Using local goods is key to reducing environmental impacts caused by hauling materials. Our pavement materials are no different.

Illinois Center for Transportation and Illinois Department of Transportation aim to increase the use of local pavement materials in a joint project, “R27-216: Optimizing the Use of Local Aggregates in Stone-matrix Asphalt.”

Imad Al-Qadi, ICT director and University of Illinois Urbana-Champaign Grainger Distinguished Chair in Engineering, leads the effort with Brian Hill, IDOT’s hot-mix asphalt operations engineer.

Jim Trepanier, IDOT’s engineer of HMA, aggregate and chemical tests, led the project until his retirement in December 2022.

Thinking local

The two agencies seek to evaluate the use of locally available aggregates — materials such as gravel or crushed stone that are bound together to form roads, among other uses — from Illinois in stone-matrix asphalt, a resilient and flexible pavement mixture.

Illinois, which does not produce the high-strength materials used in SMA, currently imports aggregates for SMA from surrounding states — with transport costs generally leading to increased mixture costs.

“Using local aggregates is a challenge because they are not as strong as aggregates that we import from other states,” Al-Qadi said. “We wanted to come up with a new design that will allow us to use local aggregates while maintaining performance.”

Local aggregates available in Illinois generally include dolomite, limestone and gravel.

Effectively using local materials in SMA will not only decrease construction costs, but also lower environmental impacts through reduced fuel and energy usage as well as emissions.

“SMA mixtures have performed well in Illinois, which has been documented in several ICT projects,” Hill said. “IDOT is really interested in the use of locally available coarse aggregate to identify if we can see a reduction in mixture costs and reduction in environmental impact while maintaining mixture stability and performance.”

Selecting materials

Through a series of performance tests, the project team selected four SMA mixes made with aggregates from various parts of Illinois and two control mixes made with imported aggregate.

A construction crew, led by Open Road Paving, assembled a 450-foot-long test area at ICT, where they placed the six SMA mixes on top of concrete and hot-mix asphalt layers on May 18 and 19.

The construction crew used a ground-penetrating radar technique developed by Al-Qadi and colleagues to measure the pavement’s density in real time, a patent of which is pending as part of a Federal Highway Administration project.

Underneath the SMA layer, Al-Qadi and his students installed 53 sensors, which measure strain from simulated traffic loads as well as temperature, and 12 pressure cells, which measure vertical pressure at the bottom of the SMA layer.

Evaluating mixes

The next steps?

Use ICT’s upgraded Accelerated Transportation Loading Assembly to collect data and to compare the mixes’ performance.

ATLAS simulates a pavement’s long-term performance in an accelerated period using real-life traffic and environmental conditions, allowing researchers to determine how a pavement will perform in real life.

Al-Qadi’s team will test the pavement sections using different loads with acceleration and deceleration as well as shear stresses at specific angles.

If the ATLAS test sections demonstrate promising results, IDOT aims to update their specifications and supporting documents for SMA using local materials.

“If proven successful, those specifications and supporting document updates should allow for more options for mix designers and potentially increase the use of locally available coarse aggregate in SMA mixtures, which could lead to reduced mixture costs and reduced environmental impact,” Hill said.

Al-Qadi credits the success of the effort thus far to the collaboration between UIUC, IDOT and industry members.

“The Technical Review Panel of this project, led by Jim and Brian, has been wonderful in providing feedback during the whole process, from coming up with the mix designs to the plan for construction,” Al-Qadi said. “The contractor who worked with us, Open Road Paving, has also been amazing.”

For ICT’s graduate students, the project had an additional benefit: the chance to get hands-on experience.

“This has been a very nice experience for my students to be involved in construction, instrumentation and testing while paving,” Al-Qadi said. “The students got to have hands-on experience of something they usually learn about only in class.”

The joint ICT-IDOT project will conclude March 2024.