Overlay tester: A simple performance test for thermal reflective cracking

Reflective cracking is a major concern when placing an asphalt overlay on a cracked pavement. The opening and closing of joints and/or cracks and associated thermal stresses produced by daily temperature cycles is a major contributor to reflective cracking. This mechanism has been simulated in the laboratory using the Texas Transportation Institute's Overlay Tester. The test specimens used are 150 mm long and can be conveniently cut from either field cores or from laboratory molded specimens. The test is simple to conduct and the results are obtained generally in less than 1 hour. Over the past decade the focus of asphalt mixture design has been largely to address the rutting concern. The Overlay Tester device described in this paper appears to be a tool to let the designer balance the competing requirements of both rut and crack resistance. Preliminary studies on replicate specimens found the test to be repeatable. Sensitivity studies indicated that the Overlay Tester provides reasonable results, in that raising the asphalt performance grade and decreasing the testing temperature will lead to shorter reflective cracking life. However, increasing asphalt content will significantly improve the reflective cracking resistance. Furthermore, in a series of controlled tests on typical Texas mixtures it was determined that asphalt absorption by aggregate appears to have a major impact on overlay test performance. In this paper the Overlay Tester concept is validated on five case studies in Texas. In each case cores from the pavement were tested and their laboratory results correlated well with actual field performance. A preliminary framework for using the overlay tester as part of a mixture design process is proposed. The Overlay Tester is recommended to characterize the reflective cracking resistance, and the Hamburg wheel tracking test and/or APA is proposed for evaluating rutting resistance. These devices provide the designer with tools to balance the conflicting rutting and cracking requirements. This framework is used to select the optimum asphalt content for two projects in Texas.