When Maryland first began pioneering the use of Stone Mastic Asphalt (SMA) in the United States nearly two decades ago, Maryland Paving General Manager and Executive Vice President Jeff Graf was there.

“Maryland was a leading state in the use of SMA,” he said. “And Maryland Paving was among the first companies to be paving SMA in the state.”

It was only fitting when Maryland wanted to test out fiberless, warm mix SMA that Maryland Paving would be involved.

Maryland Paving has been supplying hot mix asphalt since 1964. The company operates six asphalt plants across the state of Maryland.

Since the Timonium, Maryland-based company first started making SMA in the early ’90s, they’ve produced between 750,000 and one million tons of SMA total. And, in late May 2017, Maryland Paving embarked on a new SMA concept.

It performed a pilot project incorporating warm mix SMA with Evotherm on Interstate 195, the main connection between Baltimore/Washington Thurgood Marshall International Airport and highways like I-95, I-97 and Maryland 295 that lead to the cities of Baltimore, Annapolis and Washington, D.C.

Step Up Your SMA Game

It’s no surprise that SMA mixes require a higher AC content than traditional mixes.

“SMA is a stone-on-stone contact mix, so it supports extreme traffic loads,” Graf said. “Unlike a conventional dense graded mix that’s evenly graded coarse to fines, SMA is a gap-graded mix with a lot of stone. We need a high asphalt cement content and filler to make a thick paste to hold the aggregate together.”

“With that, you have to stabilize that during the mixing, hauling and laydown process until you get it in place and compact it,” Graf said. “And we used cellulose fiber to do that.”

However, it’s also possible to employ warm mix processes to stabilize SMA’s higher AC content while still achieving compaction. In an effort to go green, save money and eliminate the need for fiber at the plant, Maryland Paving was eager to test out the process.

Don’t Fix What Isn’t Broken

Despite the potential of warm mix SMA, it took some time for the state to be ready to try out the new process.

“We’ve had some very, very good performing SMA pavements in Maryland,” Graf said. “When something works that well, you’re more reluctant to make a change. But if we can do a better job protecting the environment while doing the same class of work, then it’s time for a change to happen.”

When Maryland Paving began to see the success of warm mix processes for dense graded mixes, they decided to start “dipping our toe in the water with specialty mixes, including SMA,” Graf said.

Two years prior to its pilot project on I-195, Maryland Paving used warm mix SMA on a portion of Interstate 83 into Baltimore. The crew paved the travel lane with conventional fiber-laced SMA and paved the shoulders with warm mix technology.

Working alongside the State Highway Administration, the crews paved the warm mix SMA at multiple temperatures, monitoring the compaction and workability of the mix at each temperature. After the project was paved, SHA and Maryland Paving continued to monitor the shoulder’s performance.

“We haven’t seen any differences between the shoulder and the roadway,” Graf said. “So we took that experiment and said, ‘Let’s move it onto a more traveled roadway’.”

The I-195 project turned out to be a perfect candidate for the next pilot project.

“We didn’t want to test it out on a large scale project, so we were really fortunate to win a 6,000-ton project on a major artery to BWI Airport,” Graf said. Plus, the 6000-ton project was more of an improvement project–there was no milling on the existing surface–it was a straight 2-inch overlay.

“It was a relatively small project when you look at interstate work, but it was an ideal project for this investigation,” Graf said. “It was the perfect opportunity to see what a heavy traffic load would do to this mix.”

Maryland Paving met with SHA to ask to test out the new method on the project.

“We looked at a lot of research out of Rutgers University to validate what we wanted to do, and SHA thought it would be worthwhile to investigate its performance,” Graf said.

Maryland Paving Goes Fiber-Free

In addition the environmental and cost-saving benefits of running warm mix SMA, the process also meant that Maryland Paving could cut the extra step of adding fiber to the mix. Removing the handling and storage of fiber also saved the company money.

To produce fiberless SMA, Maryland Paving added Evotherm, a warm mix additive from Ingevity.

“We’ve done other experimental projects with Ingevity, and we’ve had good results working with them,” Graf said. “They have great technical staff. And with our history with them, we were convinced that they were the right partner to go into this project with.”

Maryland Paving purchased Evotherm that had been pre-blended at the refinery in an effort to make the production process as normal as possible.

“Anything we can do to make a process run our plant more like normal, we’ll do because it’s what our guys are used to,” Graf said. “And I had a confidence that the refinery had the equipment to better blend it and they were certifying that end-result product.”

Using fiberless SMA also allowed Maryland Paving to eliminate a second unnecessary job at the plant: cleaning out truck bodies. Due to the high AC content of SMA mixes, they’re already a bit harder to haul and more likely to stick to truck beds.

“We were totally surprised by how clean the truck beds came out time and time again,” Graf said.

Practice Before You Pave

Of course, to prepare for the new mix, Maryland Paving performed many test trials, including its I-83 project.

