By Amy Chiconas
“This is an exciting time to be in asphalt recycling research,” Dr. Amy Epps Martin said. She is a professor and the A.P. & Florence Wiley Faculty Fellow in Texas A&M’s Zachry Department of Civil Engineering. “My dad began his research career in the field of asphalt recycling in the 1970s and 80s when oil prices were changing dramatically. And now, under similar global circumstances, I have the opportunity to build on and continue his research.”
Almost 50 years after the Federal Highway Administration began encouraging asphalt pavement recycling, the industry today is well aware of its economic, environmental and engineering impacts. The National Asphalt Pavement Association reported that about $2.8 billion of asphalt binder and natural aggregates were conserved in 2013 by using reclaimed and reused asphalt binder materials, versus virgin material. But dare mention the word “recycling” to a room full of highway officials or agency personnel, and looks of frustration are likely to ensue.
Epps Martin explained: “Two of the most common challenges experienced with recycled asphalt mixes are a stiff, brittle mix that is prone to cracking and the unknown effects of aging on rejuvenated asphalt.”
Challenge and Rejuvenation
“Although pavement recycling can have positive economic, environmental and engineering impacts, increasing the amount of reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) in asphalt mixtures also increases the potential for construction and performance issues as the mix becomes stiffer and more brittle,” Richard Steger, P.E., explained. Steger is the platform manager of rehabilitation technologies at pavement additives manufacturer Ingevity, based in North Charleston, South Carolina.
Some of the issues include decreased workability; low temperature cracking; fatigue cracking; reflection cracking; and raveling with subsequent aging or moisture damage, as listed in Interim Report Project No. 9-58, The Effects of Recycling Agents on Asphalt Mixtures with High RAS & RAP Binder Ratios.
“The use of an asphalt rejuvenator that allows the use of more recycled materials in pavements can address some of these challenges by restoring the aged asphalt characteristics to a consistency level that is appropriate for construction purposes and for the end use of the mixture,” Steger said. Rejuvenators are designed to restore the aged asphalt to its optimal chemical characteristics for durability, as well as to provide sufficient additional binder to coat new aggregate and to satisfy mix design requirements, as discussed in the presentation “Rejuvenator Characteristics, Blend Characteristics, and Proposed Mix Design Method,” at the AAPT Symposium, March 2015.
Ingevity chemist Dennis Muncy is also familiar with asphalt rejuvenators. He explained: “When oxidized binders are fully incorporated into mixes, asphalt producers can maximize the full value of their reclaimed materials. As more RAP and RAS are used, the difficulty of effectively blending the recycled binder with virgin materials also increases. This is where a rejuvenator excels.”
Asphalt rejuvenators are designed to improve the contribution yield of binder from recycled materials. “Rejuvenators like Ingevity’s Evoflex® CA can offset the potential negative impact of increasing the use of highly oxidized materials,” Muncy said. “And greater amounts of reclaimed products can be added because a rejuvenator maintains flexibility and low temperature crack resistance.”
At Ingevity’s laboratory in North Charleston, chemists are engineering asphalt rejuvenators to improve the performance of high recycle mixtures. Ingevity is working closely with agencies across the United States to positively impact recycled mixtures in three key areas: improved workability, meeting in-place performance grade (PG) specifications, and mixture performance.
Improve Mix Performance Characteristics
“What we see with many contractors in the upper Midwest is that their high RAP and RAS mixtures are too lean and dry,” Bob Siffert said. He’s the technical marketing manager at Ingevity. “We experience challenges with coating, workability and excessive raveling.”
Siffert said that in these cases, rejuvenators can create an asphalt mixture that is better coated and better performing. The mix looks better aesthetically and it is far more workable. Rejuvenators are making issues like long hauls and cold weather a thing of the past for contractors’ high RAP mixes. “These don’t have to be deal breakers anymore,” Siffert said.
Contractors can blend an additive directly at the plant, and plant operators can then optimize dosing depending on ambient conditions, the percentage(s) of RAP and/or RAS, haul distance, and any other necessary considerations. “Ultimately,” Siffert said, “rejuvenators are helping contractors improve the compaction of recycled mixes, as well as move mix through their plants faster.”
The Missouri Department of Transportation (MODOT) is a stand-out example of an agency with a specification that allows increased recycled content using binder dumping and/or extraction to determine the in-place performance grade of asphalt binder in the total mix. MODOT allows increased recycle percentages for both Superpave and non-Superpave mixes, but also considers many other factors related to mix contents, volumetric characteristics and performance.
“Our specifications have continually improved over the last few years,” Dan Oesch, field materials engineer with MODOT, said. “And our focus is addressing the concerns of increased recycle amounts. Recycle content in our asphalt mixes is a big reason why we’ve been able to accomplish much within our limited budgets.”
“We see contractors in Missouri really optimizing the economics of the mix,” Victoria Woods explained. She’s in technical marketing at Ingevity. “Here, we focus on the total binder stiffness added to a mix with increased recycle.”
The in-place PG binder of the total mix can be improved by incorporating a rejuvenator. Woods said that adding or increasing the amount of rejuvenator helps control both high and low PG temperatures of in-place binder that may be introduced with increased recycle contents. Selection of the virgin binder may also be optimized while considering all components of the mix that contribute to actual PG binder in place.
Robert Lee, P.E., and flexible pavements director at the Texas Department of Transportation (TXDOT), is leading the state’s efforts behind balanced mix designs that use performance tests as a way to improve the performance of high recycled mixtures. “We are currently using the Hamburg Wheel Tracking Test to measure rutting and the Texas Overlay Tester to measure fatigue,” Lee said. “Our goal is to provide the contractor with flexibility in building a mixture that fits between the bookends of the performance tests. Contractors will be able to optimize recycle content with appropriate virgin binder grade selection, along with a rejuvenator and/or warm mix additive if the performance criteria of the balanced mix design approach are met.”
The National Cooperative Highway Research Program (NCHRP) is also studying the effects of rejuvenating agents on asphalt mixtures with high RAP and RAS binder ratios. Epps Martin, the principal investigator on NCHRP’s Project 9-58, said, “We’re evaluating whether higher recycled binder ratios up to 0.5 can be utilized if rejuvenating agents are used. Our team has developed a method that balances RAP and RAS contents with their corresponding aging states and rejuvenating agent dosage determined at a maximum to partially restore cracking resistance while maintaining rutting resistance.”
Epps Martin also noted a few industry and academia trends to watch as the use of RAP and RAS continues to grow. “We expect to see broad use of the dosage selection method and an associated set of guidelines for characterizing binder blends and corresponding mixtures, and we are already incorporating our research into civil engineering courses here at Texas A&M.”