![]() 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☌).
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