Reclaiming Roads
A national effort is underway to capitalize on the economic and environmental benefits of recycling asphalt pavements.
According to the National Asphalt Pavement Association (NAPA), more than 90 percent of U.S. roads and highways are paved with hot-mix asphalt (HMA). As the Nation's infrastructure ages, Federal, State, and local transportation agencies face a growing demand for the raw materials needed to maintain and rehabilitate these roads. Responding to the shrinking supply of raw materials and the rising costs of aggregates and binders, HMA producers are turning to reclaimed asphalt pavement (RAP) as an alternative to virgin materials and a valuable ingredient in HMA.
RAP is the product of milling and removing asphalt from roadways and stockpiling it at HMA plants. A stockpile can include RAP from multiple roadways. If a contractor does an effective job of milling RAP from a roadway, often no further processing is needed before its addition to new HMA. In many cases, however, especially where large amounts of it are to be used, contractors further process RAP through sizing or fractionating (separating RAP into at least two sizes, typically a coarse fraction and a fine fraction).
Departments of transportation (DOTs) and contractors use the RAP as a substitute for a portion of the aggregate and asphalt binder in paving projects. They mix the RAP in with virgin aggregate and binder — in varying proportions — and transport the material to jobsites, where crews place it on roadways. DOTs also use RAP in other construction applications such as granular base or subbase, stabilized base aggregate, or embankment and fill material.
According to a Federal Highway Administration (FHWA) survey, as of 2007, the average amount of RAP incorporated into HMA mixtures by State DOTs was 12 percent by weight. Historically, State DOT specifications have set limits on the maximum amount of RAP permitted in HMA and further restrict the material's use in surface layers, certain mixture types, and large or critical projects. The restrictions largely stem from the fact that RAP amounts greater than 15 percent can require changes in the binder performance grade and additional tests. In addition, DOTs are concerned about the consistency, quality, durability, and performance of high-RAP pavements, and whether mixtures with high amounts of RAP might yield inferior pavements with shorter service lives.
In-Place Recycling ResourcesThe three most common methods of recycling RAP into HMA are hot-mix recycling at the plant, hot in-place recycling, and cold in-place recycling. To learn more about in-place recycling techniques, consult these Federal Highway Administration (FHWA) guidance documents:
|
In 2008, however, sharp increases in asphalt costs coupled with diminishing supplies of acceptable quality aggregate prompted the asphalt paving industry to push for using higher percentages of RAP. Evidence from a variety of real-world applications indicates that proper mix design, material processing, and production best practices can create high-quality, durable mixtures with higher RAP percentages.
"Now, most State DOTs are seriously considering the economic and environmental benefits of using RAP in greater proportions," says Peter Stephanos, director of the FHWA Office of Pavement Technology. Increasing public concern for the environment and stricter regulations are driving demand for greener construction practices and sustainable pavements. "The challenge now facing DOTs is permitting greater amounts of RAP while maintaining high-quality pavement infrastructure," Stephanos says.
The Case for Greater RAP Use
The two leading reasons for using RAP in asphalt pavement are its economic and environmental benefits. Reusing asphalt pavement reduces the amount of more expensive virgin aggregate and asphalt binder required. Using RAP typically lowers the unit cost of HMA, both in terms of reuse of aggregates and asphalt binder. The most economical use of RAP is in asphalt mixtures that go into the intermediate and surface layers of flexible pavements, where the RAP actually replaces a portion of the more expensive virgin binder.
Because aggregate, the most mined resource in the world, is nonrenewable, using RAP conserves the virgin resource itself, as well as the energy and transportation costs that go into obtaining it. In addition, using RAP greatly reduces the amount of construction debris going into landfills. Because RAP can be used again and again, it creates a cycle of asphalt reuse that optimizes use of natural resources and helps sustain the asphalt pavement industry.
How Much Progress?
According to NAPA, U.S. production of asphalt pavement material is around 500 million tons (454 million metric tons) per year, including about 60 million tons (54.4 million metric tons) of reclaimed material — RAP — that transportation agencies reuse or recycle directly into pavements. In addition, agencies reuse or recycle about 40 million tons (36.3 million metric tons) of RAP into other pavement-related applications every year. Thus, about 100 million tons (90.7 million metric tons) of RAP are used each year, compared to 72 million tons (65.3 million metric tons) used annually in the early 1990s. Asphalt pavement has the distinction of having the highest recycling rate, by percentage, of all recycled materials because virtually all reclaimed asphalt is reused or recycled.
