infrasense from Woburn, MA) to collect some falling weight deflectomer (FWD) and ground penetrating radar (GPR) data to help confirm what they built.
“We learned from their investigation that our average structural coefficient was 0.28 per inch less than 2 weeks after recycling and just before the HMA paving of the wearing course,” Patenaude says. “Understanding that the foamed asphalt stabilized layer will gain strength over time (one study by VDOT determined a 25% to 50% strength gain after 2 years), we and our client are both pleased with the results.”
In all, the paver-placement SFDR methodology can cut the recycling time by about half. And although pricing is extremely volatile in our industry this year, it appears the cost savings is about 5% to 10% with SFDR. The extra money invested in the stabilizer additives is less than the savings achieved by being able to use a reduced thickness of HMA on top of the recycled layer.
"We are very happy with the results of this project," Pacheco says. "It's sometimes scary being the first to try a new method of preservation but the team at indus did their research and training and provided us with a nice smooth road and an economical repair. We plan to do a longer stretch using this method next year.
And others should consider following suit.
“With all the recent emphasis on rebuilding America’s decayed infrastructure, paver-placed SFDR provides road owners with a unique opportunity to save both time and money repairing bad roads and we expect to be doing a lot more of it in the coming years,” Patenaude says.
