Vacuum Degassing: Yes? No? Maybe?
In 2011, after an AASHTO subcommittee on materials vote, vacuum degassing of PAV conditioned binders residue used for DSR and BBR testing became optional and if you listened carefully, you could almost hear technicians across the U.S. cheering.
Changes to the published standards frequently add time and complexity to the test methods, not simplify them. Vacuum degassed PAV residue is still required for use in the Direct Tension test. The Asphalt Institute no longer performs Direct Tension testing so this option sounded like an excellent way to shave precious minutes off of the turn-around time we promise our clients for standard PG testing. We quit cold turkey.
If the degassing option is chosen, sections 10.15, 10.16 and Table 1 of AASHTO R 28 give detailed instructions on procedure and elevation-related vacuum gauge corrections. Given the fairly strict guidelines and tolerances described in these sections, it could easily be assumed that the degassing process is of vital importance if any reduction in variability for PAV residue test results can be expected.
Far less guidance is provided if vacuum degassing is not performed. R 28 Section 10.14 mentions stirring the residue in the pan to “…assist in the removal of air bubbles…” but this step is required whether or not degassing is performed. There is no description of an acceptable residue sample in the absence of vacuum degassing. Given this rather vague language, it could easily be assumed that degassing is not vital to reduce residue testing variability. So, why are we given an option? Here is an excellent (and funny) article
that gives more insight into that question.
Prior to the addition of vacuum degassing in the late 90’s, technicians had developed various methods for ensuring PAV residue was heated adequately and was effectively free of bubbles - brushing with a light flame after careful heating, additional heating if necessary followed by another application of the flame, for instance. There was a general understanding that bubbles = bad. After degassing became a requirement and there was a prescribed method for degassing PAV residue, many of these techniques were forgotten as technicians moved on. Why is this an important point? When the requirement for vacuum degassing was removed, no guidance was given for sample prep in its absence.
As previously mentioned, the Asphalt Institute binder lab stopped vacuum degassing. That lasted approximately six months. Soon, our technicians began reporting an increase in variability between replicants in a set of two BBR beams. Naturally, the absence of vacuum degassing became the suspected cause. After we resumed vacuum degassing the variability appeared to return to ‘normal levels’ (by normal levels we mean well below the published precision and bias limits).
So, was it the lack of vacuum degassing that increased our variability? Perhaps, but maybe not. Maybe it wasn’t the vacuum but the time and temperature in the oven. We strongly believe and teach that test samples must be heated for the shortest possible time and the lowest possible temperature that produces a properly formed specimen. PAV residue that is vacuum degassed is stored in a 170o
C oven for a minimum of 45 minutes with and without vacuum after the pans are scraped whereas non-degassed residue is typically heated at 163o
C or below for a maximum of 30 minutes. In the case of PAV residue used for forming BBR test specimens, does this reduced time and temperature allow sufficient time to pour two identical specimens before the sample cools down?
This anecdotal increase in variability reported by several binder labs after stopping vacuum degassing has spurred new discussions about its importance and many theories have come out of those discussions. Should we vacuum degas? Yes? No? Maybe? We would love to hear from technicians reading this piece. What do you do and what have been the results of that decision? Please take a couple minutes to let us know by completing the short survey below.
Who is the manufacturer of your PAV?
Does your lab vacuum degas PAV residue?
If no, do you heat the residue for less time and at lower temperatures than when vacuum degassing?
yes, we do
no, we don't
If no, have you noticed an increase in testing variability in PAV conditioned specimens?
yes, we have
no, we haven't
Encroachment. When your lab counter space was designed, it looked massive. Huge vistas of stainless steel large enough to support every instrument you already own and plenty more for those future research projects. The shiny expanses will accommodate training, overflows of summer COA samples and whatever else you choose to throw at it…Ample, expansive, generous. Until the encroachment begins.
It starts innocently enough. A new scale and hot plate tuck nicely into a couple square feet of your laboratory dominion. No big deal. Then the two new sample prep ovens and fourth DSR arrive. Biggish deal. You soon notice that your realm of repeatability has receded…until one day it happens: You are (gasp!) out of space! So take a cue from Manhattan or our friends at Associated Asphalt and build up!
Recently we took a trip to visit Stephanie Richards and the rest of the Associated Asphalt gang. We are always on the lookout for a new tip to pass on to our readers. I noticed a neat little stack of aluminum squares.
You’ve probably noticed what space hogs Loaded Pressure Aging Vessel pans can be before they get loaded into the PAV. Your former stainless expanse is now a comparatively pathetic little square on which you must multi-task so let’s make a little elbowroom.
The aluminum squares are used at Associated to stack and cover (AMRL likes covered things) filled PAV pans in preparation for loading the PAV. Using these dividers, ten pans now cleverly take up the same real estate as one.
You can use almost anything you’d like. In the photo above we used cardboard. Just cut the squares slightly larger than the diameter of your pans, alternate pan and divider and get ready to return a chunk of counter to its former glory.
- Mike Beavin, Asphalt Institute Technical Training Coordinator
Bituminous Technologies and Mariani Asphalt
Deni-Maire has worked in the asphalt industry for 2 years. In her current role she performs quality control testing and analyzes asphalt and emulsion samples.
She says the NBTC program helped her better understand the mathematics and concepts behind the formulations of testing conditions and how they affect the outcomes.
"The program has given me a thorough understanding of how to interpret the results of binder testing."