Empirical and Numerical Modeling of Viscoelastic-Viscoplastic Constitutive Equation for Asphalt Mixture Compaction

Mohammad Karimi Hosseinabadi, Mohammad | 2017

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 50878 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Tabatabaee, Nader
  7. Abstract:
  8. The numerical simulation of asphalt concrete and pavement has gained considerable attention in recent decades. Information about the response of asphalt pavements during construction and in-service traffic loading is required by pavement specialists for improvement of pavement analysis and design methods. The goal of this study was to propose a constitutive relationship and a numerical modeling method for asphalt concrete compaction in the laboratory and in situ. Asphalt binders exhibit rate-dependent viscoelastic and viscoplastic mechanical behavior in asphalt concrete. The current study examined the viscoelastic and viscoplastic mechanical behavior of asphalt concrete under various loading patterns. The proposed constitutive relationship takes in to account the microstructural changes of asphalt concrete, such as aggregate reorientation in cyclic loading and stress transfer under tension/compression. The proposed numerical framework has been derived using the finite element method to integrate the proposed constitutive relationship. Experimental observation showed that the viscoplastic strain induced in asphalt concrete under cyclic loading was considerably greater than that under creep loading. In the unloading stage, the aggregates reorient and pack into a form having minimum internal energy. Under such conditions, the potential for viscoplastic strain accumulation in asphalt concrete increases. This phenomenon is referred to as hardening relaxation in this research. It was found that the rate of viscoplastic strain increased as the rest period and recovered viscoelastic strain increased. The constitutive relationship for hardening relaxation is proposed as a function of recovered viscoelastic strain. The results show that the proposed constitutive relationship substantially improved strain prediction compared to the commonly used viscoplastic constitutive relationship (e.g., Perzyna). The viscoelastic mechanical behavior of asphalt concrete under tension and compression was also studied. Stress transfer within asphalt concrete occurs in response to different mechanisms. In a pure compressive stress mode, stress is transferred by aggregate and asphalt mastic; however, in a pure tensile stress mode, aggregate interlock plays a limited role in transferring the stress, while the major contribution comes from the mastic phase. The asphalt concrete would be expected to exhibit different viscoelastic mechanical behaviors in the tensile and compressive stress modes. Experimental observation showed that the ratio of tensile to compressive creep compliance was about 2.5. When different stress modes are applied in the laboratory or in situ to the asphalt concrete, the mechanical behavior can be expected to differ in different coordinate directions. Under such conditions, anisotropic behavior is induced in the asphalt concrete. For this reason, an anisotropic viscoelastic constitutive relationship which is sensitive to tensile/compressive stress mode was developed to model the viscoelastic responses of asphalt concrete under multiaxial stress states. Comparison of the numerical modeling and experimental observation showed that the proposed constitutive relationship improved the accuracy of viscoelastic modeling of asphalt concrete for multiaxial tension/compression stress states. Laboratory experiments under extreme boundary conditions were devised to test the necessity of incorporating the multiaxial state of stress and viscoelastic and viscoplastic strains during the compaction of asphalt concrete at high temperatures. One experiment used compacted asphalt in a flexible rubber mold and the other in a rigid steel mold. In the rubber mold, the large viscoplastic strains could be attributed to aggregate slippage which caused permanent deformation in both the axial and radial directions. In the steel mold, however, the aggregates could not undergo slippage because of the rigid confinement. It was observed that, immediately after a small amount of compaction, the aggregates locked together and prevented further compaction. This induced a considerable level of hydrostatic pressure such that the distortion stress could not overcome the volumetric pressure and resulted in minimum vertical and zero lateral deformation. A large-deformation thermodynamic-based framework was developed to simulate the dependence of viscoplastic deformation on the multiaxial state of stresses. Using the relevant Helmholtz free energy and rate of energy dissipation functions, the rate-dependent constitutive relationships were derived and used to model the response of asphalt concrete during compaction. A straightforward method that allows the calibration of the proposed model against laboratory compaction data was developed. Numerical algorithms associated with the proposed constitutive relationship were implemented in finite element code. The model was calibrated with the deformation data from the Superpave gyratory compactor for different numbers of gyrations. The calibrated model was utilized to predict the field compaction of asphalt concrete. Comparison of the model predictions and field measurements show that the model is capable of accurately predicting the compaction of asphalt concrete material both in the laboratory and in situ
  9. Keywords:
  10. Finite Element Method ; Asphalt Concrete ; Numerical Modeling ; Viscoplasticity ; Viscoelasticity ; Asphalt Mixture ; Asphalt Compaction

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