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A matter of degrees: Stantec’s warm-mix asphalt research paves the way to landing safe—and sustainable—airfield runway pavement

Part seven of our 10-part Stantec R&D Fund 10th Anniversary Series dives into the sticky world of asphalt airfields

Airports and asphalt, like the Logan International Airport in Boston, go hand in hand. Stantec specialist Dave Dargie, his team, the Massachusetts Port Authority, and others spent nearly a decade studying how to make the use of asphalt at airports cooler and greener. (Photo courtesy Massachusetts Port Authority.)

The only time most of us think about the asphalt on our airfields—if we ever think of it at all—is when the pilot tells us to buckle up and “prepare for landing.” And if we are thinking of asphalt, then it’s usually to wonder one thing: will the pavement withstand my plane landing?

It’s a valid question. Airfield runways must endure the weight of hundreds of loaded airplanes taxiing, landing, and taking off from their surface every day, in all kinds of weather and extreme temperatures. Few materials better meet these conditions than asphalt, which is why many US runways are paved in the stuff.

Now consider the total area of registered US runways—roughly 11 billion square feet (1 billion square metres)[1]. That’s a lot of runway pavement—and a sizeable environmental footprint. Which is why the Federal Aviation Administration (FAA) and many airport authorities are concerned not only about asphalt pavement durability, but also its environmental impact and sustainability.

The heat of the matter
Airport Infrastructure sector leader, Dave Dargie (Scarborough, Maine), knows that a more sustainable asphalt is a matter of degrees. “The cooler the temperature at which you mix and place—or spread and compact—your asphalt, the less energy you use and the fewer volatile compounds you release into the environment,” Dave explains.

Related Item: Stantec’s Creativity & Innovation Program

What kind of savings are we talking about? A 2014 report released by the National Cooperative Highways Research Program (NCHRC)[2] concluded that using warm-mix asphalt (WMA), which is heated to significantly lower temperatures than traditional hot-mix asphalt (HMA), consumes about 22% less energy when you reduce mix temperature by 48 degrees Fahrenheit (27 degrees Celsius). The same report determined that these energy savings translate directly into emission reductions of carbon dioxide, a greenhouse gas.

There are other benefits to working with asphalt at lower temperatures, too. “Workers are exposed to lower emissions during construction, and are less likely to suffer burns during placement and handling,” Dave says. “And because WMA can be placed at much lower temperatures than HMA, we can haul WMA longer distances and extend our construction season into the cooler months. These are huge benefits in colder climates.”

Table I - Typical Production and Placement Temperatures:

PRODUCTION

PLACEMENT

HMA:  285 – 325˚ F

HMA:  250 – 300˚ F

WMA: 200 – 285˚ F

WMA: 175 – 250˚ F


Imagination takes flight
Back in 2006, before Dave and his team engaged in their research project, WMA was successfully being used to pave highway surfaces in Europe and some areas of the United States. But WMA had never been used at a US airport. The FAA, which typically funds up to 90% of US airport construction work, couldn’t approve funding for airport WMA until it had developed an approved specification.

Related Item: Airport design trends at The Airports Hub

To do that, the agency needed evidence that WMA was a safe and suitable pavement choice for airports. Specifically, they wanted proof that the lower temperatures and modifiers used in the production of WMA would not adversely affect pavement strength, durability, and longevity.

That year, Logan International Airport (Boston, Massachusetts) became the first airport in the United States to use WMA pavements—but only as a subsurface (binder course) layer. Could the material also be used to pave the top (wearing) layer of an active airstrip? That’s what Dave and his team, with help from Stantec’s Research & Development (now Greenlight) fund and in collaboration with the Massachusetts Port Authority (Massport) and industry experts throughout the country, set out to find out.

Mapping a flight plan
The research team wanted to answer three questions:

  1. Could local manufacturers consistently produce WMA to design specifications?
  2. Could contractors readily produce and place WMA as effectively as HMA?
  3. Would WMA perform as well as traditional HMA over an extended period?

In search of the answers, the team set out to find a suitable test location.

Luckily, in 2007, Logan International Airport was rehabilitating (removing and replacing) the asphalt pavement surface on Taxiway A, one of the airport’s busiest taxiways. With as many as 800 airplanes traversing Taxiway A each day, the taxiway seemed to be a perfect test ground to prove WMA’s resilience. So the research team and their partners set out to intersperse six WMA test strips in-between the standard HMA pavement during the rehabilitation project.

Several partners were key to the success of this research project. Massport paid for the WMA pavement, and provided the test site. The asphalt contractors agreed to be paid the HMA tonnage rate and absorb any cost differences associated with WMA. Dr. Rajib B. Mallick, a research scientist and professor at Worcester Polytechnic Institute, donated his time and laboratory for some preliminary testing. ATC Associates provided compliance testing. And Stantec R&D funding covered Dave and his team for their time, expenses, and laboratory materials testing services over the 10-year course of the R&D effort. 

A paraffin wax modifier was added during asphalt placement, making it easier to work with. Think molasses at room temperature versus molasses straight out of the fridge.

Prepare for take-off
“Test strips were laid on Halloween day 2007,” Dave says. “We conducted a couple trial runs to figure out how best to produce and place the asphalt.” Ultimately, the material proved relatively easy to work with once the contractor added the paraffin wax modifier, Sasobit. This modifier temporarily softens (reduces internal friction) the bituminous asphalt liquid so it flows and mixes more easily at the lower temperature. “Once mixed, we placed the WMA with the same effort we’d have expended placing HMA.” 

