ASPHALT ANSWERS

Asphalt Article December 28, 2000
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Bulk specific gravity of the fine aggregate is used in Superpave volumetric mix design calculations to determine the amount of asphalt binder absorbed by the aggregate and the percentage of the voids in the mineral aggregate.

The National Center for Asphalt Technology (NCAT), in cooperation with the Federal Highway Administration, has completed preliminary study to develop an automated equipment and method of determining the saturated surface-dry (SSD) condition of the fine aggregate so that its bulk specific gravity can be measured with satisfactory precision and accuracy.

NCAT has developed a prototype device for establishing the SSD condition of the fine aggregate. The wet sample of the fine aggregate is placed in a rotating drum with flights and is subjected to a steady flow of warm air. The temperature gradient of the incoming and outgoing air, and the relative humidity of the outgoing air, is monitored continuously to establish the SSD condition. As soon as the fine aggregate particles have no moisture on the surface (SSD condition), both temperature and the relative humidity of the outgoing air are reduced. At that time, the fine aggregate is taken out of the drum to measure its SSD mass in air and its SSD volume in a calibrated pyonometer. NCAT tested seven fine aggregates for bulk specific gravity with the existing standard test method using the cone as well as the NCAT SSD device. The test data has shown that the concept of temperature gradient and relative humidity is reasonable and valid.

Equipment manufacturers have been invited to build the commercial version of the NCAT SSD device. If the commercial version has the capacity of continuously monitoring the mass of the fine aggregate while it is drying in the drum, the sample does not have to be taken out of the drum for weighing when it reaches the SSD condition. The drying can then continue until the sample is completely dry to obtain its dry mass while in the drum. The difference between the SSD mass and the dry mass is the water absorption of the fine aggregate. The apparent specific gravity of the aggregate, which does not involve the SSD condition and is reasonably reproducible, can be measured in a separate test. The bulk specific gravity of the fine aggregate can be calculated by a formula using its percent water absorption, obtained in the NCAT SSD device and its apparent specific gravity.

About the author: 
Kandhal is the Associate Director at the National Center for Asphalt Technology at Auburn University. You may write him in care of the editor.
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