Moisture susceptibility evaluation of zeolite-foamed asphalts based on the Surface Free Energy (SFE) concept

Zeolite-Foamed Asphalt (ZFA), as a typical-used Warm Mix Asphalt (WMA) in practice, has significant advantages in environmental friendliness and construction convenience, but its moisture susceptibility is relatively weak. Most of the existing research regarding its moisture susceptibility was carried out by performing macroscopic tests, which are incapable of providing theoretical bases for moisture susceptibility optimisation. This article studies the moisture susceptibility of ZFA from a mesoscopic perspective based on the Surface Free Energy (SFE) concept. To determine the SFE of each composition, sessile drop tests were first conducted on three types of aggregates (i.e., basalt, diabase and granite) and three types of bitumen binder (i.e., virgin binder, SBS modified binder and rubber modified binder), respectively, considering four different zeolite incorporation contents (i.e., 0%, 3%, 6% and 9% by the binder mass). Several SFE-based factors, including work of cohesion, dry work of adhesion, work of debonding, spreading coefficient and a SFE-related energy ratio (EP₁), were calculated. These factors were correlated with the mass loss ratio of the binder-aggregate systems obtained through boiling water stripping tests to validate their feasibility in evaluating the moisture susceptibility of ZFA. The results indicate that the increase of the polar component and the decrease of the dispersion component of the binder's SFE resulting from the zeolite foaming process affect the moisture susceptibility of the ZFA either positively or negatively, depending on the dominated failure mode and the aggregate's SFE. An aggregate with a higher polar component of the SFE, such as the granite in the present work, is more recommended for preparing ZFA when an interfacial adhesion failure occurs. Dry work of adhesion and EP₁ demonstrate good correlations with the mass loss ratios and the correlation coefficients are not lower than 0.8057. ZFA prepared with the rubber-modified binder, granite and 9% zeolite is suggested in the present work. The results can theoretically guide the optimisation of the moisture susceptibility of ZFA in practice.