Although the stresses and demands on roads and airport runways are very different, the methods used to analyze the asphalt they are made from are very similar. Although several dedicated methods exist to analyze road asphalt, no standardized methods exist for airport pavements. Researchers in The Grainger College of Engineering, University of Illinois Urbana-Champaign have now developed a practical, simpler test and analysis for airport asphalt pavements.

“The Federal Aviation Administration wanted something specific for airport asphalt,” said John Lambros, professor in the Department of Aerospace Engineering. “Roads have continuous use and typically along the same path. In contrast, airport asphalt doesn’t have as much traffic, but the loads it feels can be much, much higher and of varying configurations.”

Lambros said one problem he and Imad L. Al-Qadi, who is a professor in the Department of Civil and Environmental Engineering, addressed was the complexity of the current strength test. One of the outcomes from their work is the Disc with O-Notch for Uniaxial Testing, or DONUT, and a cracking index.

He explained that some traditional tests just compress, or squeeze, the sample. Others require several cuts to the asphalt samples, which are slices taken from a 150-millimeter diameter cylinder. The cut or notch starts the crack in a performance test.

“Making cuts into asphalt with a saw is difficult to do in the field. A drill is much simpler to use. So, we experimented with putting a hole through the center of the round asphalt slice to create a stress concentration. All you need is a drill for this, but it still gives you a concentrated damage initiator.”

After developing the new method, they tested it on nine asphalt concrete samples that had been mixed and compacted in a lab or in a factory setting.

The final validation used multiple field cores from test lanes at the FAA accelerated pavement testing facility in New Jersey. The DONUT index successfully distinguished lanes that exhibited cracking under accelerated pavement testing from those that didn’t crack.
https://uofi.box.com/s/6cwtzwamu004rkno2jayee3kjv7c6tjq
A video shows how the circular hole that is introduced in the specimen sucks up all the energy provided during the test to lead to fracture. It shows the evolution of strains as the DONUT is compressed and more and more with progression of the test.

Al-Qadi’s Ph.D. student, Akash Bajaj, also collected nine asphalt concrete mixes from pavements across the United States to evaluate. One was a highway mix, and eight others were airfield mixes sampled from seven airports: Newark Liberty International Airport, Teterboro Airport, Tampa International Airport, Reno Stead Airport, Sacramento International Airport, San Francisco International Airport and Chicago O’Hare International Airport. In addition, in-field cores were collected from University of Illinois–Willard Airport, Kansas City International Airport, and the National Airport Pavement and Materials Research Center.

“It’s important that the samples were from all around the country because airport pavements there undergo different temperature, humidity, and sizes and amount of aircraft traffic,” Lambros said.

Lambros said he and Al-Qadi have worked together for more than a decade developing protocols for asphalt testing for roadways. Their flexibility index for roads was adopted as the standard by the State of Illinois.

“Along with other findings, we submitted the final report proposal for the DONUT method and the index to the FAA. This is a new method that is scientifically and technically sound. Now, it’s up to them whether to adopt it.

“But this is the original proposal. If this were to become standardized, one would need to do more studies before we can say we’re 100% certain that this is a good metric. It is very promising because the result from the DONUT index were similar to the flexibility index we developed for road asphalt tests that the State of Illinois adopted.”

This project was a collaboration with the University of New Hampshire and Arizona State University and The Heritage Research Group.

The study, “A novel fracture-inspired cored Brazil disc test to characterize cracking potential for asphalt concrete,” by Akash Bajaj and Imad L. Al-Qadi from the Illinois Center for Transportation, Department of Civil and Environmental Engineering, and John Lambros, is published in the International Journal of Pavement Engineering. DOI:10.1080/10298436.2026.2670562