Highway engineers in Texas have shown that discarded household roofing shingles can be recycled into durable road surfaces that perform as well as standard asphalt after years of heavy traffic. In 1997, the Texas Department of Transportation (TxDOT) built two 1,000-foot test sections along State Highway 31 in the Dallas District using hot mix asphalt containing processed shingle waste. Long-term monitoring showed that the recycled road surfaces performed as well as traditional asphalt control sections. The results helped lay the groundwork for state-wide rules allowing recycled asphalt shingles to be used in road construction. The trial was part of a wider research project documented by the Center for Multidisciplinary Research in Transportation at Texas Tech University. It was also covered in TxDOT report 9-1509-2, Road to Recycling: Findings and Recommendations, written by research supervisor Phillip T. Nash and project director Rebecca Davio.
Testing post-consumer and factory waste
The State Highway 31 trial tested two different sources of waste shingles to find out whether their origins or processing affected pavement performance. One 1,000-foot test section used five per cent post-industrial scrap collected directly from manufacturing plants. The nearby 1,000-foot section used five per cent post-consumer shingles removed from residential roofs. A third 1,000-foot section contained no shingles and used a standard Type C asphalt mix as the control. Each test section needed about 600 tonnes of asphalt mix, including 18 tonnes of shredded shingle material supplied by Thelin Recycling Company. Before being added to the asphalt, the shingles were processed to remove nails, metal fasteners and other debris. Industrial magnets and air blowers were used for cleaning, after which the material was ground into particles smaller than half an inch. Roofing shingles are made from a fiberglass or organic felt mat covered with petroleum-based asphalt and mineral granules. Adding the old shingles to paving mixtures therefore reduces the amount of new asphalt binder needed for road construction.
Measuring pavement endurance under traffic
After construction, TxDOT engineers and university researchers monitored the test sections using visual inspections, core samples and non-destructive structural tests. They checked the roads for long-term wear, rutting and cracking. Research results published in the Texas Tech University report 9-1509-2 found that both post-industrial and post-consumer shingle additives performed reliably under real traffic conditions. "Both post-consumer and post-industrial roofing shingles were successfully demonstrated as additives to hot mix asphaltic concrete pavements," the researchers recorded in the report summary. "Performances of the demonstration project test sections are comparable to those of conventional mixes." The recycled shingles also provided engineering benefits beyond similar durability. Synthetic fibres and polymers in the old roofing material increased the stiffness and resistance to rutting of the asphalt mixture during hot summer weather. This helped reduce pavement deformation under heavy freight traffic.
Expanding state-wide recycling guidelines
The strong performance on State Highway 31, along with later field trials across Texas, led TxDOT to create formal rules for using recycled asphalt shingles in state highway projects. In an official TxDOT technical publication titled From Roofs to Roads, transport officials outlined quality requirements for using shingle waste. The rules allow up to five per cent processed shingle material in hot mix asphalt. To protect pavement quality, the specifications require all recycled shingle particles to pass through a half-inch sieve and contain less than 1.5 per cent unwanted material. "Several benefits of recycling asphalt shingles include an economic cost savings on the price of hot-mix, an increase in the strength and stiffness of HMA mixes from the fibres and polymers in RAS, conservation of raw materials required for mixes and a reduction in the consumption of landfill space," TxDOT documented in the publication. With millions of tonnes of damaged and old roofing shingles sent to landfills across the United States each year, using post-consumer shingles in road construction gives transport departments a cost-effective way to turn construction waste into strong, long-lasting highway surfaces.
