By Dan Krivit
The State of Texas never does anything in a small way and has emerged as a leader in the recycling of asphalt shingles. The infrastructure is in place and the players are ready to accelerate the recycling of shingles.
In Texas today, there are at least 32 recyclers processing the material into recycled asphalt shingles (RAS) product and 11 hot mix asphalt (HMA) producers using the RAS.1 This rapid growth in shingle recycling is in part fueled by the fact there are eight asphalt shingle manufacturing plants in or near Texas.
The Texas Department of Transportation (TxDOT) has a specification on the books that provides the critical technical requirements for the quality and use rates of RAS in HMA. The Texas Commission on Environmental Quality (TCEQ) has a technical memo that provides the necessary guidance on testing and environmental protections. Continuing research and development is ongoing under the Texas Transportation Institute that will help further the understanding of the benefits and applied methods for using RAS in HMA. Even the Texas Asphalt Pavement Association (TxAPA) has helped in promoting the use of RAS in HMA and providing important technology exchange and information services.
This soup of multiple organizations continues to provide the tools and create a very positive business environment for the strong growth of asphalt shingle recycling. It is no coincidence the Construction Materials Recycling Association (CMRA) elected to host the 5th Asphalt Shingle Recycling Forum in Dallas October 27–28, 2011. Texas agencies and companies will have a lot of new information to share with each other and the rest of the industry.
TxDOT Specs Lead the Way
Senior Materials Engineer Robert E. Lee has been a part of the Construction Division TxDOT team leading the recycling charge for many years. TxDOT has continued to take a stepwise but very deliberate approach to the development of shingle specifications. “Recycling of asphalt shingles is not only an economically viable addition to our hot-mix toolbox, but an environmentally conscientious change that can benefit everyone,” said Lee.
In Texas, the first documented use of RAS in HMA was in 1997 when Duininck Brothers placed two test sections on SH 31 just outside Corsicana. One test section was constructed with manufacturers’ shingle scrap and the second section included used, tear-off shingles. The pavement performance was very good compared to the control section. This early research helped TxDOT establish its first specification parameters that are still applicable today. For example, RAS was limited to a maximum of 5% of the hot mix by weight.
The TxDOT research added to the general knowledge base in the industry about the benefits of using RAS in HMA. When current engineering specifications and operating guidelines are carefully applied, these benefits can include:
- Economic cost savings by replacing a significant portion of virgin asphalt oil (or binder) can help reduce the ultimate price of HMA pavement;
- Increase in the strength and stiffness of HMA mixes from the fibers and polymers in RAS;
- Conservation of raw materials required for mixes such as virgin asphalt oil and mineral aggregate; and
- Reduction in the consumption of landfill space.
The new TxDOT statewide special provision allows a maximum of 5% processed shingle waste in HMA. It limits the percentage of recycled binder from recycled asphalt pavement (RAP) and/or RAS to 35% for surface mixtures and 40% for non-surface mixtures. More importantly with respect to proper shingle sourcing and processing, the amount of deleterious material in the RAS must be less than 1.5%.
TxDOT requires the RAS to be processed down to ½ inch or less. When the freshly laid asphalt mat looks as if it is in need of a shave, this is usually an indication the shingles are not getting ground down to the proper size or there is a problem with the screening operations.
Here are a few tips from TxDOT for designing HMA with RAS:
- Measure the asphalt content (AC) of the RAS. This will vary by types and age of shingle. Generally the older tear-offs and cellulose backed shingles will have higher AC content. The HMA volumetric design will be directly impacted by this AC content.
- Try to use RAS with a consistent gradation, preferably ½ inch minus or smaller. Also the amount of fines in the RAS will impact the HMA mix design.
- Consider starting with a softer virgin binder. Multiple studies have shown that adding 5% RAS is roughly equivalent to adding 20% RAP in terms of mix stiffness. Typically, both will result in one PG grade bump.
Therefore, an unadjusted mix will be stiffer. Adjusting the mix design to use a softer virgin binder may be initially more costly, but the results could lead to a better product long term and a life-cycle cost savings.
