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141 CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE RESEARCH 5.1 Conclusions Two model components: (1) a VECD-based crack initiation model and (2) an HMA-FM- based crack propagation model, and one integrated system (i.e., a simplified fracture energy- based approach for crack initiation integrated with the HMA-FM-based crack propagation model) were developed in this project. The primary role of the VECD-based model is to account for damage zone effects prior to cracking and to identify the time and location of crack initiation sites. Several important material property sub-models (including aging, healing, failure criteria, viscoplasticity, and thermal stress models) were developed, modified, and/or investigated, and then incorporated into the existing VECD model. These material sub-models were then converted into and/or combined with the structural sub-models. The integrated sub-model was implemented in the VECD-FEP++, and an extrapolation method was developed for predicting top-down cracking initiation in HMA pavements. A parametric study was undertaken and shows that the VECD-based model provides reasonable predictions and trends for crack initiation. The primary role of the HMA-FM-based model is to account for macro crack effects during crack propagation and to predict the propagation of cracks over time. This model consists of the following key elements: (1) a critical condition concept that can accurately capture field observations and significantly reduce the computation time required for long-term pavement performance prediction; (2) material property sub-models that account for changes in near- surface mixture properties with aging, including increase in stiffness (stiffening), reduction in fracture energy (embrittlement), and reduction in healing potential, which together make pavements more susceptible to top-down cracking over time; (3) a thermal-response model that
142 predicts transverse thermal stresses, which can be an important part of the top-down cracking mechanism; and (4) a pavement fracture model that predicts crack growth over time, accounting for the effect of changes in geometry on stress distributions. A simplified fracture energy-based approach for predicting crack initiation (i.e., a crack initiation model that does not consider damage zone effects) was developed and integrated with the HMA-FM-based model to form a top-down cracking performance model to demonstrate the capabilities of a completed system. A systematic parametric study showed that the integrated performance model provided reasonable predictions and expected trends for both crack initiation and propagation. A limited calibration and validation using data from field sections indicated that the performance model reasonably represents and accounts for the most significant factors that influence top-down cracking in the field, but it is not ready or intended for immediate implementation because (1) the model eventually implemented in the MEPDG should capture damage zone effects, for which the VECD-based model is needed, and (2) further verification of sub-models is needed. In summary, the work performed indicates that the VECD-based crack initiation model and the HMA-FM-based crack propagation model developed and evaluated in this project can form the basis for a top-down cracking model suitable for use in the MEPDG. Furthermore, given the general nature of the component models, they can also form the basis for the next-generation MEPDG performance model to predict multiple cracking distresses simultaneously, including top-down cracking, bottom-up cracking, and thermal cracking. 5.2 Recommendations for Future Research It is recommended that the two primary model components developed in this study (i.e., the VECD-based crack initiation model and the HMA-FM-based crack propagation model) be fully integrated to form a top-down cracking performance model suitable for implementation in
143 the MEPDG. Full integration of the two models is necessary for accurate performance prediction to establish the confidence needed for implementation of a reliable MEPDG. Furthermore, the resulting model would be a suitable candidate for implementation as the next generation performance model for bottom-up cracking and thermal cracking. The general nature of the targeted system, which can predict damage and crack propagation anywhere in the pavement system, considering the combined effects of both load and environment, forms the basis for a single system to deal with multiple cracking distresses simultaneously. It is emphasized that the simplified integrated system developed in this study is not ready nor intended for immediate implementation, because (1) it is necessary to evaluate damage zone effects on performance predictions, for which the VECD-based crack initiation model is needed; (2) it is necessary to further verify the material property sub-models developed in this study (i.e., models for aging, healing, damage, and fracture criteria); and (3) validation and calibration of the integrated performance model needs to be performed on a broader range of pavements and environmental conditions. The intent of the simplified integrated system was to demonstrate the potential of a fully integrated system, while gaining experience in dealing with issues relative to integration of model components that involve very different functions (e.g., how to build a reasonable framework for integration) as well as issues relative to calibration and validation of a full system (e.g., issues associated with collection and/or use of field performance data). The experiences gained were of great benefit in helping to formulate a plan for integrating, calibrating, and validating the two model components. It is proposed that future development progress as described in the following two phases:
144 Phase One: As a minimum, the following considerations are recommended to develop a top-down cracking performance model for the MEPDG based on the two primary model components developed in this study: ⢠Aging model: There is a need to unify the material property aging submodels developed for the VECD-based and HMA-FM-based component models, including submodels for viscoelastic properties and damage and fracture properties. ⢠Healing model: There is a need to unify the healing submodels developed for the VECD- based and HMA-FM-based component models. Also, the unified healing model should be verified using data under various conditions that are needed for top-down cracking simulations. ⢠Failure criteria: It is recommended that the energy-based failure criteria defined and used in the HMA-FM-based model be incorporated into the VECD system, so that a unified and more advanced failure criterion is used by both. ⢠The near-tire mechanism: There is a need to unify the near-tire mechanism considered by the VECD-based and HMA-FM-based component models. Also, the effects of tire type and cross slope on shear-induced tension near the tire edge should be considered for incorporation into the unified near-tire mechanism. ⢠Refinement of analysis program: There is a need to reduce the computer run time of the VECD-based model through the improvement, refinement, and calibration of the sub- models which are included in the analysis system. ⢠Model calibration and validation using field data: A calibration and validation effort with a broader range of field sections is needed to ensure the accuracy of the integrated top-down cracking performance model. Phase Two: The following considerations are recommended to develop a more advanced system for future use in the MEPDG, ⢠Advanced material models: The model developed in Phase One includes modules for linear-elastic unbound paving materials. For more accurate predictions, enhancements are needed to: include a nonlinear-elastic material model, consider the effects of moisture gradients in pavement systems, and incorporate anisotropy of the HMA and slip elements at the pavement-base interface and/or between HMA layers. ⢠Traffic: Constant ESAL loading distribution was used in Phase One. It would be more realistic to use an equivalent daily loading history that has a more representative load spectra and distribution (i.e., more trucks during certain times of the day). Also, load wander likely plays a role in performance associated with the near-tire mechanism. Therefore, it should be considered in future model development.
145 ⢠Generalized cracking mechanism: The VECD-FEP++ models top-down and bottom-up damage propagation simultaneously, but an algorithm to account for the interaction of these two cracking mechanisms in macro crack propagation (or, simultaneous damage and macro crack propagation) needs to be developed for determination of critical conditions.
