Asphalt is one of transportation's workhorse materials, but it faces an uncertain future in a world being remade by carbon emissions and climate change.

The efforts of DOTs and the FHWA have involved major university studies, and highways used as laboratories for materials experiments and cracking and fitness tests. Yet dissent exists among stakeholders regarding the results. 

The tests, in this current environment, could change as much as the asphalt mixtures.

The laudable goal of keeping materials such as shingles, old rubber tires and plastics out of landfills—or worse, out of incinerators that cause air pollution—exists in an environment where elected officials have pushed for more adoption of newer, greener materials by contractors.

They also simultaneously demand better performance from the pavement and seek contributions and support from several links in the agency and industry supply chain that's managing new materials research. Experts from Illinois, Indiana, Missouri and California all stressed that trying to study and measure the performance of a new road material across different climates and conditions is difficult and challenging.

The engineers and materials scientists tasked with finding the right application for cutting asphalt's carbon footprint and recycling everything from tire rubber to asphalt shingles in the process have had to deal with demands from several constituencies throughout 30 years of experiments in the field.

The cost of funding research and shepherding new materials through the regulatory process was often paid by trade groups eager to see their materials specified.

Since its earliest days, the Illinois Dept. of Transportation was part of the experiment when it started using old, recycled asphalt and concrete decades ago. It added recycled rubber, mostly from tires, to the asphalt mix in the 1990s and, in 2010, decided to allow recycled roof shingles.

By 2017, 1,451,675 tons of reclaimed and recycled materials had been used in Illinois highway projects. In 2018, the last year figures from the department are available from, that figure dropped 31% to 995,343 tons.

Earlier this month, former State Senator Martin Sandoval (D) agreed to a guilty plea for bribery and tax evasion in federal court in Chicago. The case shined a light on his relationship with a recycled asphalt shingle provider and measures Sandoval took as the Illinois Senate's transportation committee chairman, including sponsoring bills such as a 2011 law, House Bill 1326, that allowed recycling facilities to count each ton of asphalt they recycle as two tons toward their EPA-mandated recycling quota.

He also sponsored legislation requiring more recycled material in road projects, one that regulated who could operate a shingle recycler, and one that forbade dumping used shingles anywhere within 25 miles of a recycling facility.

The RAS Experiment

When dealing with new materials like rubber, reclaimed asphalt shingles (known as RAS in the industry) or plastics, issues such as cracking were studied after installation and consensus on the cause of it was hard to come by.

Most of the experiments on RAS and other new asphalt mixtures occur on existing roads that are exposed to real-time factors, such as use by millions of motorists that drive on them, severe weather and inconsistent application over existing pavement or other maintenance factors.

The situation makes results across states and climates anything but consistent. Shoulder mixes were used on some Illinois highways because those are the areas that get less use but still need to perform. One industry expert likened the process of identifying how to use additives such as rubber, RAS and reclaimed asphalt pavement (RAP) as baking a cake and needing to get it right so it would rise.

"Most of my research is pushing the envelope on recycling in asphalt," says William Buttlar, Glen Barton Chair in Flexible Pavements in the Civil and Environmental Engineering Dept. at the University of Missouri, Columbia. He oversees the Missouri Asphalt Pavement and Innovation Lab that is funded, in part, by the Missouri Asphalt Pavement Association.

"The latest and greatest is waste plastic and finding a use for waste plastic in asphalt. But roofing shingles were kind of in the middle of this story," he says. "So, at the beginning of the story it was just taking old asphalt, melting it and putting it in new asphalt."

Several transportation engineering consultants interviewed for this story said the main difficulty that arose from using RAS as a material in road asphalt, is that while the asphalt in RAS has about five times the asphalt content of traditionally created road asphalt—making it a rich source—that asphalt also is very concentrated, making it stiffer than asphalt created just to be used in roads.

Engineers and contractors have to balance out the fact that roofing asphalt is stiff and, while it is abundant in shingles, it gets even stiffer as it ages on a roof. By 2013, cracks began to show in stretches of Illinois roads that had used the product such as Chicago's Michigan Avenue.

"If you don't account for its stiffness, you end up with a mix that's too stiff and also brittle," Buttlar says. "It's not really the rigidity or stiffness that gets you. It's the brittleness, the ability for cracks to move through it. So, then you have to find a way to mitigate that brittleness in your design, and that's just on the mixture side."

In a 2015 study Buttlar wrote that was commissioned by an industry trade group, he said the RAS material had not caused the road cracking problems observed in Illinois roads; these were caused instead mainly by underlying pavement that had been left there, and a thin layer of the new asphalt mix applied on top.

The study,  "Stability with Crack-Resistance in Modern Mixes: Performance-Based Mix Design," recommended that relaxing some testing parameters in light of performance specifications would allow for innovation, cost savings and enhanced sustainability.

"What I saw in Chicago is that they almost never build brand new roads," Buttlar says. "They always are just laying set amounts of new asphalt on a relatively old and thick pavement that already is just loaded with cracks and joints and other places where the pavement can move around."

Road Tests

Previously, the most comprehensive study of RAP in highway projects was "Performance of Recycled Asphalt Shingles in Hot Mix Asphalt," a 2013 investigation led by Christopher Williams, associate professor of civil, construction and environmental engineering at Iowa State University. It was sponsored by the FHWA and its Transportation Pooled Fund partners, DOTs of California, Colorado, Illinois, Indiana, Iowa, Minnesota and Wisconsin with data from projects in every state but California.

