| This dissertation presents a new type of uniaxial viscoplastic rate model based on viscoelastic convolution integrals for explaining the behavior of asphalt concrete in compression under repeated loading. Triaxial compression cyclic tests carried out for long rest periods, with different loading times and two different pulse shapes, square and haversine by researchers at North Carolina State University were used in developing the model. These tests demonstrate that the evolution of permanent deformation depends on load history. This history-dependent behavior is not captured accurately by some of the existing Perzyna-type viscoplastic models in which permanent deformation evolution depends on the current values of stress and viscoplastic strain. Therefore, in this study, viscoelastic-like integrals were used in the rate model to capture the effect of history.;As a simplification, and to better understand some aspects of the rate model, an incremental model was derived from the rate model by making assumptions of steady state conditions and slow hardening growth. This incremental model is capable of capturing material behavior observed from the cyclic tests mentioned above thereby verifying the rate model.;The proposed uniaxial viscoplasticity rate model is applicable to compressive creep and recovery experiments at 54°C with 1) several hundreds of cycles of loading including the secondary creep region, 2) haversine loading shapes at three different peak deviatoric stress levels, 620 kPa, 827 kPa, and 1034 kPa, and square loading shapes at 827 kPa peak deviatoric stress, and 3) long rest periods that allow complete viscoelastic recovery. A notable advantage of the model is the ability to predict the primary creep region behavior without the need for a separate model. Compared to many of the existing viscoplasticity models, the proposed uniaxial viscoplasticity has a relatively simple model form with a physical mechanical analog that can be used to intuitively understand the model behavior. Yet, unlike the existing viscoplasticity models, the model has been successful in capturing the behavior of asphalt concrete in compression under a large number of cycles of creep and recovery loading with long rest period. |
