UMass Dartmouth creates new sustainable asphalt overlay from unique blend of recyclables
We would like to thank Paul Montenegro for suggesting this article to Baystate Roads.
The Department of Public Works (DPW) in Wellesley, Massachusetts recently overlaid the pavement at the town recycling and transfer center with a new sustainable asphalt maintenance mix that incorporates recycled materials and newly developed “green chemistry.” Designed by Professor Walaa S. Mogawer, P.E., F. ASCE, Head of the Highway Sustainability Research Center (HSRC) at the University of Massachusetts Dartmouth, the experimental mix was produced by Benevento Companies of Wilmington, Massachusetts, and installed over a 2,200 square yard area at the facility by U.S. Pavement Services Inc. of Woburn, Massachusetts.
Installed just three-quarter inches thick, the dense-graded mix is designed to resist cracking and preserve the underlying pavement with an engineered blend of virgin asphalt binder and aggregates. This mix consists of recycled asphalt pavement (RAP), recycled asphalt shingles (RAS), a new polymer containing recycled ground tire rubber (GTR), asphalt rejuvenator, and warm mix asphalt technology (WMA).
GreenDOT Backs Warm Mix
The use of sustainable, or green, components in the new asphalt mix is in keeping with the GreenDOT initiative launched several years ago by the Massachusetts Department of Transportation (MassDOT). GreenDOT aims to incorporate sustainability into all of MassDOT’s activities, with a focus on reducing greenhouse gases that contribute to global warming. One of the measures taken by MassDOT is the promotion of WMA for its asphalt paving projects.
WMA is produced by adding such materials as zeolites, waxes or other proprietary chemicals to the asphalt binder. These additives allow significantly lower temperatures for producing and installing asphalt mix, resulting in lower consumption of fossil fuels leading to the production of less carbon dioxide and other greenhouse gases. Furthermore, the WMA technologies help ease mix compaction and allow for paving in colder weather and longer hauls between asphalt plants and job sites.
In a move designed to take advantage of these benefits, MassDOT established WMA pavement as the standard state specification while phasing out hot mix asphalt. As an illustration of this commitment, approximately 68 percent of the 891,000 tons of asphalt mix placed by MassDOT on its projects in 2013 was WMA.
A Growing Trend
MassDOT’s switch to more sustainable asphalt mixes reflects a growing trend across the U.S., according to the National Asphalt Pavement Association (NAPA). In partnership with the Federal Highway Administration (FHWA), NAPA conducted a mid-2014 survey of asphalt mix producers and state asphalt pavement associations. Survey results indicate that 106.4 million tons of WMA – almost a third of all asphalt pavement mix production – was used during the 2013 construction season. NAPA notes that this is a 533 percent increase in the use of WMA since 2009. NAPA reported that the use of reclaimed and recycled materials in asphalt pavements in 2013 totaled almost 72 million tons of RAP and 1.7 million tons of RAS in new asphalt pavement mixes in the U.S. Reclaiming and reusing the asphalt binder in RAP and RAS saved about $2 billion in 2013 than if 100 percent virgin asphalt binder was used. The use of RAP also conserved more than 68 million tons of virgin aggregate according to the survey report.* As reported by NAPA, other waste material was also repurposed into pavements. Survey respondents reported using nearly 1.2 million tons of steel and blast furnace slag and GTR in 2013 in the production of more than 6.6 million tons of asphalt paving mixes. (*National Asphalt Pavement Association, Annual Asphalt Pavement Industry Survey on Recycled Materials and Warm-Mix Asphalt Usage: 2009–2013.)
