An ICT team, led by director Imad Al-Qadi, has developed high-performing stone-mastic asphalt using materials from Illinois.

Stone-mastic asphalt is a highly resilient pavement mixture that requires strong, durable aggregate made from materials such as gravel or crushed stone, polymer-modified binder to bind the aggregate together and fiber.

Illinois, which does not generally produce the high-strength aggregates currently used in SMA, typically imports its SMA aggregate from nearby states, raising monetary and environmental costs from material transport and preventing more widespread use.

Al-Qadi’s team, in an Illinois Department of Transportation-sponsored project, sought to develop SMA using aggregate from Illinois while maintaining the durability of mixes with imported materials.

“SMA in its current form has proven that it can perform durably in Illinois,” said Brian Hill, IDOT’s Engineer of Hot-Mix Asphalt and Aggregate. “We thought there was potential for more locally available aggregate in SMA while maintaining durability in terms of cracking, rutting and moisture resistance.”

Al-Qadi’s team developed and evaluated SMA designs with Illinois aggregate at ICT’s lab, using typical SMA with imported aggregate as a reference. After 18 months of developing reliable designs, they measured the mixes’ ability to interlock the aggregate without damaging it as well as the mixes’ resistance to rutting and cracking.

A key focus of their mix design was to reduce the number of gyrations needed in a gyratory compactor, which simulates compaction during road construction. After completing the designs, they decreased the number of gyrations needed to achieve appropriate aggregate interlock from 80 to 50, reducing the possibility of damaging the aggregate.

To evaluate their real-world performance, Al-Qadi’s team selected six SMA designs, including two reference sections, for full-scale accelerated pavement testing. They instrumented the six test sections with 65 sensors, including pressure cells, strain gauges and thermocouples, to monitor the pavement’s response to axle loading and temperature.

They used ICT’s Illinois Accelerated Pavement Tester, which simulates long-term pavement loads over a few months, to evaluate the mixes’ responses under various axle loads, tire pressures, speeds and space between axles as well as under excessive loads up to 14,000 pounds.

The results showed no significant structural damage after I-APT testing. Any aggregate breakdown to SMA with local aggregate occurred during mix design and construction, highlighting the importance of producing and placing SMA properly.

“This is an unprecedented breakthrough: using aggregates for SMA that are generally lower strength than those used for SMA across the country to produce the most resilient highway mixes,” Al-Qadi said.

“We may now make a mix in the state, which is a big advantage for Illinois,” he added. “It’s important for our transportation system, and at the same time, it saves significant costs and reduces emissions.”

Switching to locally available aggregate in SMA, according to Al-Qadi, is expected to provide significant savings, as a ton of local aggregate costs approximately $15, while those imported from nearby states range from $45 to $60 per ton.

Al-Qadi also calculates that shifting to Illinois materials will reduce emissions from transporting SMA materials by up to 60%.

“If we can make something more cost-effective, that’s beneficial for everybody,” Hill said.

Read the report for the IDOT-sponsored research project, Optimizing the Use of Local Aggregate in Stone-Mastic Asphalt:
https://doi.org/10.36501/0197-9191/26-004

Learn more about the Illinois Accelerated Pavement Tester on ICT’s YouTube Channel.