Two years ago at TRB's annual meeting, the Federal researchers held an REOB workshop and presented the findings of their laboratory evaluations to a standing room-only crowd.
Although some agencies do not specifically ban REOB, they do impose physical tests that preclude its use--effectively a ban.
To execute the plan, TFHRC provided the initial test method and 45 blends of various REOB modified asphalt binders with REOB concentrations of 2, 5, 8, 10, and 20 percent.
The unanswered question that remains is whether REOB negatively influences pavement life.
The pavement without REOB on one segment of Highway 655 showed no distress after 9 years of service.
Similarly, a section of test pavement in Minnesota (MN1-4) found to contain REOB also cracked prematurely.
At a level of 3-5 percent REOB, the variation in the physical test methods was greater than the effect of REOB.
Some evidence suggests that the presence of REOB may be detected using the bending beam rheometer.
Two independent study teams, one from AASHTO and the other from the Asphalt Institute, concluded that more research is needed on the use of REOB in asphalt.
Chemical analysis indicated it contained approximately 1.7 percent phosphoric acid, 10 percent ground tire rubber, and 19 percent REOB. The addition of 1.7 percent phosphoric acid likely would make the asphalt very stiff.
X-ray fluorescence spectroscopy can find REOB and phosphoric acid, and the handheld spectroscopy works for spot checks.
The TFHRC team will soon submit to AASHTO the draft test methods that transportation agencies can use to test for the presence of REOB in asphalt mixes.