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Field Validation of a Thermal Cracking Resistance Specification Framework for Modified Asphalt

Current Superpave® performance grade (PG) binder specifications, such as those used in Wisconsin and many other states, were developed primarily for unmodified binders and are valid only within the linear viscoelastic range of properties. Further, performance benefits from modification are not limited to continuum rheological properties because modified binders often demonstrate superior damage resistance not captured in the current PG system. This paper discusses the results of a recently completed research project sponsored by the Wisconsin Department of Transportation in Madison; the objective was to identify promising procedures through a comparison of test results with field performance and the development of appropriate modified binder specification criteria for thermal cracking resistance. A number of procedures have been introduced in recent years to address the need for damage characterization test procedures for proper assessment and selection of modified binders. In this study, the single-edge notched bending (SENB) procedure was found to correlate well with the observed field thermal cracking when tests were conducted at the project location low-temperature PG specification temperature. The device used was a modified bending beam rheometer (BBR), which could perform displacement-controlled loading with higher load cell capacity, and accommodated notched BBR beams. On the basis of the results of this study, a procedure was introduced for thermal cracking resistance potential of modified binders with the BBR-SENB test at an average annual minimum pavement temperature of +10°C. A preliminary failure limit and acceptance criterion were defined to qualify binder results in terms of the failure properties measured.