Summer and softball, lamb and mint, fast horses and bluegrass. They all make the best of pairings.
But now -- thanks to recent research conducted by the Federal Highway Administration, and publicized this spring in FHWA's Focus newsletter -- we can add reclaimed asphalt pavement (RAP) and Superpave to that list.

Yes, we at Wirtgen America Inc. concede that asphalt millings and Superpave asphalt mix designs don't have the same enticing aura that sports, food and racing do. Yet few recent developments in civil engineering may have more impact than this on the future of asphalt cold milling and asphalt recycling -- and the futures of our customers -- because this new research further institutionalizes asphalt recycling, moving it even deeper into the mainstream of roadbuilding.

New Attention To Old Problem
The new attention to RAP's compatibility with Superpave comes at an opportune time, because questions lingered as to whether RAP could be used in the new high-performance Superpave mixes.
That's why it was gratifying to see that in April, FHWA's Focus newsletter cover story proclaimed RAP and Superpave "an excellent blend". The article cited recent research conducted for National Cooperative Highway Research Program [NCHRP 9-12], Incorporation of Reclaimed Asphalt Pavement in the Superpave System.

Superpave -- an acronym for SUperior PERforming Asphalt PAVEments -- is a performance-based system of specifications for designing asphalt pavements to hold up to the demanding roadway needs of the new century. This performance-based approach offers more durable pavements that are specifically designed with local temperature extremes and traffic loads in the equation. Using performance-graded [PG] binders, Superpave designs are thought to provide longer-lived asphalt pavements that will stand up to local climate and traffic volumes at lower costs. Final specs are due in 2005.

In the meantime, asphalt recycling using RAP still is overcoming prejudices and reluctance among some state DOTs to spec RAP in all mixes. While the April Focus article sanguinely says that nearly 30 million tons of RAP are recycled in hot mix asphalt (HMA) pavements each year, and "routinely used in all 50 states" -- it's our experience that much more RAP can be used both in terms of volume and in various specifications.

Frankly, we'd like to see RAP used more frequently in riding or friction course mixes, not just leveling or base courses. But that's for each state to decide, based on their experience and comfort level, and we respect both their expertise and diligence in making sure their citizen customers get the best mileage for their tax dollars. The NCHRP report itself says use of RAP in Superpave pavements is desired because

• RAP has performed well in the past and is expected to perform well in Superpave mixtures, if properly accounted for in the mix design

• Use of RAP is economical and can help to offset the increased initial costs sometimes associated with Superpave binders and mixtures, and

• Use of RAP conserves natural resources, and avoids disposal problems and associated costs.
  While the original specs for Superpave did not include RAP, the industry has been vigorously working to see if RAP could fit into Superpave. That's why this new research demonstrating the applicability of RAP to the demanding requirements of Superpave is so intriguing.

RAP Just A 'Black Rock'?
RAP's presumed role in Superpave mixes hinged on whether it served as nothing more than an aggregate -- the so-called "black rock" -- or whether the aged, residual liquid asphalt on the surface and in the pores of RAP interacted in a physical manner with the Superpave binder and mix design.

• Thus, a "black rock study" investigated the question of whether RAP acts like a black rock or whether there is, in fact, some blending that occurs between the old and new binders.
  

• Also, a "binder effects study" examined issues related to RAP binder testing, applicability of certain tests to RAP binders, and the effects of RAP content and stiffness on blended binder properties.
  

• Finally, a "mixture effects study" was directed at assessing the effects of the added RAP on totalmixture properties as measured by shear, indirect tensile and beam fatigue testing so important to Superpave criteria.
  

"Significant findings include the conclusion that RAP is not [just] a black rock and significant blending does occur," reports NCHRP 9-12. "This means that the use of blending charts is appropriate." Other findings say there is a threshold level of RAP below which its effects are negligible, between 10 and 20 percent, depending on RAP binder stiffness. Ultimately, a three-tiered approach for RAP usage was recommended by the Mixture Expert Task Group. This approach says up to 15 percent RAP could be used with no change in PG binder grade; between 15 and 25 percent RAP, the virgin binder grade should be decreased one grade (6 deg increment) on both the high and low temperature grades; and above 25 percent RAP, blending charts should be used to determine how much RAP could be used.

Also, the properties of the aggregate in the RAP may limit the amount of RAP that can be used. Aggregate properties should be considered as if the RAP is another aggregate stockpile, and if needed blended with virgin aggregates so that the final blend meets the consensus properties.
In addition to the research and rationale for these findings, the report includes a manual for field and laboratory technicians, an implementation plan for moving these results into practice, and a procedure to to verify the PG grade of a binder in a sample of hot mix asphalt.
The report can be downloaded in its entirety at http://trb.org/trb/publications/nchrp/nchrp_w30-a.pdf. We hope you'll give it a closer look.

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