|Ph.D Student||Levenberg Eyal|
|Subject||Constitutive Modeling of Asphalt-Aggregate Mixes with|
Damage and Healing
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Emeritus Jacob Uzan|
Asphalt-aggregate mixes are multiphase heterogeneous materials; they are a composite blend of air-voids, asphalt-cement (bitumen) and aggregates of a range of sizes. These materials exhibit extremely complex mechanical behavior that is very difficult to capture and model. Mainly for this reason available pavement-performance models are empirical, as no rigorous constitutive models were yet formulated for asphalt mixes.
The motivation underlying this research work was to improve material modeling and characterization techniques for asphalt-aggregate mixes. An up-to-date review of literature revealed that current characterization efforts are limited principally because they deal with material behavior in uniaxial tests and provide essentially one-dimensional models.
This dissertation presents the development of a triaxial viscoelastic-viscoplastic constitutive model for asphalt mixes including the effects of damage and healing. The model is confined to the description of pre-peak load response under isothermal conditions. It is based on additive separation of the total strain into viscoelastic and viscoplastic components and provides individual constitutive treatment to each part.
The viscoelastic formulation is nonlinear, cross-anisotropic, and characterized by one unique time-function. Three nonlinear isotopic effects are modeled: I) damage, i.e. loss of stiffness under load; II) stiffening, i.e. increase of stiffness under compression conditions, and III) healing, i.e., a decrease in the level of damage during rest periods. The viscoplastic equations resemble the kinematic-hardening formulations used to describe creep of metals. Neither damage nor healing is included in the viscoplastic model. Coupling is introduced between the individual formulations, making the viscoelastic response dependent also on the viscoplastic component.
In order to support the development of the constitutive formulation, new experimental procedures were designed and executed using the triaxial apparatus. Creep and recovery test results are presented and analyzed, providing means to calibrate and validate the model for biaxial stress-conditions and one test temperature. Good reproducibility and forecast-ability were obtained in the analyses of versatile test-data for both small and large strain load-cycles; indicating that the model is suitable for simulating the 3D load-response of asphalt-aggregate mixes.
The constitutive development in this study constitutes the first attempt to describe the triaxial (viscoelastic-viscoplastic) load-response of asphalt materials including damage and healing. Some aspects of this development were found limited - specifically the ability to rigorously describe the viscoplastic behavior after large rest periods. Further research is needed to try and resolve this limitation and remove some of the other formulation restrictions.