TxDOT is co-sponsoring ongoing research by the Texas Transportation Institute (TTI) led by Fujie Zhou, Jon Epps and Joe Button. The research project is still in progress and will help identify improved mix designs for HMA that contains RAS. RAS binder stiffness and RAS/RAP mix ratios relative to virgin binders are being studied.2
Lee said TxDOT is in the process of making several more changes to its HMA specifications. These may include amendments to its stone mastic asphalt (SMA) specification. The 2011 changes will likely allow for use of RAS. According to Lee, TxDOT does not need to wait for the TTI research project to be completed before the new specifications are adopted. “We like to get something out there and try it,” said Lee. “You can always improve upon the details later once you have some field experience and feedback from the HMA industry.”
HMA Plant Operating Tips
Normal plant operating adjustments may need to be made when using RAS to help optimize the benefits and mitigate the impacts of this new, recycled material. These adjustments can include:
- Mixing temperature—if the paving crew notices an increase in mix stiffness behind the paver, a slight increase in mix temperature can help. A mix with a PG 64-22 binder will behave as if it has PG 70-22 or higher grade binder. Remember, the RAS mix will be stiffer due to type of binder in the shingles.
- Regulating gate adjustments in the feed hoppers (e.g., to help better meter the feeding of RAS into the drum).
- Mixing retention time in the drum may need to be extended slightly to more fully take advantage of the binder in the RAS.
With extended stockpiling, RAS can stick together resulting in a crust on the outside of the pile or clumps that disrupt the regular conveyance of aggregates into the HMA drum. This clumping, sometimes call “reagglomeration,” can happen quickly, especially in the summer heat. TxDOT has a few tips for RAS stockpiles:
- Use the RAS as fresh as possible. While it may not be practical to always employ a “just in time” method of RAS inventory, the longer it sits in the stockpile, the more likely it is to chunk up.
- Do not build the RAS stockpiles too big.
- Pre-blend with other materials that will be used in the mix. Bituminous sand can help the processed shingles from sticking back together. RAP can also be used to keep the RAS from sticking back together.
- When RAS is pre-blended with sand or fine RAP, show the materials as two separate bins on the mixture design job mix formula even though the combined materials are added using a single cold feed bin.
- As an alternative to pre-blending the RAS, install a second cold feed recycled material bin to allow RAS to be fed in separately but commingled with RAP from the first bin. This may allow more uniform RAS blending ratios, but is more difficult to verify continuous mixing performance.
- Agitate the RAS just before loading into the RAS feed bin. Some plants will use a declumping mill for both RAS and RAP to keep these recyclable materials flowing freely as fed into the HMA drum.
- Use grizzly screens over the RAS feed bin to help screen out any large chunks of RAS.
In a recent article in the TxAPA Hot Mix Asphalt Center newsletter, details were reported on the latest TxDPT specification change.3 TxDOT announced in early 2011 a new special specification received approval. This special specification, which is listed as SS 3224, will be used in lieu of Item 341 on all future projects. This is the culmination of months of work led by Dale Rand and the staff at the Flexible Pavement Branch of TxDOT’s Construction Division. TxDOT worked along with members of TxAPA’s Specification Committee and members of the Texas chapter of the Association of General Contractors to review several draft versions of this special specification.
SS 3224 addresses all aspects of HMA production and asphalt paving operations. RAS may be used separately or in conjunction with RAP. One of the new requirements changed the RAS gradation from a minimum of 90% passing to a minimum of 95% passing the 3/8 sieve. The gradation must be tested in accordance with TxDOT specifications (Tex-200-F, Part I). Perform a sieve analysis on processed RAS material prior to extraction (or ignition) of the asphalt.
It is critical shingle recyclers satisfy all regulatory requirements as another means to continue strong and steady market development. TCEQ has oversight authority for allowing and regulating the use of recycled shingles in the state. TCEQ originally first allowed the use of manufacturers’ RAS to be used in HMA in March 2006. In February 2009, TCEQ gave the authority to allow residential post consumer (or tear-off) shingles to be recycled into hot-mix asphalt. The latest TCEQ memorandum allows a maximum of 5% RAS in HMA. The tear-off shingles must come from residential structures, not commercial buildings. The amount of deleterious material in the RAS must be less than 1.5%. This includes anything that is not part of the shingle and means that wood, paper, metal and plastics must be removed prior to grinding. During the shingle grinding operation, a little water is recommended to help with dust control issues. RAS suppliers must certify that the material is from residential buildings and does not contain asbestos or other hazardous waste. A quality control plan to prevent asbestos contamination is required. For example, suspect loads of roofing waste should be rejected for recycling. It is important for shingle recyclers to maintain good documentation including chain of custody records.