In 2007 the North Carolina Department of Transportation (NCDOT) conducted a survey on behalf of FHWA and the American Association of State Highway and Transportation Officials (AASHTO) to determine the level of RAP use across the United States. The survey also revealed the potential for increasing RAP use across the Nation. For example, only 10 States were using up to 30 percent RAP in the intermediate layer, and only 2 States were using up to 30 percent RAP in the surface layer. NCDOT conducted the survey again in 2009, and half the States reported greater RAP use after 2007.
Although many State DOTs used more RAP in HMA in 2009 compared to 2007, high proportions of RAP (greater than 25 percent) are still not common. Many State DOTs permit more than 25 percent RAP in HMA layers; however, fewer than half actually use more than 20 percent RAP.
The question remains: Why do more than half the States use less than 20 percent RAP in HMA? The survey asked participants to identify the major concerns and obstacles that limit or preclude RAP use. The two issues cited most often concerned the blended virgin and RAP binder qualities, especially for high-RAP mixes (greater than 25 percent RAP by weight) and polymer-modified binders, and stiffening of the mix with high-RAP quantities, resulting in cracking or other performance defects.
Several States are concerned that using RAP with polymer-modified binders could compromise the quality of the binders. Respondents worry that high amounts of RAP could affect binder properties and create an overly stiff mix that is subject to cracking in low temperatures. For pavements undergoing high deflections due to heavy traffic, an overly stiff mix might not be as resilient and could crack prematurely. Because softer virgin binders might be necessary to compensate for additional stiffness due to the aged RAP binder, State DOTs want guidance on when and how to select softer binders.
To evaluate necessary binder grade changes and address low-temperature properties of RAP mixtures, FHWA is working with the North Central Superpave Center at Purdue University in West Lafayette, IN, to improve understanding of HMA mixtures. This study is unique in that the mixtures being evaluated are plant-produced, high-RAP mixtures. The study expands on earlier research to incorporate multiple HMA producers and varying RAP contents. So far, the results appear to agree that, in Indiana, it might be possible to employ higher RAP contents (around 20 percent) without changing the performance grade of the virgin binder. The research should yield valuable insights on producing high-RAP HMA mixtures.
In addition to the issue of cracking, several States expressed concerns about the durability of higher percentage RAP mixes used in surface layers, especially in terms of raveling, which occurs when aggregate separates from the pavement surface. In particular, 10 States cited the skid resistance and polish values of RAP aggregate as an obstacle when using RAP in HMA. Other key concerns include the presence of dust in RAP that might make it difficult to meet the dust-to-binder ratio specifications required by Superpave, the consistency/variability of RAP, and lack of quality control by contractors.
Binder Selection Guidelines for RAP Mixtures (AASHTO M 323)
|
Recommended Virgin Asphalt Binder Performance Grade (PG) |
RAP Percentage |
|---|---|
|
No change in binder selection |
<15 |
|
Select virgin binder one grade softer than normal (Select a PG 58-28 if a PG 64-22 would normally be used.) |
15-25 |
|
Follow recommendations from blending charts |
>25 |
Source: AASHTO.
Remaining Challenges
The mix design process for HMA with RAP is similar to mix design for virgin HMA, except when the RAP percentage is greater than 25 percent. For RAP percentages more than 25 percent, additional testing of the RAP binder might be necessary to use blending charts for determining the required virgin binder performance grade. Despite the similarity in mix design, some challenges remain for maximizing RAP use and routinely using high RAP content.
First, the current binder selection guidelines for RAP mixtures (AASHTO M 323 Standard Specification for Superpave Volumetric Mix Design and AASHTO R 35 Standard Practice for Superpave Volumetric Design for Hot-Mix Asphalt) are based on the assumption that substantial mixing occurs between the virgin binder and RAP binder. But no method exists to determine the amount of blending that actually occurs.
To estimate the blending of high-RAP mixtures, the AASHTO standards specify blending charts. The charts optimize the amount of RAP to use if the performance grade of the virgin binder is known. The charts call for expensive, time-consuming binder extraction and recovery tests that use hazardous solvents. Many highway agencies are reluctant to specify amounts of RAP that require this additional testing, and many contractors are not equipped to perform tests that involve hazardous solvents.
