Low-Temperature Evaluation of Kentucky Performance-Graded 70-22 Asphalt Binders

A study was initiated to ascertain whether all performance-graded (PG) 70-22 binders would perform the same in an asphalt mixture in Kentucky, regardless of method of manufacture of the binder. Five PG 70-22 binders and one PG 64-22 binder (control) were incorporated into an asphalt mixture through a hot-mix asphalt plant. Test sections were placed for each binder. The mixture was sampled at the plant and returned to the laboratory for mechanical property testing. The mechanical property tests included repeated shear test at constant height, frequency sweep test at constant height, and indirect tensile creep (ITC) test. These tests were chosen to evaluate the high-, intermediate-, and low-temperature properties of the asphalt mixtures. Only low-temperature properties are discussed. Thermal stress analysis techniques permitted the critical mixture temperature for single-event thermal cracking to be compared with critical binder temperatures. Although the critical binder temperature [calculated from bending beam rheometer (BBR) stiffness] matched the critical mixture temperature reasonably well, the critical binder temperature calculated from the BBR m-value did not match as well. The m-value critical temperature is generally higher than indicated by the low-temperature ITC testing. Although not all PG 70-22 binders performed the same in mixture testing, the research did not clarify whether differences in mixture properties for the PG 70-22 binders are related to differences in binder stiffness, differences in method of manufacture, or both.