Performance Assessment of a Warm Asphalt Binder in the Presence of Water by Using Surface Free Energy Concepts and Nanoscale Techniques

Abstract

Warm mix asphalt (WMA) has been gaining attention in the past few years because it offers several advantages over traditional hot mix asphalt (HMA) during production and placement. The advantages include: (1) reduced energy consumption; (2) reduced emissions; (3) reduced binder aging; and (4) extended construction season in temperate climates. Moisture damage in a compacted HMA/WMA mixture can be defined as the separation of the asphalt binder from the surface of the aggregate in the presence of water. In this study, atomic force microscopy (AFM) was used to investigate the effects of water on the adhesion and cohesion forces in three asphalt binders (one HMA, two WMA) using four different cantilever tips with different materials. AFM is a microscope that can be used to makes high quality three-dimensional images for smooth and level surfaces. The asphalt binders were the HMA PG 70-22 binder; PG 70-22 mixed with a proprietary Sasol wax at 4% by weight to make a WMA binder; and PG 70-22 with a proprietary white powdered zeolite (similar to Aspha-min) at 5% by weight. The four different cantilever tip materials used in this study were: (1) SiO2 coated with a carboxylic acid functional group (─COOH) to measure the cohesion forces between two asphalt molecules; (2) CaCO3 (representing calcite in limestone) particle cantilever tip to measure the adhesion forces between asphalt binder molecules and an aggregate molecule; (3) Ohio limestone tip to measure the adhesion forces between asphalt binder molecules and limestone; and (4) SiO2 particle tip (representing sandstone) to measure the adhesion forces between asphalt binder molecules and sand. Several researchers have used AFM to examine the surface free energy of adhesion or cohesion between various substances. In this study, AFM has been shown to be useful tool to evaluate the strength of bonding between three types of asphalt binder on four types of surfaces in the presence and absence of water. The adhesion/cohesion energy of warm binders was similar or better than the adhesion/cohesion energy of hot binder by using different cantilever tips. The performance of the Sasol wax was higher than the performance of the proprietary white powdered zeolite using certain cantilever tips which can be concluded from indirect tensile strength (ITS) results.