Stormwater runoff control and beneficial use of scrap tires are two environmental problems resolved with the installation of porous walkways.  State parks, municipalities, and private construction are using forward thinking and installing these walkways to benefit the environment and the people that walk on them.

By  Tom McGill

If two environmental problems could be resolved with one idea, it would be considered forward thinking and an amazing accomplishment.  Porous walkways, that incorporate scrap tire rubber as a major constituent, are that type of forward thinking.

Scrap tires have been an environmental problem since the day the first car rolled off the assembly line. People have tried burying, burning, and even stockpiling these tires and the problem remains. Today, with many more scrap tires being generated, uses for these tires have not kept up with the need for disposal in a beneficial manner.

Uses for Scrap Tires

One beneficial end use for scrap tires is tire-derived fuel (TDF).  Scrap tires that are shredded into TDF become a very high-energy fuel source that can stabilize BTU energy values (approximately 12,000 to 16,000 BTU per pound) for mixed fuel sources. According to the U.S. Tire Manufacturers Association, the TDF market used 117 million tires in 2015 (more than 48 percent of the total annual scrap tire generation for the year).  While burning old tires is better than landfilling, other beneficial uses for shredded rubber products do exist.

Tire derived aggregate (TDA) is another beneficial use for scrap tires and it includes many civil engineering applications. TDA consumed approximately 25 percent of the scrap tire market in 2015. With a minimum of processing, shredded tires can be used for road stabilization, backfill for retaining walls, and backfill for landfill leachate lines and septic field lines. If the TDA undergoes further processing to remove the steel and tire cord, it can be made into crumb rubber and this material has the greatest potential for growth in the U.S.

Crumb Rubber

Crumb rubber is produced in sizes as large as 3/8-inch to 30+ mesh sieve size and can be used for molded and extruded products, synthetic turf rubber infill, rubberized asphalt, playground safety products, porous walkways and agriculture applications. Manufacturers are becoming more aware of the potential benefits of using crumb rubber in products that will enter a marketplace that is demanding the use of scrap tires in a beneficial manner.

One of the most environmental friendly uses for crumb rubber is porous walkway installation that uses rubber as a major component of the design. These walkways are made from crumb rubber (usually 3/8 inch in size), a rock aggregate of the same size and a polyurethane binder.  The materials can be either mixed by hand in a plastic trough or mixed in a portable cement mixer. The choice of mixing methods will depend on the size of the project.

Porous Walkways

Porous walkways have an inherent flexibility in the design mixture.  The flexible properties of the porous walkways are created by the use of rubber in the matrix of the mixture.  These characteristics make the walkways less likely to crack under normal conditions and more cost effective due to lower maintenance requirements.

Because these walkways are porous, the requirement associated with National Pollutant Discharge Elimination Systems (NPDES) Stormwater Program can be reduced because of the porous properties of the walkways. Runoff problems that plague concrete and asphalt walkways do not exist because little or no stormwater runoff is associated with the porous walkways.

The porous design also allows water to enter the soil through the walkway with no run off under normal wet dry seasonal weather conditions. This benefits trees that may have root systems near or under the walkways by allowing the roots to absorb water in a similar manner as the soil, with no walkway present. Water can be absorbed by the roots under the walkways and this decreases the probability of an enlarging root system. Because the root system will have a lower chance of enlarging and due to the flexible nature of the rubber bearing material, the risk of cracking is greatly reduced.

Another benefit is the fact that the use of rubber bearing porous walkways is a great way to recycle scrap tires. Walkways that use rubber as a major component of the construction material use approximately one tire for every three square feet of installed material. Basically, if a walkway is installed that is one mile in length, six feet wide and three inches thick, approximately 2,640 scrap tires will be used. Hundreds of thousands of tires will be used when this recipe for rubber in walkway design is embraced by architects and engineers.

The demand for porous rubber walkways is increasing because of the green movement that is sweeping the nation. Projects that include recycled materials often can receive grants to offset costs associated with the installation of walkways. Add these incentives to earning points toward Leadership in Energy and Environmental Design (LEED) certification for using recycled materials and the value for using rubber in walkways becomes even more attractive.

A Good Balance

Examples of using this technique are small project walkways that have been installed in some of the Tennessee State Parks as pilot projects with more substantial projects being scheduled. The use of grant opportunities such as the Federal Highway Administration’s Recreational Trails Program (RTP) and Tennessee’s Tire Environmental Act Program (TEAP) is allowing state parks and municipalities to move forward with projects that are cost competitive. Michael Meister, Trails and Vista Administrator with Tennessee State Parks, stated, “Tennessee State Parks is dedicated to promoting recreational opportunities for all, while protecting and preserving our precious natural resources. Flexible porous walkways allow us to provide access to recreation for people of all ability levels due to their compliance with the Americans with Disabilities Act, all the while still being good stewards of our environment by altering an areas hydrology as little as possible.” Meister went on to say, “For us (Tennessee State Parks), we have found these recycled walkways to be the best balance between access and environmental consciousness.”

Stormwater runoff control and beneficial use of scrap tires are two environmental problems resolved with the installation of porous walkways.  State parks, municipalities, and private construction are using forward thinking and installing these walkways to benefit the environment and the people that walk on them.

Tom McGill is an Environmental Specialist with the Tennessee Department of Environment and Conservation’s Office of Sustainable Practices. He manages the Tire Environmental Act Program (TEAP), assists the pharmaceutical take-back program and assists with Radon program. Tom can be reached at (615) 532-8739 or e-mail thomas.mcgill@tn.gov.

Sponsor