Editor’s note: This article is adapted from a version published Sept. 17, 2026, by the Center for Transportation Studies at the University of Minnesota.
Old asphalt roads are continually being torn out and replaced, and recycling that asphalt could conserve natural resources while stretching limited road budgets. However, local agencies have traditionally capped recycled content at 25 percent, fearing that higher percentages will make pavements brittle, hard to compact, and prone to winter cracking. In a new study, CTS scholar and Professor Jia-Liang Le of the UMN’s Department of Civil, Environmental, and Geo- Engineering examined whether mixes could safely use up to 50 percent recycled content without sacrificing pavement life. The project was sponsored by the Minnesota Department of Transportation (MnDOT) and the Minnesota Local Road Research Board.
“Finding ways to use recycled asphalt effectively is crucial because it directly reduces material costs, cuts environmental impacts, and decreases demand for virgin aggregates and asphalt binders,” Le says. “Our goal with this study was to build the foundational understanding needed to design durable, high-recycled pavements for real-world roads.”
To best simulate real-world conditions, researchers sourced recycled asphalt from a local contractor and collected high-grade pavement millings from MnDOT’s MnROAD cold-weather test track. In the lab, they used these samples to create test batches containing 25, 40, and 50 percent recycled asphalt. Feeding these samples into the lab’s gyratory compactor mimicked the heavy rolling machinery used on real construction sites, and the researchers counted how many gyrations it took to pack the material into a suitably dense, solid block. Surprisingly, they found that the 50 percent recycled mix compacted with fewer squeezes than the 25 percent or 40 percent mixes.
“We found that when thoroughly heated, the old asphalt binder would be fully activated and act as a lubricant during compaction,” Le says. “In fact, assuming the old binder won’t activate can actually backfire — if too much fresh oil is added, it can cause the pavement to bleed excess oil at the surface.”
A second set of lab tests examined the recycled asphalt’s cold-weather performance. Researchers chilled the asphalt blocks to freezing temperatures and measured the crack resistance and overall strength using bending experiments. Again, the higher-recycled-content pavements delivered, matching the crack resistance of virgin asphalt while showing even higher overall strength.
Finally, the research team explored how additives might improve the recycled asphalt mixes. They stirred in a small amount of microscopic carbon flakes known as graphite nanoplatelets to determine if they could make higher-recycled mixes easier to compact at lower temperatures (dropping from 135°C down to 115°C and 95°C). The findings suggest that adding nanoplatelets could improve compaction and may allow for some reduction in heating and compaction temperatures.
The next step in this research is bridging the gap to real-world roads. While the laboratory ovens allow enough time for old asphalt binders to fully warm up and melt, commercial mixing processes happen much more quickly. As a result, replicating these results on actual roads may be challenging and costly.
“These results provide promising evidence for potentially increasing the use of recycled asphalt pavement materials, but more work is needed to implement it for use in the field,” says Eddie Johnson, the project’s technical liaison and researcher with the MnDOT Office of Materials and Road Research.
Future research will focus on calculating how much old binder reactivates across different recycled sources, refining heating temperatures and mixing times, and testing whether the benefits seen in the lab in terms of compatibility and temperatures hold up during actual asphalt plant production. If these field trials succeed, local agencies will have a practical blueprint for increasing recycled asphalt content without sacrificing pavement quality.