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. 

The construction process of a thin overlay.

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 (UIUC) 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 (OST-R).

“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 and UIUC Professor Hasan Ozer said. “By selecting the optimum mixtures and right thickness, the lifetime of thin overlays can also be extended.”

Micromechanical model of the SCB test used to evaluate cracking resistance of various overlay mixes.

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 to predict the crack propagation in the 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. 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.”

Finite element model used to understand mechanics of thin overlays and predict service life.

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.”

Posted July 2018.