Nonlinear viscoelastic analysis of unaged and aged asphalt binders

This paper presents analyses of the nonlinear viscoelastic behavior of unaged and aged asphalt binders tested using a dynamic shear rheometer (DSR) at several temperatures and frequencies. It was not possible to conduct all DSR tests at the same range of stresses, which is necessary for establishing the master curve for nonlinear viscoelastic materials. Therefore, the stress levels for each test, at a given temperature and frequency, were normalized by the ultimate stress level of that test. Consequently, all test results were transformed to a common range of normalized stresses that were used in establishing the master curve. A phenomenological model was used to obtain the creep response of the binders in the time domain from the normalized frequency domain measurements. Then, the Schapery single integral equation was used to model the binder nonlinear creep response. A master curve at a reference temperature of 30 °C was formed using the time-temperature superposition principle (TTSP) at selected normalized stress levels. The Schapery's nonlinear stress dependent parameters (g₁g₂) were determined by vertical shifting the master curves at the different normalized stress levels. An aging shift factor was used to obtain the aged binder response from the properties of the unaged binder. The aging-time shift factor was found to be a function of temperature, but independent of stress level. The nonlinear viscoelastic model was implemented in the ABAQUS finite element (FE) software and used to back calculate the creep response of the unaged and aged binders. The FE results were in very good agreements with the experimental measurements.