Alternate Shift Factor Relationship for Describing Temperature Dependency of Viscoelastic Behavior of Asphalt Materials

Traditionally, various forms of shift factors such as Arrhenius, Williams Landel-Ferry (WLF), and polynomials have been used with asphalt materials. Shift factors have also been estimated with binder viscosity parameters. Successful extrapolation of viscoelastic functions requires a robust form of shift factor temperature relationship. This form is important for performing calculations at the extremes of temperature found in practice. A preliminary analysis of complex modulus E* data of mixtures obtained from the Mechanistic-Empirical Pavement Design Guide (MEPDG) database demonstrated that the Kaelble form of shift factor could describe the functional form of the shift factor more accurately than the Arrhenius, WLF, or polynomial-fitting functions. However, the Kaelble shift function as originally described uses the same temperature as a reference temperature and as an inflection temperature. This factor creates a problem when attempts are made to implement the function in a design method or when materials are compared at a given temperature. Since 2008, additional work has investigated the use of this shift function to describe the properties of asphalt materials, particularly mixes and materials that require a wide range of property description (both above and below the glass transition or some other defining point). A modified form of the Kaelble function has been implemented in analysis software and thus make; multiple calculations more rapid. Additional analysis working with MEPDG E* database materials has shown that shifting works best with the Kaelble modification of the WLF equation. The same method has been applied to other asphalt materials.