Highway pavements take flight for Illinois nonprimary airports

Illinois, a hub for air travel, has 107 airports — 90% of which are nonprimary, or serve fewer than 10,000 passengers per year.

Ensuring those airport pavements perform well while maintaining safety and cost-effectiveness is key.

Illinois Center for Transportation and Illinois Department of Transportation seek to do just that in a joint project, “R27-231: Evaluation of the Asphalt Mixture Design Framework for Airfield Pavements in Illinois.”

William Eves, IDOT’s Bureau Chief of Airport Engineering, leads the project with Uthman Mohamed Ali and Imad Al-Qadi, ICT’s senior research engineer and director, respectively.

The three wanted to transfer the technology of IDOT’s existing highway asphalt mixes for use in nonprimary airports — a process allowed for airports serving aircraft less than 60,000 pounds by the Federal Aviation Administration, which regulates requirements for federally funded airport projects.

“We could use IDOT Illinois materials that have been vetted by IDOT highways on our airports,” Eves said. “We can control that, and we know we’re going to get a good product.”

Airport pavements are currently modified from highway pavements to meet airports’ unique needs such as larger loads, less traffic volume and greater pressure from tires on pavement.

Here the researchers evaluated unmodified highway mixes, which may increase the efficiency of the mix-production process as well as lead to more sustainable construction practices.

“We were basically trying to see if we could get it ‘off the shelf,’ exactly the highway mix,” Mohamed Ali said. “By having it off the shelf, the contractors don’t have to go through any of the hoops of having it reapproved, and there’s a sustainability benefit, as highways use recycled material.”

Increasing the efficiency of producing mixes for airport pavements will reduce construction delays and drive down costs.

“Every time you switch a mix during the construction season at a plant, the plant has to shut down and recalibrate to get these mixes right,” Eves said. 

“If we could use the mix that’s in production all summer long, that’s a win for everybody: it’s a win for the plant, it’s a win for the scheduling, it’s a win for the price, and ultimately that should translate into lower costs and fewer delays,” he added.

Mohamed Ali and Al-Qadi’s team evaluated 18 mixes designed for IDOT highways and state airports as well as FAA airports.

To evaluate the mixes, they worked with three tests used in the approval process for highway mixes: Hamburg wheel track, Illinois flexibility index (developed by ICT) and tensile strength.

The three tests measure rutting (dents left in pavement by heavy loads), cracking potential and the maximum tensile stress a material can withstand, respectively.

Because airport pavements experience fewer load repetitions and are compacted to a higher density than highways, the research team designed all mixes to airport and highway pavement densities.

The research team determined that using highway mixes would provide stronger airfield pavements for nonprimary airports without sacrificing safety or flexibility.

“We found that airport pavement mixes didn’t perform well in the Hamburg test, which looks at heavy loading, but they did really well with cracking, which is from I-FIT,” Mohamed Ali said. “The highway pavement gives you the optimum somewhere in between, so you gain both.”

Pending FAA approval, IDOT aims to incorporate highway mixes into its nonprimary airport pavements as well as explore the use of recycled material.