The Federal Highways Administration (FHWA) Asphalt Binder Expert Task Group (ETG) advanced recommendations for several test methods and asphalt specifications to be modified to include the use of recycled tire rubber and other non-soluble modifiers in asphalt.
Ballots were drafted for consideration by members of the American Association of State Highway Transportation Officials (AASHTO) Subcommittee on Materials (SOM) in the fall of 2012. The SOM manages the development of standards and specifications for the AASHTO members, which are also used by many highway agencies worldwide.
Changes were recommended for the following standards and test methods: M320 – Performance Graded (PG) Asphalts, T44 – Solubility Test, MP19 – PG Asphalt Using MSCR, T315 – The Dynamic Shear Rheometer.
Although several states already use PG rubber modified and other rubberized asphalt materials, some were not familiar with the material and requested more information and clarity be added to the recommended changes.
Some of the states requested more specificity to which non-soluble modifiers would be included and permitted for use. For example, why not name or list the modifiers that have been and can be used? Another rationale for several non-consenting votes focused upon statistics related to the new tests. What is the precision bias between operators, labs and equipment? Some members voiced opinions that completely new standards and test methods should be developed that address particulate additives such as rubber in binder systems.
These issues were recently discussed at the spring 2013 FHWA ETG meeting and the responses are being developed for consideration at the next SOM voting opportunity this summer, August 5-8, 2013 in Nevada.
The recommendations to modify the existing specifications and methods will continue as planned but with the comments addressed. New separate standards and methods will not be developed at this time. The particulate modifiers that are allowed will be listed and described.
Look for continued discussion this summer as AASHTO moves toward the use of recycled tire rubber as a modifier to asphalt to advance sustainability and cost effectiveness of highway paving materials.
Special thanks to John D’Angelo (FHWA, retired) and the Asphalt Binder ETG for their contributions to this article.