TCEQ Air Permits Division may also need to authorize or permit the use of RAS for each specific HMA plant depending on its current permit status and authorization to use RAP. For regulatory purposes, RAS is treated like RAP. No direct flame in the counter flow HMA drum can come in contact with the RAS.
Driving the Changes
Even with all of this government activity, in Texas, it’s not the state agencies driving the changes. The HMA industry has been advocating for these kinds of cost-saving and environmentally beneficial innovations for years. Such HMA changes include use of RAP, RAS and the new “warm mix” technology. Several leaders in the asphalt paving industry have been quoted as saying “Black; it’s the new green.” Without the HMA producers and paving contractors pushing TxDOT and TCEC, these industry improvements would not have come about.
Harold Mullen, executive director of TxAPA, presented at the 49th Annual Meeting of the Flexible Pavements Association of Ohio.4 He said the parking lots at the new Dallas Cowboy Stadium in Arlington, Texas, were paved with some of the most modern, environmentally sound asphalt pavement available on the market today. The HMA included RAS, RAP and used warm mix technology. Mullen estimates such HMA innovations can reduce carbon emissions by about 23%.
The CMRA sponsored a webinar on January 13, 2011, to highlight the shingle recycling programs in Texas. One of the speakers was David Morton, quality control manager for APAC-Texas, Inc.5 As a subsidiary of the larger Oldcastle family of HMA companies, APAC is reportedly the largest user of RAS in HMA.
The history of the use of shingles in HMA in Texas has seen rapid growth in the last four years. As one of the leaders in the industry, APAC purchased its own shingle grinder in February 2008 and got started by using manufacturers’ shingle scrap. Once the new TxDOT HMA specifications (340-003 and 341-022) which allowed RAS, came out in February 2009, the industry was free to use the material on a regular basis. For example, APAC-Texas:
- Throughout 2008, used more than 8,000 tons of shingles;
- In November 2008, more than 3,000 tons of RAP/RAS hot mix was produced and placed on IH 635 in DallasCounty;
- In 2010, used both manufactured shingle scrap and tear-off shingles;
- In 2010, produced and placed 24,000 tons of WMA with RAP/RAS on SH 183 Tarrant County.
Between the experiences of other Oldcastle HMA plants and APAC– Texas, Morton presented the following recommendations during his presentation. Morton said it is critical to develop a thorough quality control plan which includes the following elements:
- Do not to exceed the recycled binder ratio limits. (i.e., measure the AC content of the RAS on a regular basis and determine if it changes between manufacturers’ shingle scrap vs. tear-offs);
- Manufactured waste shingles usually will not stiffen the mix as much as tear-off shingles;
- Make sure to grind the shingles small enough so that 95% to 100% passes the 3/8 in. sieve;
- Blend RAS with sand or RAP to minimize clumping if needed;
- Check RAS daily to verify gradation;
- Treat RAS like you would your RAP when mixing in the laboratory;
- Use 500 to 600 g of RAS when performing ignition oven burn offs; and
- Keep IDT strengths under 200 psi, preferably under 180 psi.
The CMRA anticipates the 5th Asphalt Shingle Recycling Forum will be the best ever. This year, APAC will host a tour on the morning of Thursday, October 27. Several speakers from Texas will address the TxDOT specifications and ongoing research on use of RAS in HMA. For more information, visit the CMRA website www.ShingleRecycling.org.
As per the TxDOT list of nonhazardous recycled materials suppliers as listed on the TxDOT web page at: ftp://ftp.dot.state.tx.us/pub/txdot-info/cmd/mpl/nrm.pdf; Plus the additions on the CMRA ShingleRecycling,org list of Texas Shingle Recyclers at:
2 TxDOT Research Project 0-6614: Use of Recycled Asphalt Shingles in HMA (October 2010), a presentation by Robert E. Lee (TxDOT) and Fujie Zhou (TTI)
3 “TxDOT Approves Special Specification 3224”, Texas Hot Mix Asphalt Center Newsletter (Winter 2011) co-sponsored by TxAPA.
4 Harold Mullen, the Executive Director of TxAPA, “More Tons Through Value” presentation at the 49th Annual Meeting of the Flexible Pavements association of Ohio.
5 Power Point presentation by David Morton, QC Manager for APAC – Texas, Inc. Recycled Asphalt Shingles in Texas, (January 13, 2011)
About the Author
Dan Krivit is a senior project manager with Foth Infrastructure & Environment, LLC and can be reached at 651-288-8509; [email protected].