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Pavement researchers model cold-weather cracking

August 2004
From the CTS Report

The bitter cold of northern winters is a major cause of cracking in asphalt roadways, leading transportation agencies across the northern U.S. and Canada to spend millions of dollars annually to repair or replace pavements as they deteriorate. But while the effects of low-temperature cracking are obvious, the details of how asphalt pavements respond to low temperatures have proven difficult to model accurately.

At the University of Minnesota's Department of Civil Engineering, assistant professor Mihai Marasteanu and professor Vaughan Voller are among the authors of a new report that details their group's recent work on low-temperature cracking. Their work will serve as a foundation for further work at the University's newly established Pavement Research Institute (PRI), directed by pavement specialist Erland Lukanen, P.E.

Current specifications for asphalt pavement materials are based on strength and creep tests performed on asphalt binders and mixtures at low temperatures. In response to the limitations of these tests, many researchers have begun to search for better ways of modeling pavement characteristics. More accurate low-temperature cracking models would aid pavement engineers in developing new materials to resist low temperature cracking, and help transportation agencies choose the most appropriate construction methods for new roads.

In response to demand for better pavement specifications, the American Association of State Highway and Transportation Officials (AASHTO) used a thermal cracking (TC) pavement model in the development of their latest Design Guide. Many transportation agencies rely on the Design Guide and the specifications it contains when planning road construction.

Marasteanu, a CTS Faculty Scholar, and his group evaluated the ability of AASHTO's TC model to predict pavement cracking by comparing its results to real-world observations made at the Mn/ROAD pavement testing facility. This facility, which incorporates a segment of interstate highway as well as a specially constructed test track, is used by the Minnesota Department of Transportation (Mn/DOT) and university researchers to evaluate the performance of pavement materials and construction techniques under real-world conditions.

The researchers also compared the TC model to a new model developed during the course of their research. They found that their new model gave a reasonable prediction of crack spacing that was "not significantly different" from their field observations. The TC model, in contrast, did not predict any cracking.

The researchers also carried out additional experimental work to measure the fracture properties of asphalt mixes used at Mn/ROAD. The results clearly indicated that the fracture properties of asphalt mixtures were temperature-dependent, and that the binder material played a significant role in determining the overall mixture's fracture properties.

Finally, the researchers developed an experimental "cohesive crack model" to simulate the thermal cracking mechanism in asphalt, using the results of their tests on Mn/ROAD asphalt mixes. While their preliminary analysis indicates that the cohesive model has the potential to simulate thermal cracking, the report authors caution that it is not yet suitable for use in designing pavements. Additional experimental work is required to determine temperature-specific values for model parameters. Such a model, they conclude, is a first step toward a truly comprehensive model that can predict the low-temperature behavior of asphalt pavements.

That work, with the potential to improve the design of roadways for all areas that experience extreme cold, will be the subject of a national research effort now getting underway. The University of Minnesota's Pavement Research Institute (PRI) will be among the participants. PRI was recently established as a partnership between the Center for Transportation Studies, the Department of Civil Engineering, the Mn/DOT Office of Materials and Road Research, and the Minnesota Local Road Research Board.