The intent of AASHTO T 209-12, Standard Method of Test for Theoretical Maximum Specific Gravity and Density of Hot Mix Asphalt (AASHTO T 209), is to remove all air from a loose asphalt mix sample, thus establishing the theoretical maximum specific gravity (Gmm). During the test, the sample is placed in a container, covered with water, and subjected to a vacuum of 3.7 ± 0.3 kPa for 15 m while vibrated mechanically or manually to reduce the air remaining in the sample to the absolute minimum. If there is any air in the sample at the end of the test, the test results will reflect something less than a consistent Gmm value. Establishing a process that ensures the removal of all air is challenging because the technician cannot visually see through the sample to confirm that all air has been removed. Mechanical shaking methods have already demonstrated a more true and repeatable process over manual shaking methods. Research conducted in NCHRP Project 10-87(01),“Precision Statements for AASHTO Standard Methods of Test,” by the AASHTO Asphalt Materials Reference Laboratory (AMRL) concluded the difference in air voids between manual and mechanical agitation was in the range of 0.2% to 0.4%. However, Project 10-87(01) fell short by not determining the optimum mechanically derived Gmm. The research used single amplitude and frequency settings throughout the test, which does not guarantee an accurate measurement of Gmm. Using the research’s approach, a sample will become animated, settle (interlocking the particles), and begin rotating as a single mass. It becomes more difficult for any additional air release once particle interlock and rotation occur. Research is needed to develop criteria to ensure mechanical shaking of samples in AASHTO T 209 maintains full animation of individual particles throughout the test period. This project represents a first phase to refining the use of mechanical shakers within AASHTO T 209. Future phases will be required to verify the criteria for samples produced with a wide range of asphalt binders and sizes and sources of aggregate and to establish revised repeatability and reproducibility values for Gmm.
The objective of this research is to establish criteria for sample mechanical shaking in AASHTO T 209 that assures measurement of true Gmm values. The criteria shall include: (1) the optimum amplitude and frequency required to achieve full animation of individual sample particles for each unique mix sample, (2) adjustments to the amplitude and frequency required to prevent settling of the particles throughout the test period, and (3) adjustment to the specified vacuum range to compensate for the added mechanical energy.
The project final report is available here for download. The report was submitted to the AASHTO Committee on Materials and Pavements for consideration of possible changes to AASHTO T 209.