Improving Designs For High-RAP Mixes
The National Cooperative Highway Research Program (NCHRP) develops advanced pavement technology that supports FHWA's goal to increase the life of asphalt pavements. One ongoing NCHRP project, Improved Mix Design, Evaluation, and Materials Management Practices for Hot Mix Asphalt with High Reclaimed Asphalt Pavement Content, aims to evaluate and propose necessary changes to existing specifications, such as AASHTO M 323 and AASHTO R 35. The study will develop a mix design and analysis procedure for high-RAP HMA that ensures long-term performance.
The expected mix analysis procedure will include performance-related tests and key criteria to address permanent deformation, fatigue cracking, low-temperature cracking, and moisture susceptibility. Further, the procedure will identify any promising methods to assess the durability of HMA.
As part of the NCHRP project, the National Center for Asphalt Technology (NCAT) has joined with the University of Wisconsin-Madison and University of Nevada, Reno, to develop methods for evaluating RAP binder characteristics without using hazardous solvents. The study also will determine accurate methods for characterizing RAP aggregates. The researchers expect to incorporate the study's results into the NCHRP's mix design and analysis procedures.
National Effort to Increase RAP Use
To promote best practices for increased RAP use, FHWA initiated the Reclaimed Asphalt Pavement Expert Task Group to provide technical input to advance use of reclaimed asphalt materials in paving applications. The group provides highway agencies with critical information regarding RAP, technical guidance on high-RAP projects, and direction on research activities. The group is composed of experts from State DOTs, FHWA, AASHTO, NCAT, and NAPA.
The task group's first goal was to identify the most critical needs for increasing RAP use. After determining the top 10 needs, the group formed task subgroups to target each need, and the larger group meets twice a year to discuss progress.
One subgroup developed information for highway agencies and contractors interested in increasing their RAP use. For example, the task group developed Designing HMA Mixtures with High RAP Content: A Practical Guide, a joint publication of FHWA, AASHTO, and NAPA. The document is one of the most up-to-date publications available for designing and producing high-RAP mixtures. The guide covers materials evaluation, mix design, plant verification, and quality control when using high percentages of RAP. Also, the task group published an article, "How to Maximize RAP Usage and Pavement Performance," in the September/October 2009 issue of Hot Mix Asphalt Technology.
The expert task group's efforts already are making an impact in the marketplace. For example, until recently the Arizona Department of Transportation (ADOT) had a low risk tolerance for using RAP because of past failures. In fact, for years ADOT did not allow RAP to be used in HMA at all. In late 2007, however, ADOT began reconsidering its stance on RAP in response to requests from contractors who pointed to RAP's ability to help control costs in the face of spiking asphalt prices. That same year, the expert task group met in Arizona and invited ADOT and industry representatives to present their concerns about using RAP. ADOT was encouraged after hearing about the positive experiences other States had with using RAP. By summer 2008, ADOT had successfully constructed three projects using RAP. FHWA engineers and other task group members subsequently provided input to help ADOT draft specifications for RAP use, allowing up to 25 percent in layers below the surface and up to 20 percent in surface layers.
Another task subgroup developed a best practices document summarizing the state of the practice and providing indepth guidelines for incorporating RAP in asphalt mixtures. The document is in the final stages of development and will be published as an FHWA report.
Motivated by the expert task group's efforts to evaluate the performance of RAP mixtures, the Florida Department of Transportation (FDOT) is reviewing its pavement management database to glean information on completed projects that used RAP. "The data are showing that there is no real difference in performance as a function of RAP content," says Jim Musselman, State bituminous materials engineer for FDOT. "So far, we have not found data that would indicate high-RAP mixtures perform worse than low-RAP or no-RAP mixtures. In fact, in some cases, high-RAP pavements are lasting longer than virgin material pavements."
Top 10 Needs for Greater RAP Use
|
Mobile Laboratory Evaluates Performance
Stemming from technical discussions among members of the expert task group, FHWA and industry partners have initiated several demonstration projects across the United States that are using high proportions of RAP (25 percent or more). The objectives of the field projects are to document mixture design processes; assess procedures for production, construction, and performance testing; and highlight best practices.
FHWA, along with NCAT and other partners, is working with State DOTs to establish experimental plans for design, production, placement, and evaluation of high-percentage RAP mixes on high-volume roadways. As with virgin mixtures, the DOTs need to consider RAP's aggregate characteristics, effective binder content (volume of effective binder), asphalt binder properties, in-place mix density, and conditions during placement.
