BACKGROUND: Until recently, load-associated fatigue cracking of hot-mix asphalt (HMA) concrete-surfaced pavements that occurs in the wheel path has 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 indicate 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 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.
Recent work completed under NCHRP Project 1-42 provided further review of some of the issues related to top-down cracking (see Special Note A). However, additional research is needed to address these and other issues associated with top-down cracking and to develop mechanistic-based models for use in mechanistic-empirical procedures for design and analysis of new and rehabilitated flexible pavements.
OBJECTIVE: The objective of the research 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.
Status: The project is complete. The research reviewed available information relevant to top-down cracking of hot-mix asphalt layers, considered the factors that contribute to this form of cracking, and developed models for the initiation and propagation of such cracking. These models provide a basis for the development of performance models that can be used in the mechanistic-empirical pavement design and analysis of flexible pavements. The final report is available as NCHRP Web-Only document 162, "Top-Down Cracking of Hot-Mix Asphalt Layers: Models for Initiation and Propagation" at https://www.trb.org/main/blurbs/164029.aspx