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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Roofing shingle tabs and, to a much lesser extent, tear-off roofing material waste can be used as a replacement for a portion of the asphalt cement and aggregate in hot mix asphalt and as an ingredient in cold mix asphalt patching material. When used in asphalt paving applications, the properties of constituent materials must be well defined and consistent. Since the composition and properties of old, tear-off roofing shingles are likely to include foreign materials (such as nails, metal flashing, and felt underlayment) as well as asbestos fibers, and can vary widely, prompt scrap that has been left over from the manufacture of new roofing shingles, and which exhibits more consistent properties, is preferred for incorporation into asphalt mixtures.
Laboratory studies undertaken during the 1980's suggest that asphalt mixtures containing roofing shingle scrap could exhibit mix design properties similar to that of conventional asphalt mixtures.(1) Utilization of roofing shingle scrap in hot mix asphalt pavements began in 1990 with trial sections placed in Minnesota.(2) Since then, interest in the use of asphalt paving mixtures containing roofing shingle scrap has increased, with additional studies and trials in Minnesota, Indiana, Illinois, Missouri, New Jersey, and Ontario, Canada.
Typical addition rates for roofing shingle scrap in hot mix asphalt have ranged from 3 to 6 percent (by mass). Evaluations of a New Jersey trial pavement section after a few years of service have indicated that performance similar to conventional hot mix asphalt pavements can be expected, with no significant differences in rut depth, cracking, or skid resistance.(3)
Roofing shingle scrap has also been used in hot mix asphalt for private sector (commercial/ industrial) work in Florida, Minnesota, and Michigan. It has been cited as contributing to improved performance of cold mix asphalt repair materials (pothole repair materials) in Illinois and New Jersey.(4,5) Laboratory studies indicate that prompt roofing shingle scrap can also be used in stone-mastic asphalt applications, with the fiber contained in the shingle material effectively acting as an anti-draindown additive, preventing the asphalt cement from running off this relatively rich, coarse-textured asphalt mixture.(6)
Hazardous waste regulatory leachate test data (TCLP) indicate that the leachate from processed asphalt roofing shingle scrap is well below allowable EPA limits.(7) However, tear-off roofing shingles can contain significant concentrations of polynuclear aromatic hydrocarbons that are present in roofing tar, and there is some concern that asphalt plant air emissions could be impacted during hot mix asphalt production.
When properly designed and executed, the use of processed roofing shingle scrap in hot mix asphalt can result in similar or enhanced properties to standard asphalt concrete pavements, with reductions in requirements for virgin asphalt cement (by approximately 0.5 to 1.5 percent). However, processing costs involved must be assessed against the reduction in virgin asphalt cement and aggregates to properly evaluate the viability of using roofing shingles in pavements.
MATERIAL PROCESSING REQUIREMENTS
No special quality control techniques are required for prompt roofing shingle scrap; however, tear-off shingle scrap is much more difficult to process due to the presence of contaminants and debris such as nails, wood, insulation, etc. Any debris must be removed to prevent equipment damage during size reduction. There is no standard processing equipment to accomplish this task.
Roofing shingle scrap used in asphalt paving mixes is typically shredded into pieces approximately 13 mm (1/2 in) in size and smaller using a shingle shredding machine that consists of a rotary shredder and/or a high-speed hammermill.
Shredded shingles are typically discharged from the shredder or hammermill and screened to the desired gradation and stockpiled. Experience indicates that the size of the processed pieces should be no larger than approximately 13 mm (1/2 in) to ensure complete digestion of the roofing shingle scrap and uniform incorporation into the hot mix asphalt.(8) Scrap shingle greater than 13 mm (1/2 in) in size does not readily disperse, functioning much like aggregate. Particles sized too small can release the fibers, which act as a filler substitute.(9)
Processed roofing shingle material can resolidify during stockpiling, necessitating reprocessing and rescreening prior to introduction to the hot mix plant. To mitigate this problem, processed roofing shingle scrap may be blended with a carrier material such as sand or recycled asphalt to prevent the particles from sticking together.
To keep the roofing shingle material from agglomerating during processing, it is usually passed through the shredding equipment only once, or the material is kept cool by watering at the hammermill. However, the application of water is not very desirable since the processed material becomes quite wet and must be dried prior to introduction into hot mix asphalt.
Some of the properties of roofing shingle tabs that are of particular interest when roofing shingles are used in asphalt paving include asphalt content, asphalt hardness (penetration/viscosity), and aggregate gradation.
Asphalt Cement (AC) Content: The asphalt cement contained in prompt roofing shingle scrap is typically between 25 to 35 percent by weight. Accurate determination of the prompt shingle scrap asphalt content and penetration is not possible using conventional Abson recovery techniques.(1) Extended soaking periods are required to extract and determine the available asphalt.
