Around the country, agencies struggle with tight budgets and contractors struggle with unpredictable material prices in a competitive market.
This is not good for our nation’s highways—they continue to deteriorate—or for our economy. But it has had one benefit: It has created strong partnerships between agencies and contractors to find ways to reduce costs without compromising performance.
A strong partnership of agencies, contractors and researchers has developed in support of increasing recycling in asphalt pavements. The percentage of both reclaimed asphalt pavement (RAP) and reclaimed asphalt shingles (RAS) that may be added to mixes is on the rise.
Trying for 25
Both RAP and RAS can save valuable asphalt binder. A recent survey the National Asphalt Pavement Association (NAPA) conducted on behalf of the Federal Highway Administration (FHWA) found that the asphalt-pavement industry recycled enough of these materials to save over 3 million tons of asphalt binder in 2010. Stated another way, the combined RAP and RAS resulted in an average of about 20% binder replacement in all mixes.
The survey showed that the average amount of RAP used in all mixes was about 18% in 2010. For RAS, the average percent in all mixes was about 0.33% in 2010. Shingles typically have greater than 20% asphalt binder, about four times that of most RAP. And, the asphalt-pavement industry recycled about 10% of the estimated 11 million tons of waste shingles produced in 2010.
While these numbers are good, the asphalt-pavement industry and its partners believe we can do better. The goal of the industry is to average 25% RAP in mixes. To help meet this goal, the FHWA formed the Recycling Asphalt Pavement Expert Task Group (ETG) co-chaired by Gerry Huber of Heritage Research Group in Indiana and Lee Gallivan, FHWA’s pavement-maintenance engineer in Indiana.
Matching performance
According to the ETG’s website at www.morerap.us, “The purpose of this Expert Task Group (ETG) is to coordinate, develop, and improve national guidance and recommendations for the asphalt pavement recycling program. This group will provide feedback as well as encourage correct utilization of recycling technologies and address construction problems with current state-of-the-practice solutions.” The ETG has been instrumental in increasing the amount of RAP allowed in mixes. Figure 1 shows that since the formation of the ETG, many states have increased the amount of RAP allowed in their mixes. Recently, the ETG decided to expand its mission to include RAS.
In FHWA’s report, Reclaimed Asphalt Pavement in Asphalt Mixtures: State of the Practice (FHWA-HRT-11-021), a number of reports on the performance of mixes containing high RAP were reviewed. These reports indicate no difference in the performance of RAP and virgin mixes. One conclusion from the report stated, “Based on an evaluation of pavements containing 30 percent RAP through the Long-Term Pavement Performance (LTPP) program, it has been determined that the performance of pavements containing up to 30 percent RAP is similar to that of pavements constructed from virgin materials with no RAP.”
Looking to go higher
A recently released FHWA report, Investigation of Low- and High-Temperature Properties of Plant-Produced RAP Mixtures, evaluated laboratory properties of high-RAP mixtures (25% and 40%) from plant-produced mixes in Indiana. The study was conducted by the North Central Superpave Center at Purdue University. Performance tests were conducted on mix to evaluate rutting susceptibility, fatigue resistance and thermal-cracking resistance. Binder recovered from the mixes also was evaluated to determine performance-graded binder characteristics and the degree of blending of the RAP and virgin binder.
In general, the testing results from this report indicate that RAP has a minimal effect on the low-temperature properties of mixes and suggests that up to 25% RAP can be added to the mix without changing the binder grade. A similar pooled-fund study (TPF-5[23]) is under way in the Northeast.
At the May 2011 RAP ETG meeting, researchers from the University of New Hampshire reported on high-RAP performance case studies. These case studies looked at projects from Alaska, Arizona, Colorado, Connecticut, New Hampshire, Ontario, Washington and Wyoming. All case studies except Wyoming’s showed comparable performance to all-virgin mixes.
Raising the roof
In 2009, NAPA published Guidelines for the Use of Reclaimed Asphalt Shingles in Asphalt Pavements (order number IS-136) to provide guidance on shingle processing, mix design, construction and performance. In summarizing research on performance of mixes using RAS, the report stated, “Research and experience have shown that RAS can work in all asphalt mixes including dense-graded, stone-matrix asphalt (SMA), and open-graded friction courses (OGFC). When used at percentages where the amount of virgin binder in the mix is 60 to 70 percent, no change in binder grade is normally required. The effect of the stiff asphalt binder in RAS improves rutting resistance, but does not appreciably affect the low-temperature properties of the mix.”
According to the survey NAPA performed for FHWA, implementation of RAS has soared since NAPA published its RAS guidelines. Between 2009 and 2010, RAS use increased 57%. While figures for 2011 are not yet available, it is expected that the amount of RAS continued to climb. Numerous states have added RAS to their asphalt-pavement specifications. Some significant research projects have been completed and more are ongoing.
A recently completed report from the Virginia Transportation Research Council, Investigation of the Use of Tear-Off Shingles in Asphalt Concrete, evaluated mix and recovered asphalt binder from plant-produced mixes using tear-off shingles. It examined volumetric properties, gradation, asphalt content, rutting, fatigue resistance and recovered binder properties. The report concluded that lab-test results were satisfactory and field experience with mixes containing tear-offs was successful. They also estimated that the Virginia DOT could have saved $600,000 if 4% to 5% RAS had been used in half the tons of mix placed in the state in 2009.
At the 5th Asphalt Shingle Recycling Forum held in Dallas in October 2011, Iowa State University reported on early test results from a pooled fund study, Performance of Recycled Asphalt Shingles in Hot Mix Asphalt (TPF-5[213]). This study evaluated plant-produced mixes from California, Colorado, Iowa, Indiana, Minnesota and Missouri. Fracture energy testing, which is an indication of crack resistance, indicated equal or better performance of mixes containg RAS.
Stiffening up
Two properties that both RAP and RAS mixes have in common is that they are typically stiffer at high temperatures. In the laboratory, RAP and RAS mixes exhibit lower fatigue performance than mixes containing all-virgin materials. While these lab numbers may initially be a concern to some, they shouldn’t. Laboratory fatigue resistance only compares the estimated number of passes to failure at a given strain in the laboratory. However, in the field, the increased stiffness of mixes containing RAP and RAS will actually result in lower strains in the pavement structure compared with mixes containing all-virgin materials. This reduced strain typically results in improved fatigue resistance for pavements constructed using RAP and RAS compared with mixes containing all-virgin materials.
Once contractors become familiar with RAP and RAS, they may have the opportunity to become innovative with commercial clients where they are not constrained by DOT specifications. Instead, the constraint is to give the client what they need at an economical price and make sure it performs.
One example is a high-load industrial project that Payne & Dolan Inc., Wis., constructed for a private client. Taking advantage of the stiffening effect of RAS, Payne & Dolan designed a base mix with a high percentage of RAS and a low air-void content. The company’s John Bartoszek jokingly refers to this as a “Gorilla Mix.” He said the base is tough because of the RAS and durable because of the low air voids. On the same job, the surface mix used RAS at a lower percentage and more typical air voids. Bartoszek said the parking lot is “tough as nails.” The contractor’s out-of-the-box thinking paid off in terms of both cost and quality.
Research is showing that up to 25% RAP can be added to mixes without changing the binder grade. Research and field experience show that mixes produced with RAS with 60% to 70% virgin binder perform better than or equal to mixes using all-virgin materials. Further, studies are showing that using RAP and RAS at higher percentages, which may require a change in binder grade, also can give good performance.
NAPA’s view is that RAP and RAS use will continue to increase as contractors, agencies and commercial developers partner to increase their use. AT
