NCAT Round 8 Findings

Additives, BMD research at Auburn show progress of industry

By Ty Johnson
Editorial Director

The heavily laden tractor-trailers at the National Center for Asphalt Technology (NCAT) at Auburn University eclipsed the 11-million-mile mark during the Test Track’s eighth research cycle, which ended in spring 2024.

NCAT Director Randy West said the trucks, which every two years simulate 10 million equivalent single axle loads (ESALs) of traffic on test sections of pavement, have now logged enough mileage to have circled the earth 440 times since they began cruising in 2000.

More than two decades later, NCAT’s world renown asphalt pavement proving grounds are still the chosen laboratory for innovative public and private sector stakeholders in the industry aiming to stress test their asphalt mixes, additives, and more.

Asphalt Pavement Alliance (APA) Technical Director Buzz Powell said the conference, which this year attracted more than 300 participants from across the United States and beyond, is best known for its focus on practical research and real-world applications.

“These sessions provide valuable insights into how the industry can adapt to emerging challenges while providing safe and sustainable mobility at the lowest possible life cycle cost,” Powell said, adding that the importance of the event is evidenced by the turnout from industry experts, researchers, and stakeholders eager to discuss the latest advancements in asphalt pavement technology.

Powell, who spent more than two decades managing the Test Track before joining APA, noted the value of networking opportunities at the triennial conference.

“Participants discussed future research needs and potential partnerships, reinforcing the importance of collaboration between academia, industry, and government agencies,” he said. “Conference attendees were particularly interested in implementable findings from the latest test cycle, which emphasized sustainable practices related to mix/materials, structural pavement design, and pavement preservation. The Test Track Conference continues to be a pivotal event in the asphalt industry, driving innovation and fostering a deeper understanding of pavement technologies that benefit agencies throughout the U.S.”

Research on the eighth cycle began in 2021 and wrapped up when NCAT faculty shared their findings during the Test Track Conference in May 2024.

Of particular note were results from a group of five test sections known as the Additive Group, which compared wet and dry processes for adding recycled ground tire rubber and post-consumer recycled plastic, as well as a section containing aramid fibers. A sixth section contained a control mix.

West said agencies expressed interest in a comprehensive evaluation of the additives available on the market. Once the Federal Highway Administration (FHWA) and six states got on board, the project was born, although the selection of the additives was a collective decision based on models from the lab.

“We went to the laboratory, and we asked suppliers of these types of rubber and plastics additives to actually fund what we call a Phase One experiment.” West explained. “We shared that information back to the states that agreed to sponsor the experiment, and then they said these are the five additives that we want to put on the Test Track, plus the control.”

The NCAT test sections were built in fall 2021 and then the trafficking began. A year later, West said they repeated the process in Minnesota for MnRoad, the state DOT’s cold weather pavement testing facility near Albertville.

“Recycled plastics have been in the news a lot for the last five years,” West said in introducing the plastics portion of the experiment. “There are test sections being built around the country, you know, but we have a nice experiment here as well.”

NCAT Assistant Director & Associate Research Professor Fan Yin continued the Additive Group analysis by explaining that postconsumer recycled linear low-density polyethylene (LLDPE) was selected for its relatively low melting temperature in relation to other plastics. Because LLDPE melts at similar temperatures to asphalt mix production, it can easily be added to the mix through either dry or wet methods.

BALANCED MIX DESIGN

During discussion on a balanced mix design (BMD) and RAP test section put forth by the Oklahoma DOT, NCAT Assistant Director & Associate Research Professor Fan Yin left attendees with a composite takeaway from several test section results.

“In order to fully take advantage of the innovation benefits of BMD, we need to relax those volumetric requirements,” he said, adding that NCAT collaborated with NAPA to create a Balanced Mix Design Resource Guide. Access NAPA’s BMD Resource Guide.

“For the wet process, basically what we did was to add that plastic into the asphalt binder. On the other side, when we talk about the dry process, we add the additive into the mixture,” Yin said, describing the difference in introducing the material.

Yin said cracking began appearing on the dry method plastic test section shortly after 5 million ESALs, although it wasn’t extensive. The wet method and control showed no cracking, and none of the plastics sections showed rutting, he explained, although he cautioned that plastics can differ similar to asphalt binder, depending on sourcing and other considerations. “I want the data to speak for itself.

BROWSE NCAT FINDINGS

Over the summer, crews prepared the 1.7-mile oval Test Track for its ninth research cycle, which will include continued trafficking on a handful of test sections left in place from the eighth research cycle. Learn more about NCAT’s findings from its 2021-2024 experiments.

Right now, the two technologies we looked at did not seem to show the performance-enhancing benefit, but those are only applicable to those materials,” Yin said. “We want to be sustainable, but at the same time we don’t want to sacrifice performance. Sustainability and performance have to go hand-in-hand.”

IMPACT OF LABORATORY RAP PREHEATING ON THE BMD PROPERTIES OF ASPHALT MIXTURES

This research reveals the variations in RAP preheating methods among 39 state DOTs and 46 contractors, which can lead to differences in asphalt mixture properties. It emphasizes that different preheating procedures affect recycled binder availability and aging, which in turn influences the volumetric and mechanical properties of asphalt mixtures. Access IS-146.

NCAT Associate Research Professor & Lead Researcher Carolina Rodezno said the rubber modified test sections returned stable international roughness index (IRI) results and showed enhanced fatigue resistance compared to the control. The dry rubber section cracked earlier and more than the wet rubber section.

Assistant Director and Research Professor Nam Tran explained the results from the aramid fibers section. He held a sample of the additive, which he described as a toothpick-sized bundle of 10,000 fibers held together with wax. He said the fiber test section didn’t display the same cracking resistance as the control section.

Assistant Professor Ben Bowers discussed the life-cycle assessment (LCA) data the Additive Group experiment produced, noting how environmental product declarations (EPDs) fit into the modeling used to predict GHG emissions.

“What we’ve learned so far is that data is king: You need good data,” Bowers said. “No. 2 is you need to be mindful before you start making policy-type decisions that you need a good chunk of data so be mindful when you’re looking through that data. Think about the long-term effects of that.”

TEACHING TEACHERS

Next June, NCAT will once again bring college and university faculty members from across the country to Auburn for its Professor Training Course where participants learn directly from the NCAT researchers and professors who examine the Test Track sections each week.

The five-day course, set for June 23-27, 2025, aims to increase the pool of qualified workers in the asphalt pavement industry by sharing educational materials with civil engineering faculty, providing clear and up-to-date lecture and laboratory material for the asphalt portion of an undergraduate civil engineering materials course. Participants will learn how to teach on topics like asphalt binder, aggregates, pavement design, asphalt mixtures, and road construction.

Attendees come away with the lecture, homework, laboratory, and exam materials necessary to teach asphalt technology to undergraduates.

Faculty participants living and working in the United States as a professor or instructor at a university, college, or trade school may be eligible for a stipend to help with the costs of attending the course. Instructors from outside the United States are not eligible for the stipend but can attend for a $2,500 fee.