Study of Bonding Property of Warm Mix Asphalt Based on Binder Bond Strength and Molecular Dynamics Simulations

Abstract

The bonding property plays a crucial role in the durability of the asphalt mixture. Warm-mix asphalt (WMA) commonly is used for energy-efficient pavements. The addition of warm-mix additives makes the behavior of the bonding property more complex, especially its atomic-level mechanism. To better clarify the multiscale behavior of WMA bonding property, three typical WMA binders, namely Sasobit-modified asphalt binder, Evotherm-modified asphalt binder, and waste cooking oil (WCO)-modified asphalt binder were selected. The binder bond strength (BBS) test was carried out to evaluate the bonding fracture type and bonding strength. Based on molecular dynamics (MD) simulation, cohesion and adhesion work, radial distribution function (RDF), mean square displacement (MSD), and molecule polarity were calculated to reveal the mechanism. The tensile strength ratio (TSR) of corresponding mixtures was calculated to perform mixture-level characterization. Strong correlations (R-squared >0.9) were found between BBS testing results and interaction work derived from the MD simulation (for both cohesion and adhesion behavior). In addition, the fracture type in the BBS testing can be predicted accurately by the MD simulation. BBS tests found higher cohesion strength and lower adhesion strength with the addition of Sasobit, whereas adding Evotherm and WCO resulted in lower cohesion strength and higher adhesion. The change in cohesion is explained by RDF calculations. Sasobit wax encourages the stacking of asphaltene molecules, whereas Evotherm and WCO aid in its dispersion. MD simulation also confirmed that molecule polarity plays an important role in the adhesion work (R-squared >0.9). Polar functional groups in Evotherm and WCO facilitate better asphalt–mineral adhesion. Nonpolar Sasobit weakens asphalt–mineral adhesion. MSD calculation indicated that diffusion behavior is governed by the combination of molecular mass and adhesion behavior. Molecules with larger molecular mass and stronger adhesion exhibit slower diffusion. Mixture-level evaluation (tensile strength ratio) correlated well with adhesion work (R-squared = 0.822) but poorly with cohesion work (R-squared = 0.667), indicating that adhesion is more important than cohesion in water-damage resistance.