The project identified, based on literature review (a) mechanisms the govern initiation and propagation of top-down cracking, (b) laboratory tests of HMA mixtures for determining susceptibility to top-down cracking, (c) significant factors associated with the occurrence of top-down cracking, and (d) models for predicting the initiation and propagation of top-down cracking in hot-mix asphalt (HMA) layers.

Until recently, load-associated fatigue cracking of hot-mix asphalt (HMA) concrete-surfaced pavements that occur in the wheel path have been thought to always initiate at the bottom of the HMA layer and propagate to the surface. However, recent studies have determined that load-related HMA fatigue cracks can also be initiated at the surface of the pavement and propagate downward through the HMA layer. The penetration of water and other foreign debris into these cracks can further accelerate the propagation of the crack through the HMA surface layer. These studies indicated that environmental conditions, tire-pavement interaction, mixture characteristics, pavement structure, and construction practices are among the factors that influence the occurrence of this cracking. Hypotheses regarding the top-down cracking mechanisms have been suggested; test methods for evaluating HMA mixture susceptibility to cracking have been proposed; and preliminary models for predicting crack initiation and propagation have been developed. However, only limited research has been performed to evaluate and validate these hypotheses, test methods, and models. Thus, research is needed to evaluate these hypotheses and develop models for predicting initiation and propagation of top-down cracking in HMA layers. This project provided background information that should help in the process of achieving this objective. Further work will be performed under NCHRP Project 1-42A.

Research included a review of relevant domestic and foreign literature; preliminary evaluations of relevant hypotheses, test methods, and models; and synthesis of relevant information.