# Adhesion Evaluation of Asphalt-Aggregate Interface Using Surface Free Energy Method
## 1 Introduction  
The compaction temperatures of hot mix asphalt (HMA) are usually above 160 \({}^{\circ}\)C, which consumes a large amount of fuel energies and results in the emission of CO\({}_{2}\). Warm mix asphalt (WMA) technology has been generalized to the asphalt pavement industry for a few years. The warm mix technologies can reduce the asphalt production temperature by as much as 30 \({}^{\circ}\)C. There are two widely used warm mix technologies: adding organic additives and applying water foaming. In the water foaming, due to the lower compaction temperature of WMA, water cannot be completely evaporated out of aggregates. The remaining water can impact the adhesiveness of the asphalt-aggregate interface and lead to moisture damage. Therefore, many researchers start to study problems of water damage in WMA.  
Currently, the theories for studying the adhesiveness of the asphalt-aggregate interface include the molecular orientation theory [1], chemical reaction theory [2], surface free energy theory [3; 4] and molecular dynamics [5], etc. The surface free energy theory has been applied to research the adhesion of asphalt-aggregate interface. The theory of surface free energy could be used to evaluatewater damage and fatigue cracking of HMA [6]. The surface free energies between the asphalt and aggregates were measured using the Wilhelmy plate and absorption methods, respectively, and calculated the adhesiveness of asphalt-aggregate interface with and without water, and it was feasible to use the surface free energy theory to analyze the water damage of HMA [7]. The surface free energy components of asphalt-aggregate interface were analyzed by Wilhelmy plate and adsorption methods, respectively, noting that the surface energy theory could be useful in analyzing water damage in HMA [8]. The surface free energies between the asphalt and aggregates were tested and the adhesion energy ratio was established to predict the adhesion of the asphalt-aggregate interface [9]. The evaluation of the surface energy and moisture susceptibility of various combinations of aggregates and asphalt binders were analyzed [10]. The adhesive properties could be used to estimate adhesiveness of the asphalt-aggregate interface [11]. The asphalt-aggregate interaction for moisture-induced damage mechanisms was studied using surface free energy and predicted moisture-induced damage in HMA [12]. In 2010, the surface free energies of asphalt and aggregate were tested, and the adhesion trends of asphalt-aggregate with and without water were analyzed and calculated [13]. The adhesion models of additive (SAK)-asphalt-aggregate were established and the adhesiveness was used to predict water damage of WMA [14]. The treatment of aggregate surface with hydrated lime narrows down the energy difference under dry and wet conditions, and it helps resist moisture damage [15]. Two different waxes and three kinds of aggregates were used to study the physico-chemical surface characteristics between the aggregates and asphalt. The Dynamic Contact Angle (DCA) and Dynamic Vapour Sorption Devices (DVSD) were used to measure and calculate the components of surface energy. The analysis results indicate that the waxes can adversely affect the adhesion between the aggregates and asphalt [16]. However, there are only a few studies to predict the adhesiveness of organic additive-asphalt-aggregate under the dry and wet conditions according to the surface free energy theory. In the paper, the adhesiveness of asphalt-aggregate and organic additive-asphalt-aggregate systems with and without water is studied based on the surface free energy theory.