The State of HMA Recycling in the U.S.

By Dave Newcomb, P.E., Ph. D. and Cecil Jones, P.E.

(This article was published in HMAT magazine July/August 2008)

Hot-mix asphalt (HMA) is by far the most recycled product in the nation, with 80 percent of the HMA that is taken up each year finding its way back into roadways. While this is laudable from both an environmental and economic point of view, when one considers the amount used in producing new hot mix, it is obvious that more can and should be done to promote recycling of HMA back into HMA as its best and highest use.

Oil and energy prices are at a high levels, and this has profound consequences on the availability of virgin asphalt and the production of aggregates and HMA. Aggregate and polymer costs are rising in proportion to production and transportation costs. On top of this, zoning requirements are making it more difficult to site new plants, exploit convenient aggregate sources, and to stockpile reclaimed asphalt pavement (RAP) at existing plant sites. It is clear that an industry-agency partnership is needed to advocate technology which allows for greater recycling, and this advocacy comes in the form of the RAP Expert Task Group (ETG).

The RAP ETG was formed by the Federal Highway Administration in 2007 in order to encourage the use of RAP by agencies that do not currently optimize the amount of RAP in their mixtures or those that do not allow RAP at all in their HMA. The ETG decided that the approach to this encouragement would include:

• Defining the state-of-recycling
• Identifying barriers to increased recycling
• Initiating research to address barriers
• Promoting best practices in recycling
• Assisting in high-RAP demonstration projects

In order to accomplish the first two of these items, Cecil Jones, the State Materials Engineer for  the North Carolina Department of Transportation, conducted a survey of all 50 state DOTs and Ontario. The research addressing barriers to higher RAP content in HMA mixtures is being conducted under the National Cooperative Highway Research Program (NCHRP) Project 9-46, “Improved Mix Design, Evaluation, and Materials Management Practices for Hot Mix Asphalt with High Reclaimed Asphalt Pavement Content.”  Promotion of best recycling practices is being accomplished through presentations by ETG members and NAPA publications Designing HMA Mixtures with High RAP Content (QIP-124) and Recycling Hot-Mix Asphalt Pavements (IS-123). High-RAP demonstration projects are receiving technical assistance from FHWA and the National Center for Asphalt Technology (NCAT).

The first step for the ETG was to discover the current level of RAP usage and limitations from the states in order to find out how much room for improvement exists. Responses were gathered from all 50 states as well as Ontario. It should be noted that the survey took place between June  2007 and spring 2008. The situation is changing constantly as states move mostly to greater levels of recycling.

The survey asked how much RAP is allowed in mixtures, how much is actually used, and what are the main roadblocks to greater usage of RAP. Figures 1 through 3 show the amount of RAP allowed by the states and the amount generally used in base, intermediate, and surface courses, respectively. Figure 1 shows the survey results for base courses, and it can be seen that, in general, the majority of states have no problem in allowing RAP contents of 20 percent or more in the HMA base course, and that the actual RAP contents are generally 15 percent or above. In a number of states more than 20 percent RAP is routinely used in base mixes. It should be noted that while Illinois is shown as allowing only 10 percent RAP, that percentage only applies for mixes with polymer-modified binder, and that Illinois actually allows 30 percent in mixes with straight binders.

Figure 2 shows the results for intermediate or binder courses. Again, the majority of states allow more than 20 percent RAP in intermediate course mixtures, and while there a number of states that see average RAP contents in intermediate courses in excess of 20 percent, the majority of states see actual RAP contents in intermediate mixes to be on the order of 15 percent or more. 

Figure 3 shows the amount of RAP allowed versus the average actually used in surface courses. While there are some agencies which allow more than 20 percent RAP, many others are reluctant to allow more than 10 percent and, in fact, there are some which allow no RAP in the mixtures. In much of the country, 15 percent seems to be the amount used in surface courses. The significance of Figure 3 can be thought of in terms of where most of the HMA being produced is being used – in overlay work. It is time to address the technical issues that restrict the amount of RAP allowed and used in surface mixes.

The survey queried the states on the barriers restricting the amount of RAP in mixtures.  Basically, the responses could be categorized according to specifications, lack of RAP availability, lack of processing, and past experiences.  Three of these four issues may be addressed through changes in practice and application of technology. Availability is a local issue that tends to be divided along urban versus rural locations. There tends to be an excess in urban areas, while the available RAP tends to be completely used in rural areas. Considering the value of the binder and aggregate in RAP, there may be some economy in transporting RAP from areas of excess availability to areas deficit in RAP.

