Thin is in: But researchers study just how thin and for how long?

Researchers are looking to bridge a gap that exists in the study of a very common preservation technique that is used to maintain and preserve the roads we drive on. 

Many state and local roadway agencies routinely use thin overlays to correct cracking and rutting in pavements. A thin overlay is, as the name suggests, a fine layer of asphalt mixture that is placed over existing pavement to restore and smooth out the surface. However, despite the widespread use of thin overlays, there is only sparse research about the technique’s impact on pavement life expectancy as well as its response to traffic loads. Also, existing design methods for overlays are calibrated for thicker overlays that are more suitable for major rehabilitation.

Those were the reasons for a pair of studies conducted in cooperation with the University of Illinois at Urbana-Champaign Illinois Center for Transportation, the Center for Highway Pavement Preservation at Michigan State University, and the U.S. Department of Transportation Office of the Assistant Secretary for Research and Technology.

“The ultimate goal was to provide guidance to agencies to predict the service life of a thin overlay treatment when major variables such as thickness, existing pavement condition, and mixture characteristics are known,” Principal Researcher Hasan Ozer said. “By selecting the optimum mixtures and right thickness, the lifetime of thin overlays can also be extended.”

In Volume 1 of the study, “Micromechanical Fracture Modeling for Mechanistic Design of Thin Overlays,” researchers strayed from the traditionally empirical or semi-empirical approach to thin overlays and developed an advanced approach where the asphalt concrete mixture was made of mortar and stone aggregates. The micromechanical fracture method was used in conjunction with the finite element analysis and digital image correlation (DIC) to predict the crack propagation in the pavement mix. The Illinois Flexibility Index Test, or I-FIT, developed at UIUC, was simulated to calculate the stresses, strains, and the energy around the crack tip.

“The successful comparison between DIC measurements and model predictions was mainly due to the ability of the model to consider mortar, stone aggregate, and their interaction,” post-doctoral research associate Jaime Hernandez said. “This is a considerable improvement from models that assume homogenous asphalt concrete mixture and full bonding between stone aggregate and mortar.”

Various thin overlay mixes were evaluated using the simulations and critical mix design features to improve cracking resistance were identified.

“The effect of mixture characteristics such as gradation, fines content, mortar properties, void space, and binder content on fracture and damage of asphalt concrete were evaluated using the validated micromechanical models,” Ozer said. “It was found from micromechanical modeling that mortar properties were heavily correlated to damage and fracture characteristics of asphalt concrete.”

Volume 2 of the study, “Mechanistic Characterization of Thin Asphalt Overlays for Pavement Preservation Using the Finite Element Modeling Approach,” aimed at characterizing the performance of thin asphalt overlays using a mechanistic approach, which takes into account non-uniform and three-dimensional truck loads resulting in complex stress states and uses inputs from micromechanical simulations.

The mechanistic approach allowed for a more rational characterization of the key factors affecting the performance of thin overlays and a more accurate prediction of the service life of thin overlay treatments.

“The benefits of the study are better prediction of the service life as well as better guidance on the evaluation of the pre-exiting conditions of old pavement, the selection of new mix designs, and the effect of the thickness of thin overlay,” Principal Researcher Karim Chatti said. “A better performing treatment will extend the service life of pavement while improving safety, minimizing congestion during construction activities, and reducing energy and impacts to the environment.”

According to Professor Imad Al-Qadi, who also served as Principal Investigator on the project, designing overlays has always been challenging, “but the design of thin overlays, which represent an important preservation technique, is even more so,” he said.

“Considering the lack of a designated approach for thin overlays design, we are so pleased that our research team at the Illinois Center for Transportation has been instrumental in developing an approach for designing the thickness of overlays using advanced modeling techniques,” Al-Qadi said.