Bring Sustainable Practices to Road Construction Projects
According to the EPA, U.S. industries produce over a half billion tons of potentially usable materials annually. Some of these waste materials are now being recycled in highway construction projects.
The Pennsylvania Department of Transportation (PennDOT) and road construction companies have been developing sustainable practices for incorporating recycled materials into transportation improvement projects. These practices can assist in reducing waste in land fills, conserving natural resources, and reducing costs and emissions associated with raw materials.
PennDOT's Strategic Recycling Program was created in 1998 to systematically identify, evaluate, and implement recycling opportunities for transportation infrastructure throughout Pennsylvania. Coordinating the use of recycled materials begins with the planning and design of PennDOT projects. Overall, the performance of projects constructed with recycled materials has been equal to, if not greater than, conventional materials.
PennDOT has found that the cost of using recycled materials has been comparable to the cost of conventional methods because of several factors. Waste materials are typically less expensive to purchase and decrease costs associated with extracting and processing natural resources. If there is a large enough storage site within 25 miles of the project location, transporting recycled materials can be significantly less costly than transporting raw materials. Currently, savings can be offset by costs of the equipment needed to prepare the materials to PennDOT's specifications.
PennDOT Specifications have been developed for materials such as rubber tires, glass, and fly ash, while other materials, such as post-consumer shingles, are continuing to be evaluated.
Rubber Tires. Waste tire disposal is an issue in Pennsylvania and PennDOT has continued testing and evaluating the use of rubber in road maintenance and construction projects. Incorporating rubber tires into the road base mix has shown to improve the life cycle of the roadway. The road is more flexible and durable reducing damage caused by freezing and thawing.
Seal coating is a preventative maintenance strategy used to slow roadway deterioration by filling cracks with tar and small stones. The use of rubber tires in the tar mix has demonstrated better adhesion to the stones, reducing the amount of materials picked up and thrown by passing vehicles. PennDOT is expecting that a 2007 project using 22% recycled materials will double the road's extended life versus conventional methods.
Shredded rubber tires can also be used as a lightweight fill material that can relieve pressure on retaining walls. In addition, the rubber tires drain well and resist frost penetration. PennDOT's Tarrtown Bridge Project utilized approximately 557,000 tires in the two bridge embankments. PennDOT is currently discussing the use of tire shreds in the concrete mix for noise walls planned for construction in Chester County. The tire-blended noise walls help to absorb sounds, as opposed to conventional concrete noise walls which reflect the sound.
Mushroom Compost. Spent mushroom substrate compost is the organic material generated from harvesting mushrooms and is said to provide more nutrients than traditional topsoil. As part of 2004 improvements to the US 30 interchange with PA 113, spent mushroom substrate compost was used as part of the embankment blanket/seeding operation. According to PennDOT, performance evaluations have shown good establishment of vegetative growth. The material provider for this project was Laurel Valley Soils in Avondale.
Shingles. Shingles contain some of the same basic materials as hot-mix asphalt used in road construction. Several demonstration projects have been comprised of 5% scrap materials from manufacturers. After a six-year evaluation, the road segments incorporating shingle showed improved performance. PennDOT's use of post-consumer shingles is still pending, as the materials require additional preparation to remove items such as nails found in the construction and demolition debris.