![]() This method determines volume similarly to the water displacement method but uses a melted paraffin wax instead of water to fill a specimen’s internal air voids (Figure 3). One critical problem with this method is that if a specimen’s air voids are high, and thus potentially interconnected (for dense-graded HMA this occurs at about 8 to 10 percent air voids), water quickly drains out of them as the specimen is removed from its water bath, which results in an erroneously low HMA sample volume measurement and thus an erroneously high bulk specific gravity. This SSD condition allows for internal air voids to be counted as part of the specimen volume and is achieved by soaking the specimen in a water bath for 4 minutes then removing it and quickly blotting it dry with a damp towel. SSD is defined as the specimen condition when the internal air voids are filled with water and the surface (including air voids connected to the surface) is dry. The most common method (and the one described in the Test Description section), calculates the specimen volume by subtracting the mass of the specimen in water (Figure 2) from the mass of a SSD specimen. The difference in weights can then be used to calculate the weight of water displaced, which can be converted to a volume using the specific gravity of water. These methods, based on Archimedes Principle, calculate specimen volume by weighing the specimen (1) in a water bath and (2) out of the water bath. Each one uses a slightly different way to determine specimen volume and may result in different bulk specific gravity values. ![]() Methods of Determining Bulk Specific GravityĪlthough the Test Description section describes the standard AASHTO T 166 saturated surface dry (SSD) water displacement method, there are a number of other methods available. An incorrect bulk specific gravity value will result in incorrectly calculated air voids, VMA, VFA and ultimately result in an incorrect mix design. Correct and accurate bulk specific gravity determinations are vital to proper mix design. Bulk specific gravity is involved in most key mix design calculations including air voids, VMA and, indirectly, VFA. However, direct volume measurements are difficult, therefore weight measurements are usually made and then converted to a volume based on material specific gravities. Superpave mix design is a volumetric process key properties are expressed in terms of volume. Therefore, by definition, water at 73.4☏ (23☌) has a specific gravity of 1. Specific gravity is a measure of a material’s density (mass per unit volume) as compared to the density of water at 73.4☏ (23☌). ASTM D 2726: Bulk Specific Gravity and Density of Non-Absorptive Compacted Bituminous Mixturesįigure 1.AASHTO T 166: Bulk Specific Gravity of Compacted Bituminous Mixtures Using Saturated Surface-Dry Specimens.The standard bulk specific gravity test is: HMA bulk specific gravity is needed to determine weight-volume relationships and to calculate various volume-related quantities such as air voids and voids in mineral aggregate (VMA). Using these three weights and their relationships, a sample’s apparent specific gravity, bulk specific gravity and bulk SSD specific gravity as well as absorption can be calculated. Saturated surface dry (SSD, water fills the HMA air voids).The bulk specific gravity test measures a HMA sample’s weight under three different conditions (Figure 1): The bulk specific gravity test is used to determine the specific gravity of a compacted HMA sample by determining the ratio of its weight to the weight of an equal volume of water.
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