Relationship between Rheological Properties of RAP Binders and Cohesive Surface Free Energy

The performance of conventional or reclaimed asphalt pavement (RAP) mixes depends on the properties of the virgin and RAP binders. The fundamental characteristics of different materials often are used to explain the mechanical performance. Surface free energy (SFE) is one such property that can be considered to explain the fracture and rutting performance and moisture damage resistance of bituminous binders and mixes. The correlation between different rheological properties of binders (virgin as well as RAP blends) and the SFE, determined by the sessile drop technique, was explored in this study. Two commonly used virgin binders and different virgin–RAP binder blends (prepared by varying the RAP content in the blend) were evaluated. Penetration, softening point, viscosity, Superpave rutting and fatigue parameters, multiple stress creep and recovery (MSCR), and linear amplitude sweep (LAS) test parameters of the binders were measured and correlated with SFE. Total cohesive surface free energy of the binders was found to correlate well with (1) cohesive/stiffness properties of the virgin and RAP binders represented by penetration, softening point, and $G^{*}$ (complex shear modulus); (2) the viscoelastic response indicated by phase angle and percentage strain recovered; (3) rutting resistance, represented by the Superpave rutting parameter ($G^{*}/\sin \delta$) (where $\delta$ is phase angle) and non-recoverable compliance measured in the MSCR test; and (4) fatigue performance indicators such as the Superpave fatigue parameter ($G^{*}\sin \delta$) and the fatigue life estimated from LAS test. The study clearly established the correlation between SFE and the mechanical parameters evaluated in the study and indicates the need to conduct a more-detailed and exhaustive experimental program to develop generalized relationships applicable to a wider range of binders than considered in this study. The fact that a single binder parameter measured by a simple method can explain multiple aspects of performance is a promising prospect.