Microstructural Evaluation of the Effects of Aggregate Type, Aging, and Additives on the Moisture Susceptibility of Binder–Aggregate Systems Using Chemical and Thermodynamic Approaches

Abstract

Selection of a proper binder–aggregate combination is an important step to ensure optimum resistance to moisture-induced damage in asphalt mixes. In recent years, such selection has become more crucial as asphalt industries are using various additives and modifiers in asphalt mixes that can substantially change bonding strength of a binder–aggregate system. Also, environmental factors such as oxidative aging can affect the chemical compositions of the binder and influence moisture-induced damage resistance of a mix. In order to understand the mechanisms of moisture-induced damage, it is important to determine the chemical and thermodynamic properties of constituent materials of a mix and identify their contributions to the bond strength. The present study was undertaken to explore the effects of aggregate types, additives, and aging on the moisture susceptibility of asphalt mixes using chemical and thermodynamic approaches. For this purpose, a PG 64-22 and a PG 76-28 binder were blended with a warm mix asphalt (WMA) additive, an antistripping agent (ASA), a reclaimed asphalt pavement (RAP) binder and polyphosphoric acid (PPA). The surface free energy (SFE) components of these binder blends under unaged, short-term aged, and long-term aged conditions were determined using the dynamic Wilhelmy plate (DWP) method for measuring dynamic contact angle. Chemical analyses of the binder blends were carried out using x-ray fluorescence (XRF) analysis and Fourier transform infrared (FTIR) spectroscopy. Properties of five commonly available local aggregates were determined using XRF and universal sorption device (USD) testing. The SFE components of the binders and aggregates were used to quantify bonding characteristics of binder–aggregate systems under dry and wet conditions. Aggregate properties significantly influenced the moisture-induced damage potential of a mix. In addition, an increase in carbonyl and sulfoxide functional groups resulted in increased moisture susceptibility with aging. Therefore, the presence of amine functional groups in both WMA and ASA is expected to provide resistance to moisture-induced damage and conversely the presence of PPA may reduce resistance to moisture-induced damage of a binder–aggregate system.