Using the surface free energy (SFE) method to investigate the effects of additives on moisture susceptibility of asphalt mixtures

Conventional methods for evaluating the effects of additives on moisture damage resistance of asphalt mixtures are either empirical or unable to quantify the contributions of material component properties to the overall mixture performance. To overcome these drawbacks, this study proposes a surface free energy (SFE) method to investigate the effects of various additives on moisture susceptibility of asphalt mixtures. One neat asphalt and one acidic gravel aggregate are selected as the test materials along with 6 commonly-used additives: a warm mix asphalt additive, two nano-materials, a hydrated lime, a Portland cement and a non-amine liquid asphalt anti-stripping agent. All the additives are blended with the neat asphalt to fabricate modified asphalt binders. The SFE components of these modified asphalt binders and the gravel aggregates are measured with the Wilhelmy plate method and the vapor adsorption method, respectively. An energy ratio, defined as the ratio of adhesion of asphalt-aggregate to that of asphalt-aggregate-water, is calculated and used to rank the asphalt mixtures that consist of gravel and asphalt binders modified with different additives in terms of their moisture susceptibility performance. In order to validate the proposed SFE method, two mixture moisture susceptibility tests, the modified boiling water test and the indirect tensile strength test, are conducted on loose and compacted asphalt mixtures, respectively. A consistent moisture susceptibility ranking is obtained from the SFE method and the mixture moisture susceptibility tests, which validates the SFE method proposed in this paper can be used to accurately quantify the effects of additives on the moisture susceptibility of asphalt mixtures.