Identification and Characterization of Asphalt Concrete Phases Using Nanoindentation Creep Analysis

Abstract

Asphalt researchers have studied only the bulk state (liquid or semisolid) of asphalt binder using traditional rheometers. In roadway pavements, an asphalt binder never exists in a liquid or semisolid state; rather, it exists as a solid, as one of the three phases/components (asphalt, aggregate, voids) of an asphalt concrete (AC). While rheometric techniques used mostly bulk asphalt material, recently nanoindentation technique has brought an opportunity to indent an asphalt binder while it resides in an AC at a solid phase. One problem with indentation is that it is difficult to indent a specimen phase, and it is always difficult to identify whether the indenter is indenting a void, binder, or mastic. In this study, using a nanoindentation creep test, two Performance Grade (PG) binders, PG 70-22 and PG 64-22, were studied for their behavior at the bulk state and the phase state. To prepare the sample for the bulk state, simply liquid asphalt was placed on a glass slide as a thin film and indented. For the phase state, an asphalt concrete sample was sliced into rectangular pieces and indented at three phases. It was observed that the bulk state stiffness of asphalt binder (PG 70-22) increased by 3.8 and 8 times compared with unaged binder stiffness for 2 and 5 days of oxidative aging.