Tear-offs in Asphalt Concrete

Virginia DOT takes a hard look at the use of post-consumer shingles in asphalt concrete

By G. W. Maupin Jr, P.E.

In response to Virginia Senate Bill No. 469 in the 1990s, the Virginia Department of Transportation (VDOT) formed a committee to review materials and prepare methods to allow and encourage the use of recycled products in highway construction. Many waste materials were discussed, including roofing shingles, which contain asphalt, aggregate, and fibers. “The committee recommended the Virginia Department of Transportation [VDOT] have its Research Council conduct some research in this area for possible future use if it is found that a reasonable supply of material is available and a proper mixture can be developed.”1 The Virginia Transportation Research Council (VTRC) conducted a literature review of the use of shingle waste in asphalt, and a draft special provision was developed by VTRC and VDOT personnel in 1999 to allow contractors to use the shingles in hot mix asphalt (HMA) upon request. A small paving project using the waste shingles would allow VDOT to evaluate the process for approval in possible future larger projects.

In 2006, Rose Brothers Paving of Ahoskie, N.C., submitted a request to VDOT to experiment with the use of manufacturing waste shingles on a secondary road overlay project near Franklin, Va. This type of waste is created during the manufacturing process and includes pieces of shingles and unsatisfactory whole shingles typically created during the manufacturing process. Approximately 2,000 tons of SM-12.5 surface mix containing 5% shredded manufacturing waste shingles and 2,000 tons of SM-12.5 surface mix containing 10% recycled asphalt pavement (RAP) were placed on the same project. The conventional mix containing RAP was installed at the same location to allow a comparison of the construction process and field performance. Both mixes contained PG 64-22 virgin binder, and the shingle waste came from a manufacturing facility in North Carolina. Since the results of all laboratory tests and early performance tests were encouraging, VTRC’s Asphalt Research Advisory Committee (ARAC) recommended research also be conducted on the use of tear-off roofing shingles.

An estimated 11 million tons of waste tear-off shingles removed from roofs is generated per year nationally.1 Although records of separate construction debris are not tracked in Virginia, an estimate of waste shingles from Virginia roofs based on the proportion of population in Virginia compared to that of the United States is approximately 280,000 tons. If shingles contain approximately 25% asphalt binder, this waste product could supply 70,000 tons of binder annually in Virginia, which is enough binder to overlay or resurface 2,000 lane-miles of pavement.

However, using tear-off shingles presents several potential challenges that do not exist with the use of manufacturing waste shingles. Tear-off shingles have aged because of weathering exposure, possibly causing brittleness that could affect the durability of the pavement. In addition, asbestos was used in domestic shingles in small amounts prior to the mid-1980s. Reports of extensive asbestos testing indicated that asbestos has been detected in only very small amounts in very few samples;2,3 therefore, it may not be a huge obstacle to shingle use. One possible approach to this problem would be to specify that roofing shingles must contain less than 1% asbestos. A material containing more than 1% asbestos is categorized as an asbestos-containing material by the National Emission Standards for Hazardous Air Pollutants. Another potential problem concerns deleterious materials such as metal flashing, nails, paper and wood that may not be removed properly during the recycling process. The cleanliness of recycled shingle material ready to be incorporated into asphalt concrete will depend on the enforcement of specifications designed to provide an acceptable material.

Some states either allow the use of tear-off shingles or are experimenting with it. Missouri has used it to some extent since 2005. In 2009, the Missouri Department of Transportation (MoDOT) used a considerable amount of shingles in one-third of Missouri’s asphalt mixes (J. Schoer, unpublished data). Missouri allows up to 7% shingles with no change in the grading of the virgin binder if the virgin binder provides at least 7% of the total binder in the mix. South Carolina has a permissive specification that allows 3% to 8% shingles, but its use has been limited.4 The Construction Materials Recycling Association provides information for shingle recycling in asphalt mixes relating to the experiences of several states, references, research, recycling, etc., on its Web site.5

Generally, the early performance results of pavements containing recycled tear-off shingles have been good. Even though it is a waste material, it may offer some benefits such as rutting resistance, because of the presence of stiff binder and fibrous materials, and cracking resistance, because of the fibers.

Purpose and Scope
The purpose of this study was to evaluate the suitability of using tear-off shingles in asphalt concrete. The study was designed to determine whether mixes can be produced where excessive aging of the shingle additive is not detrimental to the mix durability. The scope of the study was limited to several installations in Virginia constructed voluntarily by three asphalt paving contractors, and tested by VTRC.

