Mixed messages: new specifications adopted by AASHTO encourage the use of the recycled shingles in making hot mix asphalt.
NEW AASHTO SPEC
Late last year, the American Association of State and Highway Transportation Officials (AASHTO) adopted a standard specification that itemizes specific requirements for utilizing recycled asphalt shingles (RAS) in hot mix asphalt. This AASHTO shingle recycling spec is significant in that it provides for a national standard guideline for state and local highway departments to use when specifying materials engineering requirements. Paving contractors and HMA producers can then design their paving material production to meet these state and local specifications if allowed to use RAS in their HMA. The new AASHTO shingle recycling specification does not address other pavement applications such as hot-in-place, cold-in-place, or cold recycled.
Most studies that have analyzed the impact of including RAS in HMA have concluded that there is potential for savings in costs of virgin asphalt binder due to the addition of RAS without sacrificing pavement performance if appropriate quality assurance and quality control procedures are followed. This is the most important economic driving force accelerating the development of shingle recycling today.
The cost of virgin asphalt binder has been rising rapidly, especially in recent months. Figure 1 on page 32 displays one representation of this trend using the New Jersey Department of Transportation published data on asphalt cement selling prices from suppliers in the northern part of the state. This chart shows the relative change over 17 years. This New Jersey data indicates there has been nearly a 70 percent increase in asphalt cement prices compared to a year ago.
It is notable that the mineral aggregates used in manufacturing shingles are also valuable commodities in HMA. Even the fiber in recycled shingles can be an asset to the pavement matrix in the right application and depending on the type of mix. Finally, the prices for landfilling construction and demolition debris, such as mixed roofing material, are also increasing. As environmental regulations and landfill prohibitions continue to increase, so will the tipping fees for roofing debris.
The new AASHTO standard specification also allows the use of either manufacturers' (post-industrial) shingle scrap or tear-off (post-consumer) shingle scrap as an additive in HMA. It is estimated that about 11 million tons of tear-off roofing material is generated each year in the United States. This is about 10 times the amount of manufacturers' scrap.
In addition, AASHTO adopted a companion recommended practice to provide additional guidance for designing new HMA which incorporates RAS. Specific considerations include: shingle aggregate gradation, performance grade (PG) of the virgin and RAS binder and relative reduction of the virgin asphalt binder due to replacement by RAS binder. There are multiple references to various quality assurance and quality control (QA/QC) requirements for processing and recycling of the RAS.
Requirements within the AASHTO specification include a number of important details. First, the final RAS product must be sized and screened such that 100 percent passes a 1/2-inch sieve screen. This is important became it was found that the size of RAS can be expected to affect the fraction of shingle asphalt binder that contributes to the final blended binder. For example, the AASHTO recommended practice states that material that is ground 1/2-inch maximum can be expected to release lower levels of available asphalt shingle binder in the range of about 20 to 40 percent of total RAS asphalt binder. RAS ground to a finer size passing a No. 4 sieve can be expected to effectively utilize as much as 95 percent of the total available asphalt. The designer must be prepared to adjust the performance grade of virgin asphalt binder to compensate for this effect.
Second, the actual maximum addition rate of RAS is left as an option for the contractor. Most states have established fixed maximum limits on the amount of RAS in the HMA, usually 5 percent by weight. The AASHTO spec provides for a means of designing adjustment of virgin asphalt binder used based on the measured effectiveness of the RAS asphalt. The HMA designer must determine the asphalt binder content of the new HMA as part of the volumetric design procedure. During the production of the new HMA, shingle asphalt binder present in the RAS will mix with the virgin asphalt binder to produce a final blended binder. The new AASHTO spec states that if the quantity of RAS asphalt binder exceeds 0.75 percent by weight of the new HMA, the RAS binder and the virgin asphalt binder shall be further evaluated to ensure the performance grade of the final blended HMA complies with the originally specified performance grade requirements.
To help maintain the engineering performance of the final HMA pavement, the new AASHTO spec limits the maximum amount of deleterious material allowed in the RAS. The AASHTO spec states that lightweight extraneous materials (e.g., paper, wood, plastics) shall not exceed 0.05 percent of the material retained on the No. 4 sieve. Heavyweight extraneous materials (e.g., metals, glass, rubber, nails, soil, brick, tars, etc.) shall not exceed 0.5 percent as determined on material retained on the No. 4 sieve. It is recognized that these are stringent limits suggested by AASHTO, but recycling professionals continuously work to develop improved systems to clean up the mixed roofing material and tear-off RAS.
