Showing papers by "Rashid K. Abu Al-Rub published in 2013"

Three-Dimensional Microstructural Modeling of Asphalt Concrete by Use of X-Ray Computed Tomography

Abstract: This paper presents a framework that combines experimental techniques and computational methods for modeling the microscopic response of asphalt mixtures subjected to various loading conditions. The basis of this framework is capturing the three-dimensional microstructure of asphalt mixtures with X-ray computed tomography and a sequence of image-processing methods to identify the microstructure components or mixture phases. This microstructure is then converted to a finite element model in which the various phases are represented with constitutive models that describe their mechanical behavior. In this study, the coarse aggregate phase was modeled as a linear elastic material, and the matrix phase (asphalt, fine particles, and air voids) was represented as a thermoviscoelastic, viscoplastic, and damage model. The analysis results showed that the model captured the effects of temperature, rate of loading, repeated loads, and mixture design on the microstructure response. These results demonstrate that the ...

Continuum Coupled Moisture-Mechanical Damage Model for Asphalt Concrete

Abstract: Despite the detrimental effects of moisture damage in asphalt pavements, few macroscale models are capable of modeling this important phenomenon. Existing models have limitations in accounting for the irreversibility and time dependency of moisture-induced damage. This study presents a moisture damage model based on continuum damage mechanics. Adhesive and cohesive moisture damage phenomena are modeled independently; this procedure allows for the introduction of fundamental mechanical properties for each process and for modeling the transition between adhesive and cohesive damage. Two- and three-dimensional simulations are performed, and the results of the simulations are presented to demonstrate the applicability and utility of these micromechanical computational models. It is shown that the proposed moisture damage model can simulate the effect of moisture damage on the mechanical response of asphalt concrete subjected to different loading conditions. The model also provides useful insight into the effe...

Cyclic Hardening-Relaxation Viscoplasticity Model for Asphalt Concrete Materials

Abstract: A cyclic hardening-relaxation model is proposed that significantly enhances the prediction of the viscoplastic (VP) strain of asphalt concrete under cyclic compressive-loading conditions at high temperatures. The hardening-relaxation mechanism is physically tied to the changes in the material’s microstructure during the rest period. A memory surface that memorizes the viscoplastic deformation history is defined in the viscoplastic strain space as the general initiation and evolution criteria for the hardening-relaxation mechanism. The proposed model is coupled to the classical Perzyna-type viscoplastic model and Schapery’s nonlinear viscoelastic model, and the associated numerical algorithms are implemented in the finite element software ABAQUS through the user-defined material subroutine UMAT. Model predictions show that the proposed model predicts well both the axial and radial viscoplastic responses of asphalt concrete subjected to the cyclic creep tests at various loading times, unloading time...

Constitutive Modeling of Fatigue Damage Response of Asphalt Concrete Materials

Abstract: The fatigue damage response of asphalt concrete is modeled with a proposed damage evolution function in the context of continuum damage healing mechanics. The uniaxial constant strain rate test is used to identify damage density experimentally during this test and to propose a form for the evolution of the damage density variable in the model. In addition, the model is formulated to include the microdamage healing effect. The proposed damage model is incorporated in the pavement analysis with a nonlinear damage approach model that includes Schapery's viscoelastic model, Perzyna's viscoplastic model, and the microdamage healing model, which is used to simulate the fatigue damage response of asphalt concrete. The damage model is validated against extensive experimental data, including constant strain rate, cyclic displacement-controlled tests, and cyclic stress-controlled tests over a range of temperatures, strain rates, loading frequencies, and stress-strain levels and amplitudes. Model predictions show th...

Effect of Confinement Pressure on the Nonlinear-Viscoelastic Response of Asphalt Concrete at High Temperatures

Abstract: Asphalt concrete materials exhibit the nonlinear viscoelastic responses at high stress/strain levels. The traffic loading induces multi-axial stress states within the asphalt concrete pavement structure. Therefore, it is imperative to characterize the nonlinear viscoelastic responses of asphalt concrete under the realistic stress states since these nonlinearities significantly affect the rutting and fatigue damage performance of pavements. Schapery’s (1) nonlinear viscoelastic model has been used extensively by several researchers to characterize the nonlinearity of asphalt concrete materials. However, the available methods for characterizing the viscoelastic nonlinearity are mostly based on the simple uniaxial creep-recovery tests. In this paper, the nonlinear viscoelastic properties of asphalt concrete materials are characterized considering the effects of confinement pressure on the variation of the nonlinear parameters. Dynamic modulus test is used to obtain the linear viscoelastic properties and the time-temperature shift factors. Cyclic creep-recovery tests are performed at high temperature (55°C) and at different confinement levels to characterize the nonlinear viscoelastic responses. The effect of confinement pressure is investigated through usage of triaxiality ratio on the variation of the nonlinear viscoelastic parameters. The analyses showed that the triaxiality ratio has substantial effect on the nonlinear strain response of the asphalt concrete. The significance of these nonlinear parameters on the complex response of asphalt concrete subjected to different loading conditions is also discussed.

Constitutive Modeling of Cyclic Viscoplastic Response of Asphalt Concrete

Abstract: Rutting (permanent deformation) is the most important distress that asphalt concrete pavements are prone to at high temperatures. However, available classical viscoplastic (VP) theories are incapable of properly predicting permanent deformation of asphalt concrete materials at high temperatures subjected to cyclic loading conditions. A physically based VP hardening relaxation model is proposed to overcome this issue. This model considers the changes in the microstructure that occur during the rest period and cause the induced hardening stress to relax and recover. This mechanism affects the VP properties of asphalt concrete before and after the rest period is applied. A memory surface is introduced as the general criterion for the evolution of the hardening relaxation mechanism. The memory surface possesses a state variable memorizing the history of the VP deformation during the loading history. The proposed VP hardening relaxation model is coupled with Schapery's viscoelastic model and the classical Perz...