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1 EXECUTIVE SUMMARY It is well recognized that load-related top-down cracking, which initiates at or near the surface of the pavement and propagates downward, commonly occurs in hot mix asphalt (HMA) pavements. This phenomenon has been reported in many parts of the United States, as well as in Europe, Japan, and other countries. This mode of failure, however, cannot be explained by traditional fatigue mechanisms used to explain fatigue cracking that initiates at the bottom of the pavement. Therefore, this project was to identify or develop mechanistic-based models for predicting top-down cracking in HMA layers for use in mechanistic-empirical procedures for design and analysis of new and rehabilitated flexible pavements. For years, many researchers have tried to identify key factors and fundamental mechanisms that may lead to top-down cracking initiation and propagation. From these efforts, it appeared that at least two major mechanisms would need to be considered to predict top-down cracking initiation. One mechanism is related to the bending-induced surface tension away from the tire (i.e., bending mechanism), which governs crack initiation in HMA layers of thin to medium thickness. The other is associated with the shear-induced near-surface tension at the tire edge (i.e., near-tire mechanism), which explains crack initiation in thicker HMA layers. The damage induced by either of the two major mechanisms becomes more critical as aging progresses. Also, top-down cracking initiation can be influenced by thermal stresses and the presence of damage zones. After crack initiation, the presence of cracks and the associated redistribution and intensification of stresses, particularly in the presence of stiffness gradients, plays a potentially critical role during crack propagation in the HMA layers.
2 In this project, two models were identified and selected for integration into a unified predictive system because of their unique features and capabilities to address the dominant mechanisms associated with top-down cracking. The two models were as follows: ⢠a viscoelastic continuum damage (VECD) model to predict crack initiation by modeling damage zones and their effect on response prior to cracking (i.e., damage zone effects), and ⢠an HMA fracture mechanics (HMA-FM) model to predict crack propagation by modeling the presence of macro cracks and their effect on response (i.e., macro crack effects). However, the existing model components required significant further development before being suitable for integration and development of a top-down cracking performance prediction model for the Mechanistic-Empirical Pavement Design Guide (MEPDG). Therefore, the research focused on (1) finalizing and verifying the reasonableness of the two model components, and (2) developing for and integrating with the HMA-FM-based crack propagation model a simplified fracture energy-based crack initiation model. Consequently, two enhanced modelsâa VECD-based crack initiation model and a HMA- FM-based crack growth modelâwere developed to serve as the major components of the targeted system. 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. 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 into the
3 VECD-FEP++ (which is a product of incorporating the VECD model into an in-house stand- alone finite element code FEP++), and an extrapolation method was developed for predicting top-down cracking initiation in HMA pavements. A parametric study was undertaken and showed 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 has the following key elements: ⢠a critical condition concept that can accurately capture field observations and significantly reduce the computation time required for long-term pavement performance prediction; ⢠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. ⢠a thermal response model that predicts transverse thermal stresses, which can be an important factor in the top-down cracking mechanism; and ⢠a pavement fracture model that predicts crack growth over time, accounting for the effect of changes in geometry on stress distributions. In addition, 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 demonstrate the capabilities of a completed system. A systematic parametric study showed that the integrated performance model provided
4 reasonable predictions and expected trends for both crack initiation and propagation. A limited calibration/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. However, this performance model is not ready or intended for immediate implementation in the MEPDG because (1) the model 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 clearly indicates that the VECD-based model and the HMA-FM-based model developed and evaluated in this project can form the basis for a top- down cracking model suitable for use in the MEPDG. Furthermore, the component models can form the basis for an improved performance model to predict multiple cracking distresses simultaneously, including top-down cracking, bottom-up cracking, and thermal cracking. The project also identified and recommended research efforts to develop calibrated/validated top- down cracking performance models for use in the MEPDG.
