Upgraded Overlay Tester and Its Application to Characterize Reflection Cracking Resistance of Asphalt Mixtures

Reflection cracking is a major concern when placing an overlay on a cracked pavement. The opening and closing of joints and/or cracks induced by daily temperature cycles is a major contributor to reflection cracking. This mechanism is currently being simulated in the laboratory at the Texas Transportation Institute (TTI) using a specially modified overlay-tester device. To evaluate the overlay tester concept laboratory results are presented on cores from four Texas projects, three of which performed very poorly and one which performed excellently. The asphalt mixture on US 175 in Dallas was placed on a cracked stabilized base and did not have a single reflection crack after 10 years in service, whereas the mixtures on two projects were badly cracked after only few months. The results clearly show that the upgraded TTI overlay tester can effectively differentiate between the reflection cracking resistance of different asphalt mixtures. It is also found that the reflection cracking resistance of asphalt mixture has a good correlation with the asphalt binder properties. In this report the upgraded TTI overlay tester is also used to quantify the benefits of modified asphalt binders. This benefit is demonstrated with a single mix where specimens were prepared with a variety of asphalt binders. The mix prepared with PG 64-22 plus 3 percent SBR latex demonstrated superior reflection cracking resistance while still maintaining adequate rutting resistance. It is proposed that the overlay tester is a practical device which can be incorporated into mixture design systems, to complement the current systems, which often focus solely on minimizing rutting potential. In many instances it is necessary to optimize both crack resistance and rutting potential to obtain adequate long-term pavement performance.