In today’s culture, to be green is to be great. Under the umbrella of its GreenDOT program, the Massachusetts Department of Transportation (MassDOT) has given its six districts leeway to employ innovative techniques to use more sustainable practices for greener asphalt pavement maintenance and preservation.
In the December 2012 GreenDOT Implementation Plan, officials spelled out the department’s stewardship goal: “Operate the transportation system in a manner that embraces our stewardship of the Commonwealth’s natural, cultural and historic resources.”
Thus May 2016 found MassDOT District 3 Construction Engineer Michael Hartnett, P.E., planning for specific sustainable action alongside Gregg Berkley, senior project manager for P.J. Keating Co., of Lunenburg, Mass., and Peter Montenegro, construction marketing consultant for Collaborative Aggregates LLC (CollAgg), of Wilmington, Mass., at the laboratory of Walaa Mogawer, Ph.D., a civil engineering professor at the UMass Dartmouth Highway Sustainability Research Center (HSRC). Mogawer’s lab won a $249,000+ grant from the New England Transportation Consortium in 2014 with the purpose of studying new ways to use reclaimed asphalt shingles (RAS) from roofs. That kind of green thinking is what MassDOT is looking for.
At the UMass Dartmouth HSRC, Mogawer introduced Hartnett and Berkley to the concept of using the specific rejuvenator Delta S to address oxidized binder concerns in mixes with high reclaimed asphalt pavement (RAP) percentages. While Mogawer’s researchers have tested the proposed mix design extensively in the lab, Hartnett identified a pavement section in Worcester where the mix design could be put to real-world use in a mill-and-fill application.
This gave MassDOT District 3 an opportunity to explore the GreenDOT’s sustainability mandate by increasing RAP use from 15% in a surface course to 40%. Take a look at what the team accomplished.
Crews shut down lanes for completely closed night work zones.
Pick and choose
For a mill-and-thin overlay project using 40% RAP in the mix, Hartnett selected an older asphalt pavement considered to be in fair condition, merely in need of resurfacing. The approximately 3,000 linear ft of the North Service Road and westbound ramp onto I-290 in Worcester had seen no preservation treatments in the past.
“It showed no signs of deterioration,” Berkley agreed. “There was some oxidation and some surface cracking.”
Berkley explained the pavement was the correct one for the treatment. “The purpose of this project is a pavement preservation instead of a rebuild.”
A thin overlay’s purpose is not to fix structural problems, but to extend an existing pavement’s life, thus the minor cracking and oxidation of the chosen pavement could be resolved with the simple 1-in. mill and well-designed thin overlay.
Originally, the test section was going to be included in a MassDOT district-wide resurfacing contract. When the budget was expended prior to completing the work in mid-2016, the North Service Road project had to be pushed to a later date. October 2016 saw the project come to fruition.
Design and produce
Mogawer prepared a Superpave 9.5-mm Sustainable Thin Overlay (SP9.5 STO) mix design, which is appropriate for lift thicknesses ranging from 0.75 to 1.5 in. The mixture comprises coarse aggregate, fine aggregate, mineral filler as needed and a PG 64-28 asphalt binder typical to Massachusetts specifications. A maximum of 40% RAP incorporates a rejuvenator at a dosage of 6% by weight of recycled binder. The draft specification allows MassDOT District 3 to increase the amount of RAP it would typically include in a surface layer.
Montenegro explained, “Nationally, on a simple percentage basis, most DOTs allow up to 20% RAP in surface course mixes,” he said. “When mixes include a higher percentage of RAP than that, the pavement can experience the early onset of premature cracking due to the stiffness of the aged binder. Industry can offset that brittleness with a softer binder grade or rejuvenators. Mogawer is using the rejuvenator in the lab, and now it needs to be put to the test in the field. MassDOT District 3 and P.J. Keating opted to use it in the field for a pavement in Worcester.”
As mentioned above, the specification used for the North Service Road mill-and-fill project is a provisional one. Once enough projects use and prove the efficacy of the specification, Mogawer can assist in developing a spec that allows for the use of whatever rejuvenator is in a producer’s arsenal.
In this case, the restorative agent P.J. Keating added to the mix was Delta S, which is a small-molecule dispersion inside a plant-based oil. The plant-based oil penetrates the asphalt binder and carries the rejuvenator in to soften the binder.
The mix had coarse aggregate, fine aggregate, mineral filler as needed and a PG 64-28 asphalt binder.
Mill and “thin” fill
P.J. Keating began milling the night of Oct. 19, removing pavement at a depth of 1 in., and completed paving the night of Oct. 23. Rain delays affected the P.J. Keating crew’s schedule, but all work took place between the hours of 8 p.m. and 5 a.m., which Berkley said allowed them to shut down lanes and detour traffic for completely closed night work zones.
“We did everything from soup to nuts,” Berkley said. “From estimating and traffic control to milling. We manufactured and placed the hot mix.”
The crew used a Wirtgen W 2100 mill with Niveltronic grade control and a Roadtec RX-10 cold planer with auxiliary compressor. After milling, the crew swept the surface with an Elgin broom with a Bearcat vacuum and a Rosco RB-48 self-propelled broom to prepare the surface for tack. They distributed a trackless tack RS-1H asphalt emulsion at a rate of 0.06 gal per sq yd with an Etnyre 2,000-gal Blacktopper asphalt distributor immediately ahead of the Cat paver. They used a Volvo DD1108 double-drum steel roller and Hypac C350D steel roller in static mode to achieve compaction.
From curb to curb, the paving widths varied from 22 ft to 40 ft, depending on the section of the project. From start to finish, the P.J. Keating crew gave attention to quality control. “This project was a new mix for us with 40% RAP,” Berkley explained. “At only 1 in., it didn’t leave any room for practice. The foreman, Roland Couto, dealt with that very well. The mix is difficult to work by hand, because of the amount of RAP, and Couto and his crew handled it very well, especially for the first go around with it.”
Although P.J. Keating is capable of producing warm-mix asphalt (WMA) mixes, they kept the variables on this project to a minimum. They went with all conventional temperatures and conventional paving practices. The use of a high RAP percentage took the mill-and-fill pavement preservation technique to another sustainable level for MassDOT District 3. The use of a plant-based rejuvenator enhanced their green even more.