By Mike Anderson, P.E.
If you’re still reading this after seeing the title then you’re either a really dedicated asphalt technologist, really curious, or have a childlike nature (“childlike” has positive connotations; “childish” does not. Trust me…I know from experience).
In any case, the “gas” that is being discussed here is the air used in conducting AASHTO R28, standard practice for accelerated aging of asphalt binder Using a Pressurized Aging Vessel (PAV).
The Pressure Aging Vessel (PAV) practice was adapted during the Strategic Highway Research Program (SHRP) from the POV (Pressure Oxygen Vessel) – a research test that used pressurized oxygen and a test temperature of 60°C for 144 hours (six days) to simulate the long-term aging of an asphalt binder. As these conditions were not really suitable for a purchase specification (six days!), the SHRP researchers elected to modify the POV procedure to shorten the time required.
The PAV procedure simulates long-term aging by using a slightly higher temperature (90-110°C) than the POV procedure and elevated air pressure at 2.1 MPa (300 psi) to force oxygen diffusion to occur in asphalt binder samples. The procedure takes approximately 20 hours. At the end, the air pressure is slowly released over a period of 9 ± 1 minutes at a constant rate to minimize the creation of air bubbles in the asphalt binder sample. Once the pressure has been released completely, the individual pans are placed in an oven set at 163°C for 15 ±1 minutes. The asphalt binder sample should then be “heated until sufficiently fluid to pour” and stirred gently to assist in the removal of the air bubbles. The asphalt binder sample is then scraped from the pan (or pans if multiple PAV pans are prepared for an asphalt binder sample) into a single container.
Now the aged asphalt binder is ready for testing, right? Well, yes, if this was 1994. As SHRP ended, the original PG asphalt binder specification and tests took the sample at the end of the aging procedure and allowed the technician to perform Dynamic Shear Rheometer (DSR) and Bending Beam Rheometer (BBR) tests. It wasn’t until the Direct Tension Test was refined in the late 1990s that researchers considered the possible deleterious effect of air bubbles in an asphalt binder sample on the failure stress at low temperatures. To improve repeatability in the Direct Tension Test, a vacuum degassing procedure was added to AASHTO R28 for all PAV-aged residue.
In time, for a variety of reasons, the Direct Tension Test ceased to be used by the vast majority of specifying agencies. Still, the vacuum degassing procedure remained part of the PAV practice.
In 2008 and 2009, members of the Asphalt Binder Expert Task Group (ETG) performed DSR and BBR testing on asphalt binder samples with and without the degassing procedure. Based on this data, the ETG recommended to the AASHTO subcommittee on materials that R28 should be modified to require degassing only if the samples were to be tested using the Direct Tension Test. As part of the 2011 Concurrent Ballot, this recommended change passed (47-yes, 2-no, 3-abstain).
Going forward, look for a future published version of R28 to have some changed wording that no longer requires degassing for DSR and BBR tests. Nevertheless, it is still important that the technician take the time to heat and gently stir PAV residue before testing to minimize the inclusion of air bubbles into the sample. This is just good lab practice…as everyone knows that even a little gas can cause problems at times.
Mike Anderson is the Director of Research and Laboratory Services at the Asphalt Institute.