Field Aging of Unmodified Asphalt Binder in Three Texas Long-Term Performance Pavements

Abstract

Binder hardening in pavements and its impact on pavement durability are becoming better understood. Binders oxidize, harden, and become more susceptible to fatigue and thermal cracking. Data are mounting that binder aging occurs to a significant degree well below the top inch of the pavement, contrary to common belief. Whereas recent data indicate that binders age at constant rates and to a significant depth in pavements, a need exists to obtain the detailed profile for understanding pavement aging as a function of air voids. The extent to which binder oxidation occurs throughout the pavement is an important issue and must be understood and quantified to understand pavement durability better and to design longer-lasting pavements. In this work, two layers in each of three pavements in three Texas districts were evaluated. Pavement measurements included recovered binder properties using a dynamic shear rheometer, size exclusion chromatography, changes in these properties over time due to oxidative aging in the pavement, and pavement total and accessible (interconnected) air voids. Many of the pavement cores were sawed into 0.5-in. layers with the binder and air void properties determined for each layer. These measurements provided an indication of binder aging in Texas pavements and suggest strongly that binders age even inches down into the pavement. It was observed that lower accessible air voids correlated with a lower rate of binder oxidation and hardening.