Determining Air Void Content of Compacted Hot-Mix Asphalt Mixtures

A project to develop a new method or adapt a current method with wide applicability for determining bulk specific gravity of compacted hot-mix asphalt (HMA) mixtures (G_mb) is described. A more reliable G_mb would result in more reliable HMA volumetric properties, specifically, percent air voids. Consequently, pavement distress types such as rutting, bleeding, stripping, and age hardening (whose occurrence is related to percent air voids, among other factors) could be avoided more often. The project goal was accomplished in three steps. In Step 1, a literature review and survey of state departments of transportation revealed 13 existing G_mb determination techniques. In Step 2, a feasibility study of the seven methods selected was conducted to evaluate cost, logistical factors, and preliminary repeatability. In the final step, 50 compacted HMA samples and four aluminum cylinders were used to evaluate the precision and accuracy of the four methods selected. The dimensional analysis (AASHTO T269) and the parafilm (ASTM D1188) methods were found to form upper bounds for true sample air voids, whereas the saturated surface-dry method (AASHTO T166) was found to form a lower bound for true air voids. Although the true air voids can never be determined, for 90% of Tennessee Department of Transportation sample groups tested, the InstroTek Corelok method yielded results in this range between the upper and lower bounds for accurate air void results. In the precision and accuracy evaluation, all four methods were found to be capable of producing high-precision results.