Examination of Gamma-Ray Method for Measuring Bulk Specific Gravity of Hot-Mix Asphalt Concrete

Much attention has been given to the determination of bulk specific gravity (G_mb) of compacted hot-mix asphalt (HMA) specimens. For relatively open-graded HMA mixtures, such as those produced by Superpave® and stone matrix asphalt design methods, perceived potential shortcomings in traditional test methods may be pronounced. Recently, a new device was introduced for measuring G_mb that features the use of gamma-ray technology. To evaluate the efficacy of the device, 221 compacted HMA specimens were tested with three test methods: saturated surface dry (SSD) (AASHTO T-166), vacuum-sealing (Corelok), and gamma-ray technology (CoreReader). Each specimen was tested three times in each of the three methods. The specimens represented field-sampled and laboratory-prepared mixtures, 12.5-mm and 25-mm nominal maximum aggregate sizes, and a variety of aggregates typically used in Arkansas. Test results indicated that statistically significant differences exist between mean G_mb-values generated by each of the three testing methods. In general, the vacuum-sealing method yielded the lowest G_mb-values, followed (in order of low to high) by the gamma-ray and SSD methods. Significant differences in the variability of test results were not observed between test methods. In ease of testing and testing variability, the CoreReader gamma-ray device does not appear to offer significant advantages for determining the bulk specific gravity of compacted HMA cores. Further, departments of transportation wishing to replace or allow alternatives to traditional SSD-based tests should consider the impact of test results on existing volumetric property specifications.