Methods
VDOT issued an invitation for contractors to use tear-off shingles in a small tonnage of experimental mix in 2009 in order to gain experience with the material and allow VTRC to evaluate the product. A special provision had been developed for the experimental sections to be placed (See Sidebar, p. 28). Three asphalt contractors elected to try the material, producing a total of three surface mixes and two base mixes (Table 1).

The two BM-25.0A base mixes were produced by different contractors. One mix contained 5% shingles, and the other contained 4% shingles. Three SM-12.5A surface mixes were produced, including HMA and warm mix asphalt (WMA) versions of the same mix design by the same contractor in a “green plant.” A green plant uses a small amount of water to foam the asphalt cement and provide workability at lower temperatures. Two of the surface mixes contained 5% shingles, and the third contained a combination of 18% RAP and 2% shingles.

Samples of mixture were taken from the truck before shipping to the paving site in each case in order to perform laboratory testing later. The target air-void content was the typical value reported through quality assurance testing by field personnel. Tests performed subsequently in the laboratory determined gyratory volumetric properties, rutting and fatigue. In addition, binder was recovered from the mix and graded in order to help estimate future pavement performance.

Limited additional testing was done in the laboratory in an attempt to determine the true amount of binder in shingles and the relation between binder content determined by the ignition furnace and extraction methods. Indirect tensile testing was also performed to indicate if blending of the virgin binder and shingle binder could be influenced by the lower mixing temperatures that are typically used for WMA. The researcher thought tensile tests were appropriate simple tests that related to the behavior of the binder in this situation.

The endurance limit was estimated for each mix using the fatigue life and strain level for each of approximately 10 specimens. The endurance limit is defined as the strain at which asphalt concrete can endure an infinite number of load cycles.8 In a practical sense, for this study, it was defined as the strain level at which asphalt concrete survives at least 50 million cycles, and it was projected from the regression of the test results for each mix. This endurance limit equates to approximately 500 million load cycles on an in-service pavement, i.e., 40 to 50 years of traffic on a heavily trafficked road. The endurance limit was roughly estimated from the 95% confidence one-sided lower prediction limit for a fatigue life of 50 million cycles.

Binder Recovery and Grading
Binder was recovered from mix samples by extraction (AASHTO T164, Method A) and Abson recovery (AASHTO T170).6 Then, the recovered binder was graded in accordance with AASHTO M320. Multiple temperatures were used in order to determine an exact grade rather than just the passing grade, which is normally obtained for acceptance testing.

Next Issue: The results.

About the Author
G.W. “Bill” Maupin recently retired as principal research scientist at the Virginia Research Council. He can be reached at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

References
1    Recycled Materials in High-way Construction Advisory Commit-tee. Report of the Virginia Department of Transportation in Response to Senate Bill 469— Use of Recycled Materials in Highway Construction. Draft Report. Richmond, 1994.
2    Hansen, K.R. Guidelines for Use of Reclaimed Asphalt Shingles in Asphalt Pavements. Information Series 136. National Asphalt Pavement Association, Lanham, Md., 2009. 14
3. Hansen, K. Shingles: There’s black gold in those roofs. Hot Mix Asphalt Technology, Vol. 14, No. 1, January/February, 2009, pp. 17-21.
4    New England C&D Recycling Issues. C&D World, Vol. 1, No. 2, March/April, 2009, pp. 30-33.
5    Construction Materials Recycling Association. www.shinglerecycling.org. Accessed November 18, 2009.
6    American Association of State Highway and Transportation Officials. Standard Specifications for Transportation Materials and Methods of Sampling and Testing, 27th ed. Washington, D.C., 2007.
7    Virginia Department of Transportation. Virginia Test Methods. Richmond, 2007. http:// www.virginiadot.org/business/materials-download-docs.asp. Accessed June 12, 2008.
8    Transportation Research Board of the National Academies. NCHRP 09-38 [Completed]: Endurance Limit of Hot Mix Asphalt Mixtures to Prevent Fatigue Cracking in Flexible Pavements. 2010. http://144.171.11.40 /cmsfeed/TRBNetProjectDisplay.asp?ProjectID=972. Accessed March 4, 2010.
9    Maupin, G.W., Jr., Diefenderfer, S.D., and Gillespie, J.S. Evaluation of Using Higher Percentage of Recycled Asphalt Pavement in Asphalt Mixes in Virginia. VTRC 08-R22. Virginia Transportation Research Council, Charlottesville, 2008.

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