Plastic is forever. The millions of tons discarded each year will never rust or rot away.

That is a nagging problem for Americans worried about the nation's overflowing landfills. For Jim Turek, represents it an opportunity to build more durable traffic signs, roads, bridges and buildings.Mr. Turek is chief executive of International Plastics Corp., a Lexington, Ky., company that last year turned 5 million pounds of discarded plastic into construction products and highway signs.

An avid outdoorsman, Mr. Turek says he founded his company partly to make a profit and partly to find practical uses for the vast quantities of plastics discarded in the United States each year.

For example, 735 million pounds of plastic soft-drink bottles were thrown away by Americans in 1988, according to the Council for Solid Waste Solutions. Only about 170 million pounds of this plastic was recycled.

"When you add in the car bumpers and the car hoods and the inner-fender wells, and all this stuff that's now being made out of non-biodegradable, thermal-engineered materials, we've got to find ways to use this," Mr. Turek said.

"Fortunately, we've found one."

Actually, International Plastics has found several uses for "regrind," chopped-up resins bought from plastic recyclers. When mixed with ground glass, the plastic from recycled soda containers becomes a rigid and durable substance, perfect for making highway signs. IPC has sold 7,000 such signs for use in the Kentucky Adopt-A-Highway program.

The company uses a denser polycarbonate plastic to make sheets that are then bonded to plywood and used as forms for concrete pouring. The plastic- coated forms are easier to clean than wood or metal, and they impart a smoother finish to concrete walls.

But International Plastics has found its biggest success in supplying plastic accessories used in pouring reinforced concrete.

Bridges and roads are built by pouring liquid concrete over a network of steel reinforcing rods. These steel rods rest on small devices called ''chairs" and "bolsters" that hold the rods a few inches above the roadbed.

Traditionally, these chairs and bolsters have been made of steel. But Mr. Turek discovered that a mixture of plastic and ground glass can do the job at least as well, and possibly better.

Metal chairs and bolsters tend to rust, which leads to corrosion of the reinforcing rods. This leads to a process called "spalling," in which the rods lose their strength, and the concrete crumbles away.

"We're spending about $10 billion a year just on reconstruction of bridges - not new bridges, reconstruction - as a result of spalling," Mr. Turek said.

He said his firm's plastic chairs and bolsters eliminate spalling, and users of the products agree.

Samuel Hale of Bush & Burchett, a construction firm in Allen, Ky., uses International Plastics' products in bridge-deck construction.

"We would rather have the plastic chairs," Mr. Hale said. "The plastic will not corrode, so there's automatically no worry about corrosion."

The chairs and bolsters have also won praise from Canadian contractors.

"The reaction that we've had by the engineers has been extremely favorable," said Gordie Smith, vice president of contracts at Ennis-Paikin Steel Ltd., a Hamilton, Ontario, firm that distributes International Plastics products in Canada.

International Plastics offers a variety of chair and bolster designs. Most have been approved for bridge and highway use in 37 states. International Plastics chairs and bolsters supplied by Ennis-Paikin were used in the construction of the new Toronto Skydome stadium, the world's largest stadium with a retractable dome.

Mr. Turek has achieved this success despite a lack of formal training either in construction engineering or plastics. Mr. Turek's first job after

college also involved mastering an unfamiliar field. He was hired by aircraft manufacturer McDonnell Douglas Corp. as a corporate financial analyst. The company deliberately chose college graduates with no accounting experience, and trained them to seek out cost savings in its various divisions.

In 1975, he left the company to found Turek Marketing, a St. Louis-based product marketing firm. In 1981, he established Termar Limited, a Des Moines, Iowa, firm which produced trade expositions. In the early 1980s, long before it was fashionable, Mr. Turek says he began worrying about America's solid- waste problem.

"I'm an outdoor person, and seeing a lot of the liter bottles laying around when you go fishing and hunting and camping with the Boy Scouts, I began conceptualizing things you could do with this plastic."

Mr. Turek contacted the nation's leading plastics manufacturers like General Electric Co., Du Pont Co. and Monsanto Co. in search of technical information about their products. "Many times the door would be closed," he recalled. "They wouldn't give you an answer."

But his persistence gradually wore down the companies' resistance. In 1985, an engineer at General Electric mentioned that his firm had attempted to make plastic chairs and bolsters for concrete construction.

The potential market was huge. Mr. Turek estimates that for every mile of two-lane highway, 450,000 chairs and bolsters are used. But General Electric found that new, or virgin, plastic was too expensive to be competitive with metal chairs and bolsters. "These kinds of resins, by the time you add the glass and you get the color, are expensive," Mr. Turek said, "and if you buy them virgin, they're damn expensive."