A Review of Aggregate and Asphalt Mixture Specific Gravity ...

Objective and Process of the ReviewEXECUTIVE SUMMARYVolumetric properties are currently used for design and acceptance of hot-mix asphalt(HMA) mixtures. Vital to the calculation of mixture volumetric properties is theconversion of mass percentages to volume proportions/percentages using specific gravitymeasurements of the mixture and the aggregates in the mixture. The accuracy andreliability of the specific gravity measurements are therefore fundamental to the businessof building quality asphalt pavements.A task group consisting of the authors under the direction of the Federal HighwayAdministration (FHWA) Asphalt Mix and Construction Expert Task Group was tasked toconduct a critical review of specific gravity measurement methods. The objectives of thisreview were to summarize problems and issues with current methods, examine possibleimprovements and/or alternate methods, and identify areas that need further research anddevelopment.This review separately examined three specific gravity determinations, namely, the bulkspecific gravity of the aggregates (G sb ), the maximum specific gravity of asphalt mixtures(G mm ), and the bulk specific gravity of compacted specimens (G mb ). The review drawsupon information from research studies, state transportation departments, AASHTO andASTM standards, and the AASHTO Materials Reference Laboratory (AMRL). The viewof the task group has been that change(s) to the current specific gravity methods may bemotivated by one or more of the following three reasons:1. The change(s) or new method(s) will provide specific gravity results closer to thetruth (i.e., greater accuracy),2. The change(s) or new method(s) will yield more repeatable results (i.e., betterprecision), and3. The change(s) or new method(s) will be faster, easier, and/or less expensive.FindingsThe primary standard test methods for determining the bulk specific gravity of coarseaggregates are AASHTO T 85 and ASTM C127. Disadvantages of these methods include(1) the inconsistency of the visual method for determination of the SSD condition; and(2) the required soak time of at least 15 hours. These disadvantages are addressed inalternative methods for determining G sb of coarse aggregates, including the vacuumsealing procedure (i.e., the commercially available CoreLok device). Recent studiesshowed that the vacuum-sealing method produced higher G sb values that weresignificantly different (both statistically and practically) from those determined byAASHTO T 85, especially for highly absorptive coarse aggregates. In addition, thevacuum-sealing and AASHTO T 85 procedures had similar reproducibility.With respect to the determination of bulk specific gravity of fine aggregates, the primarystandard test methods are AASHTO T 84 and ASTM C128. Disadvantages of these testi