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5 1. BACKGROUND 1.1 Problem Statement It is now well accepted that top-down cracking (i.e., cracking that initiates at or near the surface of the pavement and propagates downward) commonly occurs in hot-mix asphalt (HMA) pavements. The phenomenon has been reported in many parts of the United States (1, 2, 3), as well as in Europe (4, 5), Japan (6), and other countries (7). This mode of failure cannot be explained by traditional fatigue mechanisms used to explain load-associated fatigue cracking that initiates at the bottom of the HMA layer. The lack of an appropriately verified description (i.e., model) of the mechanisms that lead to this type of cracking makes it difficult to consider this form of distress in the design process. Several researchers have developed hypotheses regarding the mechanisms of top-down cracking (8). Some researchers have also proposed experimental methods that may provide the properties necessary to evaluate the susceptibility of HMA mixtures to this type of distress (9, 10). In addition, researchers have performed analytical work that has led to the development of preliminary models that offer the potential to predict the initiation and propagation of top-down cracks (8, 11, 12). However, only limited research has been performed to evaluate and validate these hypotheses, test methods, and models. Research was needed to further evaluate the causes of top-down fatigue cracking and to develop effective laboratory testing systems and models to account for it in design and allow for selection of HMA mixtures and pavement structures that are resistant to top-down fatigue cracking for the expected loading and environmental conditions. 1.2 Research Objective The research focused on:
6 1) Finalizing the two primary model components (i.e., a viscoelastic continuum damage (VECD) model for crack initiation and an HMA fracture mechanics (HMA-FM) model for crack propagation), involving development and integration of sub-models that are relevant to dominant top-down cracking mechanisms into each model component; 2) Verifying the reasonableness of the two enhanced primary model components (i.e., the VECD-based model and the HMA-FM-based model); and 3) Developing for and integrating with the HMA-FM-based crack propagation model a simplified fracture energy-based crack initiation model to illustrate the potential of a completed system and to help formulate a plan for integrating, calibrating, and validating the two enhanced primary model components. 1.3 Research Scope 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 mechanism 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 mechanism 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.
7 In this project, two models were identified and selected for further development (to form the basis of a comprehensive predictive system) because of their unique features and capabilities to address the dominant mechanisms associated with top-down cracking. The two models are 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). The capability of the VECD model to predict the performance of the pavement structure has been validated using different pavement structures. One example uses the FHWA Accelerated Load Facility (FHWA ALF) pavements and another example is the Korea Express Highway (KEC) test road in South Korea (13). 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). 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 (14). Because several factors (e.g., aging, healing, and thermal stress) and material property models (e.g., fracture energy, creep compliance rate, and characteristic damage curve) which are relevant to the dominant top-down cracking mechanisms were not addressed by the existing component models, sub-models that properly address these critical factors were developed and incorporated into each of these two models. 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. A parametric study was undertaken to show how reasonable are the VECD-based model 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. To demonstrate the capabilities of a completed system, a simplified fracture energy-based crack initiation model (that does not consider damage zone effects) was developed and integrated with the HMA-FM- based model. A parametric study was carried out to show how reasonable are the integrated
8 performance model predictions for both crack initiation and propagation. A limited calibration/validation using data from field sections in Florida and Minnesota was conducted to demonstrate how reasonably the performance model represents and accounts for the most significant factors that influence top-down cracking. A broader calibration/validation is necessary after the VECD-based model is integrated with the HMA-FM-based model.