Samples were then taken from each strip. Tests showed that WMA pavement was successfully compacted to the same densities measured in the HMA strip. Also, “wheel-rutting” tests proved that, when WMA samples were subjected to 80,000 simulated plane passes, the negligible ruts that formed in the samples were the same depths as those typically found in HMA under the same conditions.

The test section was in place. All the research team could do was wait and see how the WMA would perform over time.

“We were confident the asphalt we laid was safe, but for how long? We couldn’t be certain what would happen in two, three, or 10 years—that’s why monitoring was an important part of our research.”

Related Item: Learn more about Stantec’s airport services

Paving the way forward
Three years later, the team couldn’t discern any visible differences in pavement aging or distress between the different mixes of WMA and adjacent HMA pavement surfaces during site visits.

Nor could they after five years of service. The researchers realized that they couldn’t adequately differentiate how the three different test mixes were aging using eyesight alone. They needed to measure the asphalts’ chemical composition to assess how each mix had changed over time.

But Taxiway ‘A’ had to stay operational. It could not be closed for an extended period to allow the team to take conventional pavement cores. Nor could the busy airfield risk damage from core sampling. So aviation technical lead for pavements Alex Bernier (New York, New York) sought the expertise of Dr. Iliya Yut of the University of Connecticut. Dr. Yut helped the team conduct on-site Fourier-Transform Infrared Spectroscopy (FTIR)analysis on multiple dime-sized samples taken across each of the test strips. FTIR uses light waves to develop a rapid chemical “finger print” of a test material to determine its composition. Results confirmed what the team had already inferred through visual observations. Statistically speaking, the performances of all three mixes and six sections were indistinguishable.

So far, so good.

By 2015, the WMA had been in place for eight years. The researchers returned once more to check the WMA strips and confirm that the WMA was still in the same condition as the adjacent HMA. The test strips were performing well, and likely would for some time to come.

The team was never able to fully confirm this last hypothesis, however. The WMA, along with the adjacent HMA, was replaced in 2016 as part of the airport’s routine maintenance cycle.

“The contractor who removed the test strips told us that each looked no better or worse than the adjacent pavements,” Dave says. “That made us feel pretty good about using WMA.”

This paving train placed WMA at Logan Airport in 2007. Nine years later, the WMA test strips were performing just as well as its adjacent HMA counterparts.

Spreading the savings
WMA has come a long way since Dave and his team first considered its use in major airports. In 2015, the National Asphalt Pavement Association reported that WMA was used to pave two-thirds of asphalt roadways in the United States. Dave and Alex see more airfields going the way of highways, especially since the FAA released its Warm-Mix Asphalt for Airfield Pavements guidelines in 2014. Small airports, with aircraft loading under 60,000 pounds (27,000 kilograms), can use WMA provided they comply with the design specifications approved by their local department of transportation. Larger airports can also use WMA, with some modification to the existing FAA standard.

There’s still the matter of cost. “WMA is slightly more expensive than HMA,” Alex says. “But as the material becomes more popular, its price will come down and more people will use more of it.”

Luckily, Dave, Alex, and their colleagues will be there to help. “We've been using WMA in runway surfaces for several years now. It’s our pavement of choice, and we know it works,” Dave says.

Dave and his team’s experience has made Stantec a recognized industry leader in airport WMA applications. Stantec experts have presented their WMA research findings to Canadian airport authorities in Montreal, Quebec, as well as to the US Transportation Research Board in Washington, District of Columbia.

But what excites Dave and Alex the most is the effect this 10-year research project has had on their clients and communities. “Thanks to Stantec’s R&D funding and the support of our clients and partners, we helped make real and measurable change in the airport industry and environment,” Dave says. “We’ve shown airport clients that they can be more environmentally sustainable without adversely affecting their current operations—and that’s something to be proud of.”

About Dave
Dave is Stantec’s Aviation Infrastructure Sector lead. He has more than 30 years of airport experience and specializes in airfield design and construction administration activities, including construction phasing and implementation plans.

About Alex
Alex serves as the Northeastern region airport team’s technical lead for pavements. After completing his graduate studies in fracture mechanics and asphalt material characterization, Alex spent five years “airside” providing General, Commercial, and Department of Defense Airport clients with pavement design, evaluation, and construction inspection services.

About this article
Stantec is celebrating the 10th anniversary of our Research and Development (R&D) Fund—now called Greenlight. Through Greenlight, Stantec invests $2 million annually into our employees’ big ideas, with half the funds earmarked for scientific R&D initiatives. Greenlight is part of our Creativity & Innovation Program, which nurtures the efforts of our people to apply any idea that benefits us, our clients, or our communities, and enhances our reputation, competitive position, and ultimately our financial performance. In the coming months, we’ll be profiling 10 of our R&D grant recipients and their work, so check back often for more stories.

[1] As registered in the US Federal Aviation Administration (FAA) National Plan of Integrated Airport Systems

[2] NCHRC Report 779: Field Test of Warm Mix Asphalt, West, Randy; Rodezno, Carolina; Julian, Grant; Prowell, Brian; Frank, Bob; Osborn, Linda V; Kriech, Tony; Transportation Research Board, Washington D.C. , 2014

We’ve shown airport clients that they can be more environmentally sustainable without adversely affecting their current operations—and that’s something to be proud of.” ­– Dave Dargie

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