, Professor, Civil, Environmental and Geo-Engineering
Good fracture properties are critical for the performance of asphalt pavements built in the northern part of the United States and in Canada, for which the predominant failure mode is low-temperature cracking. In the first phase of this national pooled-fund study, both traditional and new experimental protocols and analyses were applied to a statistically designed set of laboratory-prepared specimens and to field samples from pavements with well-documented performance to determine the best combination of experimental work and analyses to improve the low-temperature fracture resistance of asphalt pavements, resulting in a number of important findings and recommendations. This project, phase two, continued the research performed in phase one, resulting in a number of significant contributions to this comprehensive research effort. Two fracture testing methods were proposed, and specifications developed, for selecting mixtures based on fracture energy criteria. A draft SCB specification was developed that received approval by the ETG and has been taken to the AASHTO committee of materials. In addition, alternative methods were proposed to obtain the mixture creep compliance needed to calculate thermal stresses. Dilatometric measurements performed on asphalt mixtures were used to more accurately predict thermal stresses, and physical hardening effects were evaluated, resulting in an improved model to take these effects into account. In addition, two methods for obtaining asphalt binder fracture properties were developed. A new thermal cracking model, called "ILLI-TC," was also developed and validated. This model represents a significant step forward in accurately quantifying the cracking mechanism in pavements compared to the existing TCMODEL. Finally, the research conducted a comprehensive evaluation of the cyclic behavior of asphalt mixtures that may hold the key to developing cracking-resistant mixtures under multiple cycles of temperature.