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600451-00006-1 Report Abstract

A Comprehensive Characterization of Asphalt Mixtures in Compression

Yuqing Zhang, Rong Luo, and Robert L. Lytton, August 2013

Permanent deformation (i.e., rutting) is one of the major distresses in asphalt pavements, and it consists of irrecoverable deformation due to viscoplastic flow and viscofracture fatigue damage. The mechanisms of rutting have not been well addressed due to the complexities of asphalt mixture including (a) distinctions between compression, extension, and tension; (b) rate and temperature dependence; (c) dilative volumetric change; (d) frictional material with cohesion; (e) inherent anisotropy due to preferential aggregates’ orientation; (f) crack-induced anisotropy due to crack growth; (g) strain hardening during viscoplastic accumulation; and (h) strain softening during viscofracture evolution.

In this project, all of the aforementioned fundamentals of asphalt mixtures were simultaneously characterized by a comprehensive viscoplastic-fracture mechanistic model, which was incorporated with (a) a modified effective stress to consider the inherent anisotropy and the crack-induced anisotropy due to viscofracture cracking in compression; (b) a smooth and convex Generalized Drucker-Prager (GD-P) yield surface; (c) a non-associated viscoplastic flow rule; (d) a rate- and temperature-dependent strain hardening rule; and (e) a viscofracture evolution that was modeled by an anisotropic damage density-based pseudo J-integral Paris’ law. The model parameters were related to fundamental material properties that were measurable and understandable for civil engineers. A systemic testing protocol including five individual test methods were proposed to determine the model parameters and material properties. The test protocol was demonstrated to be efficient, as one asphalt mixture could be completely characterized within 1 day. The GD-P yield surface model was validated by octahedral shear strength tests at different normal and confining stresses. The GD-P model was able to characterize the full range of the internal friction angles from 0 to 90 degrees. In contrast, the widely used Extended Drucker-Prager (ED-P) model can only be used for a material that has an internal friction angle less than 22 degrees due to the convexity criterion of the yield surface.

Keywords:  Asphalt Mixtures, Rutting, Constitutive Modeling, Compression, Anisotropy, Viscoplasticity, Viscofracture

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