Superpave

Superpave was adopted in the early 1990s, and being a new mix design system, many DOTs were reluctant to allow much, if any, reclaimed asphalt pavement in their HMA. Although these restrictions were generally loosened over the years, agencies and industry did not press for higher RAP contents. NCHRP project 9-12 provided guidelines on how to incorporate RAP into Superpave mixtures, but volumetric constraints within Superpave often worked to restrict RAP contents. Generally speaking, good aggregate gradation control is necessary to produce mixtures that have the required voids in mineral aggregate (VMA) in the Superpave system. This is especially true in the face of statistical specifications such as Percent Within Limits (PWL).  Until recently, contractors mostly operated out of one, or at most, two RAP stockpiles. The fines generated in RAP production combined with the inherent variability of the RAP gradation prevented RAP containing Superpave mixes from meeting the volumetric requirements.

RAP processing allows the contractor greater flexibility in manufacturing mixtures in addition to the ability to achieve greater consistency.  In some instances, agencies have required contractors to separate RAP piles according to the source of RAP. For instance, if RAP is to be used in a surface mixture, then the contractor may be required to use only RAP from sources having hard, skid-resistant aggregate. It is not clear that this necessarily improves the skid resistance of the final mixture if the virgin aggregate comes from a high-quality source, but this has been an approach used in some instances. Separating the RAP according to size is another way to more closely control the final gradation in the mix. Again, better gradation control can lead to better volumetric control in the mix. Having RAP stockpiles separated according to size also allows more of the fine RAP to be used in fine mixtures and the coarser RAP to be used in coarser mixes.

According to the survey, some agencies are reluctant to use higher percentages of RAP because of some projects that failed prematurely when high-RAP contents were employed.  In the early days of recycling, some projects incorporated RAP contents in excess of 50 percent, and while many were successful, there were notable failures. It should be noted that the controls mentioned above were not in place at the time. Also, guidelines such as those from NCHRP 9-12 or in NAPA publication QIP-124 were not available. The industry and its agency partners should keep informed on the progress of NCHRP Project 9-46. It is time to consider the origin of past failures and to resolve to change the specification and construction practices that limit RAP.

There are efforts within the FHWA Expert Task Group to explore means to successfully incorporate higher RAP through the application of best practices and improved testing techniques.

Efforts to Increase RAP include a number of demonstration projects in 2007 and 2008. In 2007, North Carolina built a project with 40 percent RAP, South Carolina had two projects with 30 and 50 percent RAP, Wisconsin built a section with 25 percent RAP, and Florida constructed a section with 45 percent RAP. There are plans to construct high-RAP sections in Illinois, Minnesota, and Delaware. By monitoring the material properties and performance for these projects, the industry will understand how to optimize the amount of RAP in mixtures while maintaining high quality.

There is currently a research effort underway at NCAT at Auburn University to develop methods to increase RAP content in HMA. The objectives are to develop mix design procedures to ensure long-term performance through performance testing and to suggest changes to existing specifications for high RAP content mixtures. This project is slated to be completed by April 15, 2010. The project team from NCAT includes Dr. Randy West, principal investigator, and Dr. Andrea Kvasnak.

The future of the asphalt pavement industry will be strongly tied to its ability to efficiently and effectively produce its material according to its customers’ needs. Maintaining high quality and versatility are keys to ensuring the HMA industry’s market share.   The technical concerns and historical performance issues need to be addressed through timely application of existing knowledge and timely performance studies. The logistics of RAP need to be understood between areas with excess RAP and those with RAP deficit. It is expected that economics will dictate the amount of RAP used in mixes, but technical barriers should be addressed and removed.  RAP needs to be used in its highest and best application, hot-mix asphalt, to take advantage of the binder and high-quality aggregate in the material. Finally, the industry and its agency partners must work to allow more RAP in HMA.

Dave Newcomb, P.E., Ph.D.,  is the Vice President–Research and Technology for National Asphalt Pavement Association.  Cecil Jones, P.E., is the State Materials Engineer with the North Carolina Department of Transportation.