Paving the way to control pavement cracks

Placing an asphalt layer — or overlay — over a concrete road is a common technique to rehabilitate roads, extending their service lives and creating smoother surfaces.

Asphalt overlays, however, are prone to reflective cracking, in which cracks appear on the surface because of movement in the underlying concrete slab.

Illinois Center for Transportation and Illinois Department of Transportation aim to control this cracking in a joint project, “R27-204: Optimized Hot-mix Asphalt Lift Configuration for Performance.”

Imad Al-Qadi, ICT director and University of Illinois Grainger Distinguished Chair in Engineering, and Hasan Ozer, Arizona State University associate professor, led the project with John Senger and Laura Heckel, IDOT’s engineer of pavement technology and asset management engineer, respectively. 

To control reflective cracking, the researchers sought optimized and cost-effective overlay configurations, including materials and layer thicknesses, for Illinois.

Al-Qadi’s team conducted tests on eight large-scale “cracked” concrete slabs with various overlay mix and thickness combinations to investigate crack initiation and propagation in overlays.

Their goal? 

Simulate how real-life cracks occur in overlays placed on jointed pavement due to the movement of vehicles. 

“Simulating truck loading in our laboratory was a big challenge,” Al-Qadi said. “There is no instrument that would do that, so we developed our own to simulate shear and bending stresses, similar to how vehicle tires traverse roads in real life.”

Their instrument was a two-actuator servo-hydraulic system with loading pads similar to those of truck tires.

The tests highlighted the importance of effective bonding between the concrete slab and asphalt overlay on cracking.

Utilizing advanced finite-element analysis, Al-Qadi’s team also developed time-saving and inexpensive models that they validated with the lab results.

They packaged the models, which predict the performance of additional overlay mixes and thicknesses, into a user-friendly tool for engineers.

“State engineers will be able to list traffic materials they plan to use in the tool,” Al-Qadi said. “The tool predicts potential cracking: if it’s safe or if it would crack.”

Key to the project was the cost and benefit analysis of IDOT overlay projects led by Ozer’s ASU team, from initial construction costs until the pavement is no longer in use.

Their analysis will allow IDOT to know which mix is most cost-effective in various overlay scenarios based on the service life it will provide.

“We saw the benefit of using certain types of high-quality mixes in the scenarios as part of both of the lifts or one of the lifts,” Ozer said. “The analysis showed the benefits of using high-quality mixes, such as stone-matrix asphalt, on the life-cycle performance of the overlays.”

IDOT aims to use the project’s results to maximize the performance of their roadways.

“As we continue to move forward in our implementation of asset management principles, it is imperative that we maximize the performance of each of our rehabilitation strategies,” Heckel said.

“The results of this project will go a long way to improve our life-cycle planning strategies while minimizing life-cycle costs on our rehabilitation projects,” she added.

Al-Qadi credits the project’s success to the hardworking people involved, including ICT students Zehui Zhu, Aravind Ramakrishnan and Izak Said and ICT engineers Greg Renshaw and Uthman Mohamed Ali.

“This project had really amazing teamwork between ICT, ASU and IDOT, especially the co-chairs of the panel — John Senger and Laura Heckel — as well as the panel members,” Al-Qadi said.

“IDOT’s role was very important from the very beginning, from the selection of different layers to critical discussions throughout the project,” he added. “They really helped us come up with a good product.”