Can Recycled Tires Make Durable Concrete?

Disposal of used automobile tires made of virgin vulcanized rubber presents a unique problem.

Just as you can never recover flour from a baked cookie, you can’t recover virgin rubber from a tire. Consequently, the three billion used tires generated worldwide every year usually end up in landfills. “Tire mountains” still exist.

In the past it was common practice to burn old tires, but that’s no longer considered ecologically viable. If you can’t burn them and you can’t throw them away, what do you do with them? The answer is, recycle. Used tires are usually shredded to produce fuel and roadway asphalt, among other things.

Researchers at Royal Melbourne Institute of Technology (RMIT) in Australia found a one-to-five ratio of shredded rubber to bitumen produces roadway material that lasts twice as long as bitumen alone. They also devised a way to make durable concrete from recycled tires.

Because traditional concrete production is the third-largest source of industrial pollution, this is potentially a big win for the environment in two ways. It helps solve the problem of tire disposal while reducing atmospheric pollution.

What Is Recycled Tire Concrete?

Each ground-up car tire produces about one kilogram of fibrous material. Recycled tire concrete combines this material with Portland cement and rock aggregate.

Canadian researchers at the University of British Columbia (UBC) in Vancouver have also been working to develop this product. In a 2017 press release, researcher Obinna Onuaguluchi said the process produces a superior grade of concrete with better resilience and longevity. To test it, the UBC team paved the steps in front of a campus building and observed its performance over time.

Recycled tire concrete isn’t the same as the rubberized asphalt concrete (RAC) developed to pave roads. Unlike a road material, concrete must have structural strength to be used for construction, and in this regard RMIT researchers discovered a problem. Air bubbles develop around the rubber fibers as the mixture sets and water evaporates, weakening its structural integrity.

There are two solutions: Include a higher concentration of aggregate rock in the mixture or compress the wet mixture in steel molds to force out the air.

A third method is under development at Rice University. Researchers there reduced the shredded tire chips to graphene, also known as carbon black, by burning it in a low-oxygen environment, extracting useful oils and flash heating the residue. They found that concrete cylinders reinforced with graphene had 30 percent better compressive strength than concrete alone.

The Difference Between Recycled Tire Concrete and Regular Concrete

Sensors placed in the mixture used to pave the steps at the McMillan Building on the UBC campus have so far shown that the addition of rubber fibers strengthens the mixture and reduces cracking by 90 percent, when compared to regular concrete.

The biggest advantage of recycled tire concrete is its beneficial environmental impact. The three billion tires discarded every year produce three billion kilograms of fiber when recycled.

According to Nemy Banthia, the UBC project supervisor, this “could shrink the tire industry’s carbon footprint and also reduce the construction industry’s emissions, since cement is a major source of greenhouse gases.” The fibers, he says, could be included in all of the six billion cubic meters of concrete used every year.

The addition of recycled tire fibers to road paving material results in a longer-lasting mixture. Recycled rubber acts as a sunscreen, reducing deterioration from ultraviolet sunlight and making road surfaces last twice as long. It holds the same promise for structural concrete.

Benefits of Recycled Tire Concrete

When recycled tire concrete does hit the market, builders will realize these benefits:

• More lightweight: Rubber fibers are lighter than aggregate, making the entire mixture lighter and easier to pour.
• More durable: Recycled tire concrete lasts longer and resists cracking better than regular concrete.
• Reduced carbon footprint: Incorporating used tires into concrete keeps the tires out of landfills and lowers the pollution created by conventional concrete production.

It’s too early to tell whether recycled tire concrete will be more or less expensive than conventional concrete. But once the infrastructure has been created to widely produce it, there’s every indication it could be.