FHWA's Mobile Asphalt Pavement Mixture Laboratory provides the mixture design replication and performance testing of RAP and control mixes, including dynamic modulus, fatigue, and low-temperature testing. The participating State DOTs also perform quality assurance testing. The mobile lab is able to provide probable performance information and evaluate binder blending based on mixture properties using the Asphalt Mixture Performance Tester (see "Evaluating the Field Performance of Asphalt Mixtures in the Lab," Public Roads, January/February 2005).
So far, the lab has participated in high-RAP field projects in Delaware, Florida, and Kansas. In each case, the State DOT approved higher RAP percentages than its specification allowed.
Using Best Practices in High-RAP Field Projects
Best practices for processing, stockpiling, and plant production are crucial to maintain the quality and consistency of the RAP aggregate, the mix, and the final pavement, especially with mixes containing higher percentages of RAP. "In fact, high-quality, high-RAP mixes could be impossible without use of processing and production best practices," says David Newcomb, vice president of research at NAPA.
The field demonstration projects highlighted some of the best practices, such as proper milling of the existing roadway, processing and fractionating for uniformity and final blend consistency, and flexibility in plant operations.
High-RAP Field Projects
|
State |
% RAP Permitted in Intermediate Layer (as of 2007) |
% RAP Permitted in Surface Layer (as of 2007) |
% RAP Used in Project |
Date of Paving |
|---|---|---|---|---|
|
Florida* |
$30% |
0% |
45% |
December 2007 |
|
Kansas |
25% |
25% |
30%-40% |
May 2008 |
|
Delaware |
20% |
10% |
30%-35% |
Summer 2008 |
Source: FHWA.
*WMA process used.
The Delaware project is a $52 million, multiyear, multiproject turnpike improvement program that includes widening I-95 near Wilmington. The Delaware Department of Transportation (DelDOT) placed more than 80,000 tons (72,600 metric tons) of HMA, including base, binder, and surface layers. DelDOT used the high-RAP mixture in the binder layers of the pavement and the surface layer on the shoulder. Eventually the shoulder will carry traffic when it becomes part of the mainline as a right lane, after another interchange project is completed in 3 years.
At the time of the bidding process, DelDOT specifications limited RAP use to 20 percent. However, the contractor requested and was approved to use up to 35 percent RAP. DelDOT was confident in the contractor's ability because the contractor had successfully used RAP for 7 years.
"Our HMA supplier on this project put a lot of money into plant and operations, and [the investment] has paid off in terms of DelDOT's satisfaction," says James Pappas, the agency's chief materials and research engineer. "The supplier maintained a captive stockpile for this project and processed the RAP, which led DelDOT to agree to higher RAP percentages."
The contractor routinely mills the RAP off the roadway and stockpiles it for future use in HMA. The contractor invested in a crushing process and screens to fractionate the RAP into three sizes: 100 percent passing the 0.5 inch (1.3 centimeters), plus #4 size materials (where #4 is a sieve with a standard designation of 0.19 inch, or 4.76 millimeters), and minus #4 size materials.
"Overall, the project went very well and DelDOT is pleased," says Pappas. "This was a pioneering project that opened the eyes for the whole State in terms of increasing RAP use." In fact, DelDOT plans to increase the amount of RAP permitted in its specifications for 2010. In terms of costs, DelDOT calculated savings of $7-$8 per ton for liquid asphalt conserved with the higher RAP use in this project. "This translates to a definite overall savings of $5 to $10 per ton of HMA for DelDOT and the taxpayers," Pappas says.
In Florida, FDOT milled and repaved a portion of State Route 11 in Deland using 45 percent RAP in combination with the water-injection, warm-mix asphalt (WMA) process. The project was the first large production in which FDOT allowed that much RAP combined with the use of the WMA process. To use up to 45 percent RAP, the contractor employed a softer binder and fractionated the milled RAP.
"Overall, we are pleased with the quality we have seen as we have moved toward using higher amounts of RAP [more than 40 percent]," says FDOT's Musselman. "There have been no construction or performance problems noted to date. Most contractors realize that it takes a little more effort in terms of paying attention to detail, but there is an economic benefit for them to use more RAP." Since January 2009, partly due to the project's success, FDOT has increased its permitted amount of RAP to 20 percent in surface layers and unlimited amounts in intermediate and base layers. As an exception, when polymer-modified binders are used, FDOT limits the RAP content to 20 percent in all layers.