Asphalt Hardness (Viscosity/ Penetration): The precise determination of the AC viscosity and penetration is not possible because the roofing shingle asphalt is much harder than that normally used in asphalt concrete paving mixtures and contains fibers (which tend to stiffen asphalt concrete mixtures). Roofing shingle scrap must be blended with a much softer (higher penetration/lower viscosity) asphalt cement binder for use in hot mix asphalt.
Gradation: Roofing shingle aggregates (granules) consist of minus 4.75 mm (No.4 sieve size) particles, which supplement the fine aggregate fraction of hot mix asphalt.
Some of the mix properties that are of interest when roofing shingles are incorporated into asphalt pavements include the stiffening influence of roofing shingle fibers, strength, and stripping.
Stiffening Influence of Roof Shingle Scrap: Due largely to the influence of shingle fibers, laboratory studies indicate that incorporating prompt roofing shingle scrap in asphalt mixes tends to improve the high temperature susceptibility and rut-resistant properties of the mix.(6,9,10) Test results also indicate improved fatigue life of pavements with increasing roofing shingle scrap content.(10)
Strength Characteristics: The use of prompt roofing shingle scrap (ranging from 3 to 5 percent by mass of the total mix) results in a denser pavement under compactive effort.(6) While cold tensile strengths are reduced by the addition of roofing shingle scrap,(6) resilient modulus testing indicates that potential for low-temperature cracking is not substantially affected.(3,6,10)
The strain capacity of mixes with scrap shingles appears to be related to shingle type. Mixes containing organic felt shingle scrap exhibit slightly higher strain capacity than those with glass felt shingle scrap.(6)
Stripping: The moisture sensitivity of asphalt mixtures (an indication of stripping potential) does not appear to be significantly affected by the addition of prompt roofing shingle scrap,(6,9) but could be adversely affected by old, used roofing shingle scrap.(6)
Asphalt mixes containing roofing shingle scrap can be designed using standard laboratory procedures. The specifications for new roofing shingles are set out in ASTM D255(11) and ASTM D3462,(12) which provide information on the material constituents.
Roofing shingle scrap (both organic felt and/or fiberglass felt) incorporated into asphalt paving mixes modifies the binder and also acts as a fine aggregate and mineral filler. Typically, an increase in filler content lowers the optimum asphalt cement content and increases the density and stability of the mix. Higher filler content can adversely affect durability due to reduction of voids in the mineral aggregate and optimum asphalt cement content. Also, excessive mineral filler can increase the surface area of the aggregate, reducing the asphalt film thickness.
Roof shingle scrap may be susceptible to moisture-related damage (stripping) and, therefore, the mix design should include a stripping resistance or retained stability test such as AASHTO T283(13) or MTO LS-283.(14)
Conventional AASHTO pavement design methods are appropriate for asphalt pavements incorporating roof shingle scrap in the mix.
Material Handling and Storage
Accommodation must be made to avoid agglomeration of processed roofing shingle scrap, which can result in poor dispersion in the paving mix. One method of controlling agglomeration is to shred the material at the asphalt plant, immediately prior to introduction into the mixer. However, this generally requires that a shredder be installed at each plant and may not be practical.
Mixing, Placing and Compacting
Processed roofing shingle scrap can be blended with fine aggregates in a pugmill (at a ratio of three parts roofing shingle scrap to one part fine aggregate).(8) The fine aggregate acts as a carrier for the shingle scrap, preventing agglomeration and producing a material that can be stockpiled, handled, and fed through the existing recycle system of a batch or drum mix plant.
The same methods and equipment used for conventional pavements are applicable to asphalt pavements containing roofing shingle scrap.
The same field testing procedures used for conventional hot mix asphalt mixes should be used for mixes containing roofing shingle scrap. Mixes should be sampled in accordance with AASHTO T168(15), and tested for specific gravity in accordance with ASTM D2726(16) and in-place density in accordance with ASTM D2950.(17)
There is a need to establish standard mix design methods for use in designing asphalt pavements using roofing shingle scrap. There is also a need to verify the long-term performance of hot mix asphalt incorporating suitably processed roofing shingle material with respect to fatigue, rutting, and low-temperature cracking. The recyclability of asphalt pavements incorporating roofing shingle material at the end of the service life of the pavement needs to be evaluated along with life cycle cost analysis to determine the economic viability of using roofing shingle scrap.
TRT Terms: Waste products as road materials--Handbooks, manuals, etc, Pavements, Asphalt concrete--Design and construction--Handbooks, manuals, etc, Pavements, Concrete--Design and construction--Handbooks, manuals, etc, Pavements--Additives--Handbooks, manuals, etc, Fills (Earthwork)--Design and construction--Handbooks, manuals, etc, Roads--Base courses--Design and construction--Handbooks, manuals, etc, Wastes, Environmental impacts, Recycling