Three main topics were researched in this work: viscoplasticity, fatigue, and rutting of flexible pavements. The literature review indicated the need for more research work in these areas, with more understanding of the damage mechanisms involved in the fatigue and rutting problems. With this in mind, models on the phenomenological and mechanistic levels have been developed to characterize and assess the above problems with more emphasis on interrelating the damaging mechanisms with material characterization.;These developed models are supplemented by appropriate experimentation phases to identify and numerically evaluate the relevant parameters. The experimentation phase is based on actual existing routine methods, with proper adjustments, modifications, or extensions to comply with the proper evaluation of the models parameters, on the one hand, and kept as simple as possible, on the other hand, in order to gain wider user acceptance. In fact, the new developed models have the merits of looking at the analysis of flexible pavements as a whole unit, and have tried to put the interrelationships between different aspects of flexible pavement design package into its perspective.;A one-dimensional combo-viscoelastic/plastic model that is composed of viscoelastic element of burger type connected in series with a friction element was used. The friction element is the mechanical representation of plasticity with Druker-Prager yield criterion. The model is solved under creep phase loading conditions. The solution is then used to develop a rutting model that incorporates densification phase represented by a relaxing spring and viscous and plastic deformation as found from the viscoelastic/plastic model. Comparison with actual measured data from the AASHTO Road Test shows consistency. The fatigue problem was solved on the mechanistic level using the one parameter, elastic-plastic fracture mechanics discipline, J-integral. The contour J-integral was interpreted as the energy release rate to enable its experimental evaluation for asphaltic mixtures using cracked disc specimens. The fatigue life is defined in this dissertation as that life required for initiation, propagation and ultimate failure of an engineering size crack. The universal slope method was used to construct the fatigue life curve that model ductile failure and in the same time that model was matched with the mechanistic model to determine the crack initiation life.
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