The Kansas project was located on U.S. Highway 83, a major north-south route heavily used by trucks. The project was unique in that the contractor set up a mobile HMA plant near the jobsite and was allowed to use up to 40 percent RAP. Also, the FHWA Mobile Asphalt Pavement Mixture Laboratory was onsite to collect data and determine volumetric properties as increasingly greater percentages of RAP were substituted into the mix. The contractor milled the RAP from the existing highway and avoided transportation costs, but ultimately used a 30 percent RAP mix. The flexibility to adjust the amount of RAP used was critical to meet quality assurance requirements. The feed system included the RAP cold feed and scalping screen to ensure no oversize particles went into the mix.
"This was one of the first high-RAP projects for the Kansas DOT," says Mike Crow, of the Kansas Asphalt Pavement Association. "The finished highway is nice and smooth."
In the Pipeline
As the public continues to push for more sustainable infrastructure and greener technologies, using RAP can play a critical role in creating sustainable highways. Many State DOTs are interested in using greater percentages of RAP for cost savings, and most State projects have the potential to use more.
FHWA, along with partners AASHTO, NAPA, and NCAT, is leading the national effort to increase responsible RAP use and provide guidance on overcoming barriers to adoption, especially for mix design and performance. Through the expert task group, FHWA will continue to document the long-term performance of RAP projects to highlight successful practices and share lessons learned in using higher percentages of the material.
"As we continue to develop more effective test methods for analyzing blending between virgin and RAP binders and evaluating the performance of high-RAP mixtures, we expect to see support for and use of RAP in roadway projects become increasingly widespread," says Randy West, director of NCAT.
Additional information is available from the Asphalt Recycling & Reclaiming Association's publications Basic Asphalt Recycling Manual, Full Depth Reclamation Manual, and Cold Recycling Manual. For more information on pavement recycling, visit FHWA's Web site at www.fhwa.dot.gov/pavement/recycling/index.cfm.
More RAP Resources1. Pavement Recycling Executive Summary and Report (FHWA-SA-95-060), FHWA, Washington, DC, 1996. 2. Recommended Use of Reclaimed Asphalt Pavement in the Superpave Mix Design Method: Technician's Manual, NCHRP Report 452, TRB, Washington, DC, 2001. 3. User Guidelines for Byproducts and Secondary Use Materials in Pavement Construction, www.recycledmaterials.org/tools/uguidelines/index.asp. 4. Recycling Hot-Mix Asphalt Pavements, Information Series 123, National Asphalt Pavement Association (NAPA), Lanham, MD, 2007. 5. Designing HMA Mixtures with High RAP Content-A Practical Guide, Quality Improvement Series 124, NAPA in partnership with FHWA and AASHTO, Lanham, MD, 2007. 6. "Can I Run More RAP?" Hot Mix Asphalt Technology, NAPA, September/October 2007. 7. HMA Recycling Expert Task Group Web site: www.moreRAP.us, NCAT, Auburn University, AL, 2009. 8. "How to Maximize RAP Usage and Pavement Performance," Hot Mix Asphalt Technology, NAPA, September/October 2009. |
Audrey Copeland, Ph.D., is a materials research engineer on the Pavement Materials and Construction team at FHWA's Turner-Fairbank Highway Research Center in McLean, VA. She manages the Binder Rheology Laboratory and is responsible for research and development activities for asphalt pavement technologies to increase the lifespan of flexible pavements. Previously she was a highway engineer in FHWA's Office of Pavement Technology, where she led the national effort to increase RAP use. She holds a doctorate in civil engineering from Vanderbilt University and an M.S. and B.S. in civil engineering from Tennessee Technological University.
Cecil Jones, P.E., recently retired from NCDOT, where he served as the State materials engineer and chaired the AASHTO Subcommittee on Materials' Recycling Task Force. His responsibilities included overall management of the materials quality system and management of laboratory and field quality assurance operations for materials and manufactured products used on North Carolina's State-maintained highways. He has a B.S. in civil engineering from North Carolina State University and is a registered professional engineer. He currently is president of Diversified Engineering Services, Inc.
John Bukowski is a senior pavement engineer and the Asphalt Technology Team Leader in the FHWA Office of Pavement Technology. He is responsible for identifying and leading development and implementation of innovations in asphalt pavement technology. He has more than 35 years of pavement technology experience through positions with FHWA, the Asphalt Institute, and the military. He has a B.S. in civil engineering from the University of Pittsburgh and a law degree from The Catholic University of America.
For more information, contact Audrey Copeland at 202-493-3097 or audrey.copeland@dot.gov, or John Bukowski at 202-366-1987 or john.bukowski@dot.gov.