Comparison of the saturated surface-dry and vacuum sealing methods for determining the bulk specific gravity of compacted HMA

Recently, a new method of determining the bulk specific gravity of compacted hot mix asphalt (HMA) has been developed. This method uses a vacuum-sealing device to tightly conform a plastic bag around compacted samples. A number of recent studies have shown that this method provides a good estimation of bulk specific gravity, even when samples have low density (i.e., high air voids). However, most of the work has focused on laboratory compacted samples (e.g., Superpave gyratory compactor samples). Also, guidance on when this new test method should be used instead of the saturated surface dry method has not been provided. The objective of this study was to first confirm that the vacuum sealing method does provide an accurate estimation of bulk specific gravity of compacted HMA. If the vacuum-sealing method did provide an accurate estimation of bulk specific gravity, the second primary objective of this study was to provide guidance when the vacuum-sealing device should be used instead of the saturated surface dry method. Two separate experiments were conducted to accomplish the objectives. First, samples prepared in the lab, with the Superpave gyratory compactor, were tested using the saturated surface dry method and the vacuum-sealing method. The second experiment included the testing of 355 cores cut from the roadway prior to traffic from 42 different field projects, again with both methods. Based upon the results of this study, the vacuum-sealing method did provide an accurate estimation of bulk specific gravity. It was concluded that for lab prepared HMA samples having gradations passing above the maximum density line, the saturated surface dry test method provided accurate results for the air void content range encountered. However, the vacuum-sealing method should be used for lab prepared HMA samples having gradations passing below the maximum density line. This included mix design samples. On average, the use of the vacuum-sealing method during mix design would have resulted in the addition of about 0.2% more asphalt binder. For field compacted samples, it was concluded that the saturated surface dry method was adequate for HMA mixes having gradations more than 10% finer than the maximum density line on the 2.36-mm (No. 8) sieve. The vacuum sealing method should be used on field compacted samples for all other gradation shapes.