“We did a lot of lab work before taking our results to the plant, and we did some test runs with smaller tonnages,” Graf said. “We at the plant had a high level of confidence that we were heading in the right direction and everything was going to work. We’ve had a lot of experience with SMA, so we knew right away if something wasn’t doing what it should.”

At the plant, Maryland Paving did a normal battery of tests, including asphalt extraction, volumetrics testing, gradation, wash sieve analysis and drain down tests on all quality control sampling.

“In Maryland, we have a PWSL spec and we do field sampling of the mix, where the split sample comes back to the plant and goes to the agency,” Graf said. “We did multiple samples on this, some of which went to Evotherm, and everyone did quite a battery of tests on them.”

On the first night of production, on the 195 project representatives from Evotherm and SHA personnel were at Maryland Paving’s plant as the new mix was made.

After the test sections were done, Maryland Paving’s plant was proven right: “It was as good if not better than conventional SMAs we’d run,” Graf said. He added that one of the most significant challenges in SMA production is avoiding areas heavy on asphalt. “But, we had virtually no heavy spots, whatsoever.”

Although Maryland Paving’s plant crew knows how to mix SMA very well, mixing it at a lower temperature did require some adjustment, not only of the mixing temperature but also of the crew’s mindset.

“It’s hard to see 300 on that heat gun and not think we’re going to have big mess, and instead realize that it’s possible to work it and compact it at that temperature,” Graf said.

When Maryland Paving runs standard SMA with fiber and PG76-22, they would mix it at temperatures between 330 and 340 degrees. With the fiberless mix, Graf estimated they were running at temperatures between 300 and 305.

Even on this small pilot project, Graf estimated that the company saved between 5 and 8 percent on fuel producing the fiberless SMA compared to traditional SMA.

“When all is said and done, the cost of Evotherm additive is offset by the cost of the fiber, and then we also save on energy costs and we reduce our environmental impact,” Graf said.

But, using Evotherm did present another challenge: if they used Evotherm in the SMA mix, the mix would only require 6.1 percent liquid AC. However, the state was requiring that SMA mixes be at least 6.5 percent liquid AC.

“We had to get their blessing, because it was going outside of their comfort zone,” Graf said. “But with the help of Ingevity and with the research from Rutgers, they were comfortable enough to try out this investigative project.”

The Proof is in the Pavement

Despite the benefits of this new process, Maryland Paving and Maryland DOT are waiting to see how the new mix design performs over time.

“As time goes by, we’ll re-core it to see if there’ll be increased density,” Graf said. SHA will be performing a whole gamut of tests to evaluate the cores from the pilot project against it performance testing criteria, he adds.

But so far, the results look good.

“Performance-wise, it’s doing just as well as conventional SMA,” Graf said. Maryland Paving collected 10 cores for every 1,000 tons of mix and when all the data was collected, the average density was 95.3 percent. “Right in the middle of the spec we wanted it to be.”

“I’m sure that after some more studying and data collection that Maryland will look into expanding this on a case-by-case basis,” Graf said. “We’re watching its performance before jumping in and going hog-wild into potentially ruining something that’s worked so well in the past.”

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“At Maryland Paving, and in the industry in general, we do anything we can to be more green,” Graf said. That’s why the company participates in NAPA’s Emerald Eco-Label Program, which aims to provide credible environmental data to customers, including engineers, architects, users and producers.

Through NAPA’s Emerald Eco-Label program, plants can create plant-specific and mix-specific EPDs, or Environmental Product Declarations. EPDs are a standardized way to quantify how sustainable a product is through a green construction rating system, like LEED, Greenroads, or INVEST initiatives.

On many products, having an EPD has become crucial and even required on some projects, like LEED version 4 projects.

For example, if a project involves an asphalt parking lot, the project can get credits toward its LEED status if the asphalt contractor doing the work can prove the mix’s environmental sustainability.

NAPA’s Emerald Eco-Label program comes in handy. A plant can input data about each asphalt mix and generate an EPD to share with their customers to help them earn green rating systems credits, like in the example above, that take into consideration every step of the asphalt production process, including materials, transportation and production.

The program can also help manufacturers identify inefficiencies in fuel use or material selection.

“We were initially put out there as a ‘bad’ industry and we’ve had to prove that we’re actually a clean industry and a green industry,” Graf said. “Anything we can do to promote that, we should.”

Maryland Paving’s six asphalt plants are also recognized as part of NAPA’s Diamond Achievement program. The program pushes members to continuously improve and begins with a self-assessment of six aspects of plant and site operations, including appearance, operations, environmental practices, safety, permitting and regulatory compliance, and community relations. The commendation is then verified by an outside third party.

“It was really good to go through the process,” Graf said. “We thought we were doing everything right, we found things we could have been doing better. If you participate with integrity, the programs will really help you through things you can improve on.”