Stress Relaxation can Perturb and Prevent Physical Hardening in a Constrained Binder at Low Temperatures

Abstract

ABSTRACT The physical hardening phenomenon that is observed in asphalt binders held under isothermal conditions at low temperatures is found to be absent when the asphalt binder exists in combination with the aggregates in the aggregate-asphalt mixes. A recent study on the effect of physical hardening on stress relaxation behavior of asphalt binders has provided a plausible explanation for this observation. Controlled experiments using the new Superpave Direct Tension Test (DTT) were used to demonstrate how stress relaxation overcomes physical hardening when the asphalt binder is held under restraint. In the present work, an attempt is made to provide a theoretical basis for the prevention of isothermal physical hardening due to stress relaxation in a constrained asphalt binder. The altered free volume concept is used for developing the mathematical formulation for the physical hardening, and a linear viscoelastic mechanical model is used for expressing the equations for the stress relaxation. It is shown that stress relaxation can, indeed perturb and prevent the physical hardening of asphalt binders that are held under restraint, as is the case when the binders are present in the aggregate-asphalt mixes.