Civil engineering applications for scrap tires are fundamentally different from most other scrap tire markets for one very obvious reason—size. ``In most scrap tire recycling applications, you use crumb rubber,'' according to Dana Humphrey, a civil engineering professor from the University of Maine.
``In civil engineering, we're using very large pieces of tires in very large quantities to solve some very large problems for civil engineers.''
Humphrey is on a year's sabbatical from his teaching job to work full time for the Scrap Tire Management Council, doing research and giving seminars on the value of scrap tires in civil engineering projects.
One of his many scheduled appearances this year was at ``Rubber Recycling '98: The North American Experience,'' an Oct. 22-23 symposium in Mississauga, Ontario, co-sponsored by the STMC and the Rubber Association of Canada.
The first thing civil engineers and government officials must understand, Humphrey said, is that scrap tires are anything but mere filler in a roadbed, highway embankment or bridge abutment.
``We're using tire shreds because we need their specific properties,'' he said. ``They're lightweight, weighing less than half of what soil weighs. They have low earth pressure, meaning that when you put them behind a wall, they don't push on that wall very hard."
``They're a good thermal insulator, eight times better than soil and limiting frost penetration to a roadbed,'' Humphrey continued. ``They are very compressible, offer good drainage and also are often the cheapest solution to a civil engineering problem.''
Price figures illustrate the economy of shredded tires as a lightweight fill, he said. Tire shreds work out to about 37 cents per pound of weight credit, compared with 66 cents for extruded polystyrene scrap and $1.60 for shale.
Tire shreds are useful in landfill designs because of their high permeability, low cost and compressibility that allows the use of 75 scrap tires in one cubic yard of space, Humphrey said. But the crucial test of their value has been in highway and bridge construction.
According to Humphrey, his home state of Maine has led the way in innovative scrap tire use in civil engineering. The three major projects in Maine to date have been fill for a rigid-frame bridge in Topsham, a railroad bridge abutment in Topsham and the Congress Street-Jetport highway interchange in Portland.
In the rigid-frame bridge project, soil was pressing against a thick retaining wall, threatening its collapse, Humphrey said. A three-foot layer of tire shreds—100,000 tires in all—lowered the earth pressure by 50 percent.
The railroad bridge was endangered by being built on crumbly clay soil, again threatening the collapse of the entire embankment and endangering a protected species of fish in the river below.
``It must be a human failing to build a city on the worst soil possible,'' Humphrey said. ``In my line of work, that's what we call an opportunity.''
Engineers excavated the upper portion of the existing slope at the bridge site, putting in 400,000 shredded tires topped with six feet of soil. This improved the slope's stability and lessened pressure on the back of the bridge.
The compressibility of the tires was the major factor in the project's success, according to Humphrey. ``To get the required 14 feet of fill, we actually had to put in 151/2 feet of tire shreds,'' he said.
The Congress Street interchange presented a similar problem as the railroad bridge, according to Humphrey—``a big city built on that awful, goopy marine clay.''
Solving the problem in this case were two 10-foot layers of tire shreds—1.2 million tires in all—separated by three feet of soil. The soil separating the tire layers is extremely important, Humphrey said.
``You don't want the tire layers too thick, because of the danger of overheating,'' he said. Early highway embankment projects using scrap tires in Washington state were unsuccessful because the tire layers' thickness caused heat buildup and combustion. But new guidelines from the American Society for Testing and Materials, of which Humphrey was principal author, set safe limits for the design and use of scrap tires in civil engineering.