Thermal Cracking Models for AC and Modified AC Mixes in Alaska

Low-temperature cracking is a major distress mode in Alaskan pavements because of the extreme temperature conditions—which range, in some instances, from about −50°C in winter to more than 40°C in summer. The use of asphalt modifiers in Alaskan pavements occurred over the past 15 years. These modifiers include Styrene-Butadiene-Styrene polymers, Styrene-Butadiene-Rubber polymers, ULTRAPAVE, and CRM [both the dry process (PlusRide) and the wet process]. Field observations and laboratory studies in Alaska and elsewhere indicate that the use of these modifiers would improve the low-temperature cracking resistance of pavements. The degree to which these modifiers provide beneficial effects for Alaskan pavements needs to be evaluated. The objectives of this research were (1) To characterize asphalt and polymer modified asphalt from a number of selected sites using Superpave PG grading system and to conduct thermal stress restrained specimen tests (TSRST) and Superpave IDT laboratory tests on field specimens; (2) To compare low-temperature cracking performance using field surveys; (3) To verify the applicability of the Superpave thermal cracking model (TCMODEL) and other available models for predicting low temperature cracking; and (4) To recommend guidelines for predicting minimum pavement temperatures in Alaska. Results of this study indicate, in general, significant improvement in low-temperature cracking resistance when polymer modifiers are used. Comparisons between predicted and observed low-temperature cracking using available crack propagation models, including Superpave TCMODEL, were poor. An improved regression model was developed using minimum air temperature, TSRST fracture temperature and strength, and pavement age to fit the observed field data for both conventional and polymer modified sections.