Identification and Verification of a Suitable Crack Growth Law

IDENTIFICATION AND VERIFICATION OF A SUITABLE CRACK GROWTH LAW
Zhiwang Zhang, Reynaldo Roque, Bjorn Birgisson, Boonchai Sangpetngam

ABSTRACT
Fatigue cracking due to repeated traffic loading is one of the major distresses in asphalt concrete pavements. A mixture’s resistance to crack development and propagation affects the cracking performance of asphalt pavement. Therefore, it is necessary to acquire more insight into the crack behavior of asphalt concrete mixture, to obtain a better understanding of the cracking mechanism of asphalt pavements, and to have a practical and reliable system to determine a mixture’s resistance to crack development and propagation.
The use of mechanics is necessary to fundamentally and accurately account for a better understanding of cracking mechanism in asphalt pavements. However, literature review conducted in this investigation showed that currently, it appears there is not a comprehensive enough law that fully describes the cracking mechanisms observed in asphalt pavements.
Traditional fatigue approach may not be adequate because it does not account for complexities such as healing and stress redistributions, which are known to have a significant effect on the fracture behavior of asphalt mixtures. Research work has shown that conventional fracture mechanics approach may provide a rational way to consider the effects of stress redistributions induced by flaws or microcracks in a material. However, the relevance of the cracking mechanism described by conventional fracture mechanics to cracking mechanisms occurring in the field is questionable. It is noted that viscoelasticity and continuum damage approach provided a more fundamental approach by analyzing cracking behavior under realistic loading conditions and healing in asphalt pavements. However, this approach only considers microcracks in asphalt pavements, and assumes that microcracks grow continuously.
Therefore, it is clearly important to understand the cracking mechanism of asphalt mixtures and to develop a more rational approach to analyze the cracking behavior of asphalt mixtures. A complete testing, data acquisition, reduction, and analysis system was developed in this study to determine fracture parameters using the Superpave Indirect Tension Test (IDT) (1). However, discrepancies were observed between measured crack growth rate and field performance. Investigation of these discrepancies led to a better understanding of cracking mechanisms, and to the determination of a suitable crack growth law. This crack growth law has the following features: (1) generalized loading conditions and healing can be considered; (2) a threshold concept, was introduced, which results in discontinuous, step-wise crack growth; (3) both crack initiation and crack propagation are addressed; (4) the dissipated creep strain energy limit was identified as a threshold to crack initiation and propagation, and (5) it was found that the value of this threshold can be determined from simple creep compliance and strength tests using the Superpave Indirect Tensile Test (IDT) system.
Finally, an HMA Fracture Mechanics model was developed based on this crack growth law. Preliminary evaluation indicated that this model could rationally predict crack growth in asphalt mixtures. Therefore, further modification and evaluation of the model, including the effect of loading history and healing, were recommended.