Building on Prior Success
With the trend to green materials use in mind, Pavement Consultant R. Paul Montenegro, based in Rhode Island, approached Wellesley DPW Director Mike Pakstis with the idea of installing a thin asphalt maintenance mix incorporating the latest sustainable technology over existing pavement at the community’s recycling and transfer center. Pakstis was responsive to the idea, indicating that the DPW was “willing to try new things,” if the cost-effective alternatives can prolong pavement service life. As a case-in-point, Pakstis had approved an earlier project involving a trial mix overlay for the road leading to the recycling center. Director Pakstis and Town Engineer David Hickey had worked with Montenegro and Dr. Mogawer on the overlay installed late in 2007,that was witnessed by officials from the state and more than 20 other municipalities. The overlay placed in 2007 had a three-quarter inch compacted thickness and consisted of a dense-graded material containing 30 percent RAP, a proprietary warm-mix formulation, and an asphalt binder modified with 1.5 percent latex polymer. Dr. Mogawer’s intent was to provide a thin wearing surface that would slow cracking.
The 2007 overlay was installed as a preventive maintenance treatment, which is one of several methods of pavement preservation defined by the FHWA. Pavement preservation treatments can be used on pavements still in good condition before the onset of serious damage from weather and wear, thus restoring pavement to almost original condition and postponing costly rehabilitation and reconstruction. Other preventive maintenance treatments are asphalt crack sealing, chip sealing, slurry sealing and microsurfacing. Pakstis and Hickey have monitored the condition of the thin overlay for the past seven years. “We’ve had good performance from that three-quarter inch mix” Pakstis said.
After reviewing the concept with Montenegro, Director Pakstis decided to try the new sustainable asphalt maintenance mix developed at the UMass Highway Research Center to resurface the pavement at the recycling and transfer center. He said he was encouraged by the fact that Dr. Mogawer was already involved in designing crack-resistant, high-RAP, WMA-based asphalt mixes under a research program for MassDOT.
Recycling-Focused Research
Dr. Mogawer was looking for a sustainable mix that would use recycled asphalt pavement and recycled asphalt shingles, and would advance cost-effective and environmentally friendly pavement preservation strategies. In addition, Dr. Mogawer wanted to develop a mix that would prevent pavement cracking propagation and have the potential to result in a service life of 12 years or more. “Using as much recycled asphalt pavement and shingles as possible conserves stone and asphalt and therefore offsets the high cost of virgin asphalt,” said Dr. Mogawer. “In addition, we are interested in achieving lower mix production temperatures resulting in emission reductions and contribute to better for the health of workers.”
He added that coincident to the research being conducted, Montenegro wanted to put together a demonstration project in Wellesley similar to the one performed in 2007, but this one would use RAS and other sustainable products. Since Montenegro’s proposal meshed well with Dr. Mogawer’s ongoing research for the GreenDOT initiative, he provided Wellesley with the mix design for the demonstration.
Tough Test for Sustainable Mix
Using the highway research center design for the Wellesley demonstration also offers researchers an opportunity to study the performance of a lab-designed, thin overlay on a 3 to 4 inch bituminous concrete pavement subjected to long-term weather and harsh traffic.
“We get between 800 and 1,500 residents’ vehicles each day and about 4,000 on Saturdays,” Pakstis said. “In addition, each day our own regular six wheel, 35,000 pound collector trucks use the facility, plus six to eight tractor-trailer trucks periodically pick up compacted refuse.”
The trial overlay places Wellesley in the forefront of testing sustainable asphalt mixes. Furthermore, it provides experience with meeting requirements of provisional specifications for sustainable asphalt mixes for municipalities being developed by MassDOT. The provisional specifications will allow municipalities to obtain affordable maintenance asphalt mixes for the state’s Chapter 90 projects, according to Montenegro. Massachusetts’ Chapter 90 entitles municipalities to 100 percent state reimbursement of expenditures for highway construction, preservation and improvement projects.
“The specification requires the mix to contain a minimum amount of asphalt from RAS and meet the new performance graded acceptance criteria,” said Montenegro. “This is a step forward, because the State will not give recipe specs. Rather, the manufacturer will meet performance tested acceptance criteria developed at UMass Dartmouth,” he said.
Shingles are a Rich Resource
The manufacture of the new mix for this particular project is unique, in that all of the green chemicals are added to the RAS at one plant, after which the treated RAS is shipped to a hot mix asphalt plant where it is in turn combined with RAP and virgin materials to make the final product.
