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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
There are two types of roofing shingle scraps. They are referred to as tear-off roofing shingles, and roofing shingle tabs, also called prompt roofing shingle scrap. Tear-off roofing shingles are generated during the demolition or replacement of existing roofs. Roofing shingle tabs are generated when new asphalt shingles are trimmed during production to the required physical dimensions. The quality of tear-off roofing shingles can be quite variable.
Approximately 10 million metric tons (11 million tons) of asphalt roofing shingle scrap is generated each year in the United States.(1) It is estimated that 90 to 95 percent of this material is from residential roof replacement ("tear-offs"), with the remainder being leftover material from shingle production ("roofing shingle tabs").
Roofing shingles are produced by impregnating either organic felt produced from cellulose fibers, or glass felt produced from glass fibers, with a hot saturant asphalt, which is subsequently coated on both sides with more asphalt and finally surfaced with mineral granules. Most roofing shingles produced are of the organic felt type. The saturant and coating asphalt need not be the same. Both saturant and coating asphalts are produced by "blowing", a process in which air is bubbled through molten asphalt flux. The heat and oxygen act to change the characteristics of the asphalt. The process is monitored, and the "blowing" is stopped when the desired characteristics have been produced.
The largest component of roofing shingles (60 to 70 percent by mass) is the mineral material. There are several different types in each shingle.(2) They can include ceramic granules (comprising crushed rock particles, typically trap rock, coated with colored, ceramic oxides), lap granules (coal slag ground to roughly the same size as the ceramic granules), backsurfacer sand (washed, natural sand used in small quantities to keep packaged shingles from sticking together), and asphalt stabilizer (powdered limestone that is mixed into the asphalt).
Specifications for roofing shingles are set out in ASTM D255(3) and ASTM D3462.(4)
CURRENT MANAGEMENT OPTIONS
Small quantities of prompt shingle scrap, typically shredded to 38 mm (1.5 in) and smaller, have been used as a gravel substitute for the wearing surface for rural roads and farm lanes. Increasing use of processed tabs or prompt roofing shingle scrap and, to a much lesser extent, tear-off roofing material is being made as a modifier to hot mix asphalt pavements, stone mastic asphalt pavements, and cold mix asphalt patching material.
Most roofing shingle scrap is presently disposed of by landfilling.
Roofing shingle tabs (prompt shingle scrap) can be obtained directly from shingle manufacturers. Tear-off shingle scrap can be obtained directly from roofing contractors or disposal sites that accept the scrap. Information on tab or prompt scrap sources can be obtained from the roofing shingle suppliers or manufacturers.
The general composition of prompt roofing shingle scrap is identical to the shingles from which it is cut except for substantially smaller dimensions. Proportions of the primary components of asphalt shingles are listed in Table 15-1.
Table 15-1. Components of asphalt shingles.(2)
Tear-off roofing shingle scrap typically contains small percentages of foreign materials, including nails, felt underlayment, metal flashings, and wood, as well as waterproofing and insulation materials. The asphalt cement binder component of roofing shingle is generally old, severely weathered and oxidized.
The specification for glass felt shingles (ASTM D3462) permits asbestos fibers. Although not widely used, processing of shingles containing asbestos fibers can present a health risk to workers handling the material.
HIGHWAY USES AND PROCESSING REQUIREMENTS
Asphalt Cement Modifier
Recent tests strongly suggest that roofing shingle tabs (prompt roofing shingle scrap) can be used as an asphalt cement modifier. The incorporation of roofing shingles into asphalt mixes results in reduced asphalt cement requirements and tends to result in stiffer mixes, with improved temperature susceptibility and rut resistance.
Prompt roofing shingle scrap is mainly produced in tabs approximately 285 mm long by 9.5 mm wide by 3 mm thickness (11.5 in by 3/8 in by 1/8 in) which must then be processed to suitable size for introduction into the hot mix asphalt. The asphalt tabs are processed in two stages. The tabs are first shredded using a rotary shredder consisting of two slow-speed blades turning at approximately 50 revolutions per minute. This reduces the chips into smaller, but still quite coarse pieces. The smaller pieces are then reduced to a nominal size of about 9.5 mm (3/8 in) or finer using a high-speed hammermill operating at about 800 to 900 revolutions per minute. 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 hammer mill, then stockpiled. 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.
Tear-off roofing shingle could potentially be used as an asphalt modifier, but tear-off shingle scrap is much more difficult to process because of the presence of contaminants and debris such as nails, wood, insulation, etc. Any debris must be removed to prevent equipment damage during size reduction.
Aggregate Substitute and Mineral Filler
Roofing shingles incorporated into asphalt paving mixes not only modify the binder, but also, depending on the size of the shredded material, function like an aggregate or mineral filler. Organic felt and glass felt particles in particular tend to function like a mineral filler substitute.
Roofing shingles are unlike other by-product or secondary materials in that they contain components of fine aggregate, mineral filler, and asphalt cement. There are also differences between the types of shingles (organic and glass felt) produced. As shown in Table 15-2, organic felt shingles can be expected to exhibit higher moisture content and lower specific gravity than glass felt shingles. Shredded organic felt shingle scrap also exhibits much higher absorption than shredded fiberglass shingle scrap.
Table 15-2. Typical physical properties of shredded roofing shingle scrap.(5)
Typical physical properties of recovered asphalt cement and the gradation of mineral granules in shingle scrap are listed in Table 15-3. Asphalt cement in old roofing shingles undergoes oxidative age hardening and stearic hardening (a hardening process in which solid compounds separate from volatile oils in the asphalt cement). Consequently, the asphalt cement in old tear-off roofing shingles is somewhat harder than new asphalt. Although the stearic hardening process has been demonstrated to be reversible by reheating and/or solubilizing,(1) oxidative age hardening is not reversible.
Table 15-3. Typical physical properties of recovered asphalt cement and gradation of recovered mineral material from roofing shingle scrap.(5)
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