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.

The bulk specific gravity test measures a HMA sample’s weight under three different conditions (Figure 1):

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.

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).

The standard bulk specific gravity test is:

Specific gravity is a measure of a material’s density (mass per unit volume) as compared to the density of water at 73.4°F (23°C). Therefore, by definition, water at 73.4°F (23°C) has a specific gravity of 1.

Superpave mix design is a volumetric process; key properties are expressed in terms of volume. However, direct volume measurements are difficult, therefore weight measurements are usually made and then converted to a volume based on material specific gravities. Bulk specific gravity is involved in most key mix design calculations including air voids, VMA and, indirectly, VFA. Correct and accurate bulk specific gravity determinations are vital to proper mix design. An incorrect bulk specific gravity value will result in incorrectly calculated air voids, VMA, VFA and ultimately result in an incorrect mix design.

Although 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. Each one uses a slightly different way to determine specimen volume and may result in different bulk specific gravity values.

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. 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.

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. 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. 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.

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