The Asphalt Roof Recycling Center in Stratford, Connecticut, grinds asphalt shingles and adds other products to the ground shingles inline at its new indoor facility. Shingles are a rich source of asphalt binder, with the asphalt cement content of fiberglass matt base shingles running between 19 and 22 percent, and asphalt cement content on felt matt base averaging between 30 and 36 percent, according to the National Association of Home Builders (NAHB). This compares to approximately 6 percent asphalt binder in RAP. NAHB estimates that shingle tear-off waste and installation scrap amounts to between 7 and 10 million tons each year.
‘Green’ Treating Ground Shingles
While the potential for recycling asphalt shingles is promising, there are limits to using RAS in pavement mixes according to PQ Corporation, the maker of Advera WMA. PQ Corporation notes that one of the challenges to using RAS in producing asphalt mix is that the RAS tends to clump together in the stockpile and in the mix. This can lead to an inconsistent feed rate, causing inconsistent mix results. Moreover, the mix is stiffer, making it difficult to handle. The company’s proprietary Advera WMA, designed to reduce clumping, contains zeolite, a mineral that occurs naturally but is also produced industrially for use in making WMA . It was one of the chemicals added at the Connecticut facility to the RAS destined for the Wellesley project.
Another contributor to the sustainable Wellesley mix is a re-engineered, waste-stream modified polymer that includes GTR. The polymer is added to RAP or RAS on the conveyor belt – hence its name, Belt Added Modifier (BAM). Produced by EVR International, BAM is designed to improve mix resistance to cracking and rutting, and enhance resistance to tire skidding.
One characteristic that limits the amounts of RAS and RAP that can be used in hot or warm mix asphalt is the tendency of recycled binders to be stiffer and less strain-resistant than virgin asphalt, and therefore more susceptible to cracking. This is primarily due to oxidation from long-term exposure to weather. One deterrent to oxidation is a class of recycling agents known as rejuvenators. Research has shown that adding a rejuvenator to high-RAP and RAS mixtures improves the crack resistance of the asphalt mix.
The rejuvenator added to the RAS for the Wellesley mix was developed by a team from Warner Babcock Institute of Green Chemistry, headed by Dr. John Warner. The Delta S rejuvenator, added to the RAS at the Connecticut facility, is designed to reverse existing asphalt aging and oxidation, and to retard future aging and oxidation. A product of Collaborative Aggregates LLC, Delta S is also a warm mix additive, and is marketed as a dual-action WMA and rejuvenator.
Mix Production and Installation
The completed RAS was transported to Benevento Companies’ asphalt plant in Wilmington. Dr. Mogawer’s design called for 5 percent RAS and 25 percent RAP (based on total weight of mix). The RAS and RAP mix was blended with three-eighths inch top size virgin aggregate and asphalt to produce the final mix for the Wellesley trial. The virgin asphalt binder was a PG64-28.
The mix was manufactured in Benevento’s 300 ton per hour batch plant at between 250 and 255 degrees Fahrenheit. Benevento’s trucking fleet delivered the mix to U.S. Pavement Services’ LeeBoy 8510B Paver at between 200 and 215 degrees. Meanwhile, the paving crew had employed a LeeBoy Tack Wagon to cover the area with an RS-1 asphalt tack coat. Mat temperatures ran between 170 and 180 degrees after breakdown rolling by the crew’s 3 ton, double-drum Volvo DD29 Roller operating in static mode.
Pakstis pointed out that the new overlay resembled the 2007 overlay in color and texture. He said the town will closely monitor the performance of this new sustainable pavement preservation treatment over the coming years, with the hope that it could achieve a service life of at least 12 years.
State Encourages More Recycling
MassDOT has expressed a strong interest in projects that utilize recycled materials such as the Wellesley overlay. Ed Naras, Pavement Management Engineer for the Highway Division of MassDOT, summed it up this way:
“The efforts to expand the use of recycled material in a responsible and sustainable manner is consistent with MassDOT’s GreenDOT Policy and other long-standing initiatives. We hope to see an increase in the use of these products in pavement preservation and maintenance activities.”
Reprinted with permission from New England Construction magazine.
Mass Interchange, Summer 2015
