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. It was also covered in TxDOT report 9-1509-2, Road to Recycling: Findings and Recommendations, written by research supervisor Phillip T. 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. 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. Nash and project director Rebecca Davio.
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. 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. 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.
“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. “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. 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. “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. 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. 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.