| It is well recognized that load-related top-down cracking commonly occurs in HMA pavements. This phenomenon has been reported to occur 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. The primary objective of this research is therefore to identify the causes of top-down cracking and to develop mechanistic-based models for predicting this type of distress in HMA layers.The HMA fracture mechanics (HMA-FM) model developed at the University of Florida (UF) was determined as necessary to form the basis of this study because of its associated threshold concept that is suitable for developing rules for both top-down cracking initiation and propagation, and its ability to model the presence of macro cracks and their effect on response. However, the effect of aging and healing on top-down cracking performance during the entire service life of asphalt concrete pavement had not been considered. Also, the effect of transverse, as opposed to longitudinal, thermal stresses had not been addressed. Therefore, the framework of the existing HMA-FM model was modified by identifying and including appropriate sub-models to address these key factors, based on which the framework of the targeted model was formed. Consequently, the targeted model/system, which was termed the HMA-FM-based model, was completed by development and integration of several key elements into the finalized framework, including material property sub-models that accounts for changes in mixture properties (e.g., fracture energy, creep rate, and healing potential) with aging, and a thermal response model that predicts transverse thermal stresses.The HMA-FM-based model placed emphasis on the bending mechanism. A systematic parametric study showed that the model provided reasonable predictions and expected trends for both crack initiation and propagation. Furthermore, a full calibration of field sections indicated that the predictive system adequately represents and accounts for the most significant factors that influence top-down cracking in the field. The validation efforts using the prediction sum of squares (PRESS) procedure further established the viability of the predictive system. In conclusion, the work performed clearly indicates that the HMA-FM-based model developed and evaluated in this study should form the basis for a top-down cracking model suitable for use in the mechanistic-empirical pavement design guide (MEPDG). (Full text of this dissertation may be available via the University of Florida Libraries web site. Please check http://www.uflib.ufl.edu/etd.html)... |
