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98 Suggested future research is presented in Figure 2. In order to arrive at a final aging protocol, the interim aging protocol should be applied to RAP mixtures, WMA mixtures, and polymer- modified mixtures. The AIPs of the binders extracted and recovered from the laboratory-aged loose mixtures would be compared to those of the binders extracted and recovered from cor- responding field cores. In addition, the compaction of loose mixtures after long-term aging for the various WMA technologies should be investigated. A diffusion model that considers the morphological properties of asphalt mixtures (e.g., binder film thickness, air voids) should be developed and coupled with the developed kinetics model to arrive at a pavement aging model. Such a model would improve the prediction of changes in asphalt binder properties with oxidative aging within pavement performance prediction frameworks, including Pavement ME Design. The diffusion model could be developed largely by evaluating the WesTrack mixtures, which include systematic changes in asphalt mixture morphology. The combined kineticsâdiffusion model would then be calibrated against field core AIP measurements at varying depths using materials and field cores from various pave- ments in different climatic conditions, including the Group B materials and pavements used in this study. The predicted changes in the binder properties with oxidative aging must be related to the changes in the asphalt mixture properties to facilitate the integration of the pavement aging model in pavement performance prediction frameworks. Therefore, a systematic study of laboratory-aged loose mixtures should be conducted, whereby loose mixtures are prepared at three levels of long-term aging and subjected to dynamic modulus and cyclic fatigue perfor- mance testing. Asphalt binders would be extracted and recovered from the long-term aged loose mixtures and evaluated. The changes in the asphalt binder properties with oxidative aging would be related to the corresponding changes in the asphalt mixture dynamic modulus values and cyclic fatigue performance. Note that the current kinetics model allows only for the prediction of G* at 64Â°C and 10 rad/s. Thus, linking changes in binder properties with oxidative aging to corresponding changes in mixture performance may require establishing a means to predict the effects of aging over the range of temperatures and loading rates actually experienced by pavements. The performance test results of the systematic aging study will be integrated into FlexPAVEâ¢ to predict pavement performance, with consideration given to the changes in mixture properties as a function of depth and time due to aging. The results will be analyzed to evaluate the effect of aging on cracking performance and to identify the depth within the pavement that is most critical to pavement performance. There is also the possibility to extend the kineticsâdiffusion modeling framework to address the impact of oxidative aging in accelerated pavement tests (APTs). The kinetics-diffusion C H A P T E R 4 Suggested Future Research
Suggested Future Research 99 modeling framework would be used to derive a tool for the prediction of the required APT conditions (i.e., time and temperature) to represent a given level of aging. In addition, a means to determine the appropriate laboratory aging conditions to produce performance test speci- mens with the same level of aging as a given APT and that has the capability to consider the effects of environmental exposure both prior to and/or after the APT and of thermal treatment during the APT should be developed.