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9 2. RESEARCH APPROACH The research approach for this project consisted of the following five phases. 1. Determination of key elements that must be part of the top-down cracking predictive system. These elements included two primary model components: ⢠A micro-damage effects model that predicts the damage induced in the HMA layer, the resulting reduction in modulus associated with the damage, and the change in stress-strain-energy response resulting from the modulus reduction. As discussed earlier, the VECD model was selected because of its ability to model damage zones and their effect on response prior to cracking. The capability of the VECD model to predict the performance of the pavement structure has been validated using different pavement structures (e.g., FHWA ALF pavements and the KEC test road in South Korea). ⢠A crack effects model that predicts the effect of cracks (which are regarded as discontinuities that reduce the effective cross-section of the pavement layer) on the stress, strain, and energy redistribution within the layer. The HMA-FM model was selected because of its ability to model the presence of macro cracks and their effect on response during crack propagation. The capability of the HMA-FM model to identify cracked from uncracked sections has been validated by using 27 field test sections selected throughout the state of Florida for evaluation of top-down cracking. 2. Identification and/or development of sub-models. Because the existing VECD and HMA-FM models did not account for several key factors and material property sub-models relevant to top- down cracking mechanisms, there was a need to identify and/or develop appropriate sub-models, as follows: ⢠Material property sub-models, including aging, healing, failure criteria, and thermal stress models for incorporation into the existing VECD model. ⢠Material property sub-models that account for changes in mixture properties (e.g., fracture energy, creep rate, and healing) with aging, and a thermal response model that predicts transverse thermal stresses for incorporation into the existing HMA- FM model. 3. Integration of sub-models into primary model components. The sub-models developed in Phase 2 were integrated into each of the existing models to form two enhanced primary model
10 components (i.e., a VECD-based crack initiation model and an HMA-FM-based crack propagation model) to form the basis of the targeted system. In addition, a simplified fracture energy-based crack initiation model was developed for and integrated with the HMA-FM-based model to demonstrate the capabilities of a completed system. This phase included the following: ⢠Integration of sub-models into the VECD model, including a sensitivity study to investigate the effect of each sub-model on pavement performance and two example simulations to demonstrate the full capabilities of the VECD-based crack initiation model; ⢠Integration of sub-models into the simplified fracture energy-based crack initiation model and into the HMA-FM model to form two core modules (i.e., the crack initiation simulation (CIS) and crack growth simulation (CGS) modules) based on a critical condition concept; and ⢠Integration of the CIS module with the CGS module (which resulted in a simplified HMA-FM-based performance model), including a sensitivity study to investigate the effects of sub-models (i.e., aging, healing, and thermal effects) on pavement performance. 4. Evaluation of the VECD-based and HMA-FM-based models. This evaluation included the following: ⢠Evaluation of the VECD-based model in a systematic parametric study to determine the reasonableness of the enhanced model. ⢠Evaluation of the simplified performance model (i.e., the product of integrating the simplified crack initiation model with the HMA-FM-based crack growth model) in a parametric study to show how reasonable are the model predictions for both crack initiation and propagation. 5. Calibration and validation of the simplified performance model. The simplified performance model was calibrated and validated using data from field sections to evaluate the accuracy of top-down cracking performance predictions in HMA layers using rheological and fracture properties of HMA mixtures. For most of the sections included in the calibration/validation studies, properties were available from tests performed in previous investigations (14). For test
11 sections for which properties were not available, Superpave IDT tests were performed on specimens obtained from field cores.