Recently, the CMRA was awarded a $74,000 grant from the U.S. EPA to examine the barriers to the recycling of tear-off asphalt shingles. The CMRA intends that the project will accelerate the national development and implementation of tear-off asphalt shingle recycling. The project partners will attempt to address the key barriers to reaching full-scale commercialization of this promising technology.
The primary goal of this new EPA project is to develop and demonstrate recommended best practices that provide for superior quality assurance and quality control that can be utilized by profitable shingle recycling operators throughout the nation. The project has three principal objectives:
* Demonstrate successful and appropriate environmental and worker health protection procedures;
* Document materials engineering benefits and methods of QA/QC to optimize their pavement performance effects; and,
* Develop operational guidelines that maximize cost-efficiency while attaining minimum environmental, worker health and safety, and engineering standards.
The project is being produced by the CMRA with key partner support from a wide variety of public and private agencies and companies. This project will build directly on the substantial efforts of other research and development efforts in order to help bring tear-off shingle recycling technology to full-scale implementation.
One spin-off benefit of this project will be an update of the Web site www.ShingleRecycling.org that was originally produced by the CMRA with funding from EPA in 1999. One of the other partners in the original Web site development was the University of Florida, Department of Environmental Engineering Sciences and it will be again be involved in this tear-off shingles recycling project.
In addition, several state departments of transportation have adopted material specifications over the years providing for the use of RAS in HMA. The CMRA Web site, www.ShingleRecycling.org, is in the process of being updated with the known recycled shingle specifications from various states. As a general rule, the states have been leading the way in the research and development of shingles recycling, even though there has been notable support from FHWA, RMRC and EPA.
The state of Minnesota has sponsored several research studies on the use of recycled shingles in HMA over the past 15 years. Continuing this trend, several recycling projects are currently underway. The Minnesota Office of Environmental Assistance (OEA) is one of several agency sponsors of an extensive lab study project. The Minnesota Department of Transportation (Mn/DOT) is directing the research to determine if and how a new tear-off spec can be developed. Mn/DOT's current spec only allows the use of manufacturers' shingle scrap in HMA. Mn/DOT is analyzing the asphalt cement (AC) content, asphalt performance grade (PG), aggregate gradation and deleterious debris in a series of tests consistent with the protocol in the new AASHTO spec. Mn/DOT is not only comparing tear-off shingle scrap to manufacturer shingle scrap, but also to other control mix type without any RAS of either type. The University of Minnesota, Department of Civil Engineering's asphalt lab is also conducting a controlled set of HMA laboratory analyses to provide empirical data on HMA strength. These indirect tensile tests will help analyze in the lab how shingles might affect the low temperature cracking properties of shingle-derived HMA. Another project is also underway in Minnesota sponsored by the Solid Waste Management Coordinating Board (SWMCB). This SWMCB project will include a new controlled road construction demonstration this year utilizing tear-off-derived HMA compared side-by-side with manufacturers'-derived HMA. Lab performance tests and pavement surveys will help analyze if there are any different impacts between the two types of scrap shingles and a "virgin only" mix without any RAS.
The Missouri Department of Transportation (MoDOT) has a new shingles recycling specification that was originally released in early 2005. Pace Construction is the primary HMA producer that is utilizing tear-off RAS in their paving products. Peerless Resource Recovery and Landfill is the recycler responsible for receiving the mixed roofing material, sorting, grinding and screening the material in a product suitable for Pace Constructions HMA plant.
MoDOT is conducting a lab analyses project similar to the Minnesota OEA study. MoDOT intends to produce additional lab data to further analyze the HMA supplemented with tear-offRAS. The University of Minnesota Department of Civil Engineering has performed similar lab analyses using its equipment to measure indirect tensile strength for as part of a cooperative study with the Mn/DOT. These two states are taking leading roles in the research and development of using recycled shingles in HMA.
The author is a consultant with Dan Krivit and Associates and can be contacted at firstname.lastname@example.org.
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|Title Annotation:||ASPHALT SHINGLE RECYCLING|
|Publication:||Construction & Demolition Recycling|
|Date:||Jul 1, 2006|
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