By John Davis
Since the early and mid-1980s recyclers have been eyeing factory waste asphalt shingles and old tear-off roofing shingles as a legitimate source of asphalt binder.
Each year, almost 10 million tons of roofing shingles are removed from rooftops or hauled away as scrap from manufacturers. Roofers and shingle manufacturers used to haul them off to the landfill, but nowadays some are sending them to the local hot mix asphalt (HMA) plant.
DOTs and road agencies around the U.S. are discovering that using these roofing shingles in an HMA mix can produce a smooth performing pavement if done properly.
Composition of Asphalt Shingles
In several ways, shingles make an ideal additive to an HMA mix. They add asphalt, fine granules and mineral fillers. They are composed of asphalt, sand and fiber. Shingles are 30-35 percent asphalt, 5-15 percent mineral fiber and 30-50 percent mineral granules. Fiberglass shingles are 15-20 percent asphalt, 5-15 percent felt, 15-20 percent mineral filler and 30-50 percent mineral granules.
Tear-off shingles from residential roofs contain a greater percentage of asphalt than new shingles because they have lost a portion of the surface granules due to weathering. The tear-off shingles are hardened from oxidation and the volatilization of the lighter organic compounds. They shred more easily because the aged asphalt in the tear-offs is harder and more brittle. Although tear-offs contain more asphalt, nails and other debris must still be removed before feeding them into the asphalt mix.
Processing Roofing Waste
To be used in paving applications, shingles must be shredded or ground. Generally, for hot mix asphalt, the shingle pieces must be smaller than ½-inch. The Texas DOT requires that 100 percent of the shingle shreds pass the 19 mm (3/4-inch) and that 95 percent pass the 12.5 mm (1/2-inch) sieve. The Georgia DOT requires that 100 percent of the shingle scrap pass the 12.5 mm sieve, while the Federal Highway Administration (FHWA) recommends that shreds be less than ½-inch.
Crushers, hammermills and rotary shredders are used to process waste shingles. Oftentimes, the shingles are passed through the processing equipment twice for size reduction.
“We grind the shingles,” says Jerry Lambert, Operations Manager for Recycling & Processing Equipment Inc.(RPE), based in Indiana. “There’s really no other way. We grind them to an approximate minus ½-inch, but we can also grind them to an approximate 3/8-inch. If the contractors are doing intermediate and base course work, they want ½-inch,” says Lambert.
Bituminous Roadways, a Minnesota shingle processor and asphalt paving company, and Minnesota DOT (MnDOT) have used scrap shingles for several years. Bituminous Roadways uses ½-inch ground shingles in base layers and surface courses on highways, low volume roads and on driveways and parking lots.
“We’ve been using shingles in our mixes since 1997-98,” says Todd Smedshammer, Plant Manager for three Minneapolis Bituminous Roadways HMA plants. “Last year the shingle manufacturer sent us 22,000 tons of factory scrap shingles. We process them and use them in our mixes almost eight months out of the year.”
Smedshammer says they have found that adding shingles to the HMA doesn’t compromise the mix. “Our goal is to grind them and mix them and place them in such a way that no one knows the shingles are in the mix.” He adds that shingles comprise about 5 percent of the mix by weight.
“Using the shingles in the mix involved a big learning curve,” says Smedshammer. “We did demonstrations and private projects to prove that recycled shingles worked. At first the state wouldn’t let us use shingles in every mix, but now they do.”
During the early 2000s, MnDOT provided provisional specifications for using asphalt shingles in pavement mix, adds Smedshammer. “But from 2003 or ’04 MnDOT has allowed us to use recycled shingles in a variety of mixes without provisional specs.”
Georgia DOT Test Sections
The Georgia DOT paved two test sections using roofing shingles in HMA in 1994. They paved a 1500-foot section of the northbound lane of the Chatham Parkway in Savannah with a 2-inch thick base course.
The fiberglass-backed shingle scrap was incorporated in the mix as RAP. Mix sampling at the time showed that the asphalt shingle HMA material properties were similar to the conventional HMA mix.
Core testing showed that the shingle cores compared well with the job mix formulas and plant mix tests. Field observations recorded that the shingle mix showed little distress and performed as well as the control sections.
Georgia DOT also resurfaced one mile of State Route 21 in Effington County with the same shingle-modified asphalt mix as it used on the Chatham project. After two years, six cores taken from State Route 21 showed that the shingle mix pavement was performing well.
Missouri Shingle Use
The Missouri DOT’s (MoDOT) laboratory research on tear-off scrap shingles resulted in a new shingles recycling specification in early 2005. Pace Construction is one of the primary HMA producers in Missouri using tear-off shingles. ECO Recycling is responsible for receiving the mixed roofing material, then sorting, grinding and screening it for use in Pace’s HMA plants.
“We use shingles in a range of HMA mixes, from large private parking lots to Superpave on county roads and city streets,” says Tim Mitana of Pace Construction in St Louis, Missouri. “We use shingles on nearly all of our projects,” adds Mitana. “This is our third year of using them. Once we learned the mechanics of handling the recycled shingle mix, the roads we pave show the same result as if paved with a virgin HMA mix.”
Mitana says that Pace uses mostly tear-off shingles and puts 2 to 5 percent in the mix, according to MoDOT specifications. ECO Recycling processes and supplies the tear-off shingles. ECO removes nails and other debris and ensures that the shingles are free of asbestos.
A typical shingle project may use up to 30,000 tons of HMA with roofing shingles in the base and surface courses, says Mitana. “These are volumetric-based projects with good densities and bonus pay—103 percent.”
The costs of producing asphalt shingles include processing, transporting shingles from the manufacturer to the processor and to the HMA plant. Cost savings from using shingles comes from reduced amounts of raw materials such as liquid asphalt and fine aggregate, and reduced disposal or tipping fees.
Savings from disposal fees—when they range from $20 to $30 per ton—are significant. The National Asphalt Pavement Association estimates cost savings to be between $1.00 and $2.80 per ton when using 5 percent shingles in HMA.
Laboratory tests and field tests performed have shown positive results when asphalt shingles are used in HMA. Many DOTs feel comfortable using manufactured scrap because of its uniform quality. And more DOTs are allowing tear-offs.
As landfill space shrinks and tipping fees increase the use of asphalt shingles in HMA becomes more economical.
“One of the big advantages of using shingles in HMA is financial,” says Mitana. “And it’s good for the environment. It keeps shingles out of the landfill and uses them to create a necessary piece of our infrastructure.”
|John Davis is a contributing editor for Asphalt magazine.