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The laboratory of the National Crushed Stone Assn. has long been concerned with the specific gravity of aggregates for bituminous mixtures. About ten years ago, a limited study was undertaken in which four samples of coarse aggregate were coated with asphalt by dipping. The results indicated that the aggregates absorbed some asphalt, but the asphalt absorption was less than water absorption. Therefore, neither the bulk nor the apparent specific gravity values of the aggregate were applicable for computing the theoretical maximum density of a bituminous mix. In 1951, experiments were begun with the direct measurement of voids in compacted specimens by use of an air meter such as is used for measuring entrained air in concrete (1). While the equipment used was not particularly suited for this application, the results were encouraging. Still we were not completely satisfied with this procedure because there was always some doubt that all of the voids were being measured. The vacuum saturation procedure was first used in connection with the air meter tests on compacted specimens. This test was performed with the air meter chamber filled with water; thus, there was a possibility that the test specimen was absorbing some water while the equipment was being prepared. In order to minimize this source of error, specimens were initially saturated with water after evacuation of air. With porous mixtures it was found that this procedure filled practically all of the measurable voids.
Rice, James M.
Research and Testing Engineer, National Crushed Stone Association, Washington, D.C.,