Researchers studied everything from mix design to cracking and recommended a 14% asphalt binder replacement when using RAS and a 20% binder replacement when using RAP.

With evidence from a test in Wisconsin, the study also said that engineers and contractors need to use softer grades of base liquid in the asphalt mix with RAS.

In light of this, many Illinois road projects using RAS switched in 2013 to PG 58-28, a light, performance-graded asphalt base liquid derived from specially selected crude oils with a controlled refining process. The product became a standard for use with RAS, RAP and other recycled materials in road projects according to several highway consultants interviewed.

The problem with buying more costly oils for asphalt is that many contractors were turning to RAS, RAP and other recycled products in hope of saving money, not spending more. Many simply did not want to buy extra tanks for holding and mixing expensive base liquid like PG-58-28.

Oil refineries try to reserve their best light crude for high-demand products such as jet fuel and gasoline, meaning that that oil used to create asphalt products such as PG 58-28 does not always meet the needs of road construction. Still, if contractors wanted to take advantage of the highest asphalt binder replacement, they would have to buy a more expensive, lighter-grade product like PG 58-28.

In January 2014, IDOT sent a memo to all regional engineers requiring higher levels of asphalt binder replacement (ABR). What previously made sense for many contractors from a sustainability standpoint for many projects no longer made sense from an economic standpoint.

In 2017, the FHWA made a further study of the issue that found projects that used the highest concentration of recycled shingles scored half as well as those using less or none at all on roads that were less than three years old.

The agency also said that "there is no widely accepted method to determine the blending of new and reclaimed binder [in asphalt]."

"The amount of blending among RAP and virgin binders is often assumed to be 100%," the technical brief said. "But for RAS mixtures, it is assumed that only partial blending occurs. To maintain similar or better performance, the virgin binder content of the mixture may need to be increased to account for this lack of blending and ensure mixture durability."

The technical brief found, however, that, "when properly engineered, produced, and constructed," RAS materials "can provide comparable levels of service as asphalt mixtures with no reclaimed materials, referred to as virgin asphalt mixtures," while noting that "care must be taken during design, production, and construction to ensure proper performance."

The study came up with a new test to measure the likelihood of pavement cracking called the Illinois Flexibility Index Test. It differs from the disk-shaped compact tension test previously used by the state DOT and others in the U.S. The disk-shaped compact tension test is one of the few tests of asphalt cracking that has an ASTM designation.

Officials of IDOT, who were contacted for this story, chose not to comment.

Richard Willis, vice president for engineering, research and technology at the National Asphalt Pavement Association, points out that the group last year issued a second edition of its "Guidelines for the Use of Reclaimed Asphalt Shingles in Asphalt Pavements," which highlights areas for caution and care in material handling and mix design.

“In general, it has been found both in laboratory and field experience that RAS can be used successfully in new asphalt pavement mixtures, but care and attention are required,” he says.

Since 2009, NAPA has worked with FHWA to track the use of recycled materials, including RAS, in asphalt pavement mixtures.

“Overall there was generally steady growth in the use of RAS through 2015 but its use declined after that, falling from a high of 1.964 million tons in 2014 to 944,000 tons,” says Willis. “During 2018, we saw a small increase in the use of RAS (1.053 million tons). We are currently surveying the industry about its use of recycled materials during the 2019 construction season, so we should know in a few months if 2017 marked a bottoming out of the decline in RAS use.”

Going Forward

In the decade or so during which agencies have tried RAS, “some states and contractors have used [it] well in small quantities, less than 5% by weight, when the material is very controlled,” Willis adds. “This includes knowing the asphalt content and ensuring that the grind of the shingle is fine and consistent. When the grind is coarse and inconsistent, or when people have tried to push the limits of RAS use, performance has been problematic.”

The latest national research on RAS was published by the National Cooperative Highway Research Program in its "Report 752." It followed some field projects and tracked early performance. The report also states that long-term performance needs to be tracked as well. If mixtures with RAS are shown to perform well, benefits would be reduced costs and improved sustainability through the reduced use of virgin materials.

Russell Snyder, executive director of the California Asphalt Pavement Association, says the group has engaged with the California Dept. of Transportation (Caltrans) and the asphalt industry to assess the FHWA report. There is also a Caltrans task force discussing specs, tests and emerging technologies, with a subcommittee focused on RAS, he adds.

Caltrans and other public agencies use RAP, but RAS is not yet a staple, says Brandon Milar, association director of technical services. “[California public agencies] typically do not allow RAS right now,” he says.

The subcommittee is addressing concerns raised in Indiana, Illinois and elsewhere to fully understand what happens when RAS levels are increased in asphalt, says Snyder.  

“There is a high degree of interest, but Caltrans engineers and our engineers want to make sure we understand the implications for RAS in asphalt mixes and will we get the performance we want,” he says. “The state is so massively diverse, with nine climate zones and different sources for rock, binder, and so on. The complexity of what we’re dealing with includes how to apply engineering principles across the state.”

The future of RAS, no doubt, will need to take into account those complexities wherever it is used.