Welcome to the Rubber Road?
When I was much younger a humorous schtick on television had an excited professor talking about his new invention. It was the inflatable rubber road, complete with painted white and yellow road stripes. An observer asked about the danger of using rubber tires on the rubber road. “No problem," explained the inventor, “I’ve also invented the concrete tire!" It seemed ludicrous at the time but in the past couple of decades the rubber road has become a reality- not inflatable and not entirely rubber- but recycled tire rubber mixed into asphalt.
Grinding Up Old Tires to use in New Asphalt
Transportation requires vehicles of some sort, and most vehicles in turn require tires. Lots and lots of tires. Annual production numbers show over 200 million tires manufactured globally for automobiles, trucks, farm equipment, and recreational vehicles. When these tires wear out there are three choices for disposal. Throw them away at a dump, try to use the worn tires architecturally, or grind them up and reuse the raw materials for other products. One such product use adds the grindings into the binder portion of asphalt paving mixtures. The fine rubber particles are first screened to remove bits of metal, plastic, and other non-rubber components used in tire manufacture. The rubber bits are then mixed into the liquid asphalt binder at a temperature high enough to swell and react the rubber with the binder. This helps keep the bits suspended in the liquid binder until it is mixed with the usual aggregates and sand at the asphalt concrete mix plant. The result is called Rubber Modified Asphalt Concrete (RMAC) and is used in a variety of pavement applications. Becoming more and more popular, many government pavement specifications now refer to RMAC as a biddable product
Advantages of Turning Scrap Tires into Pavement
Grinding scrap tires uses significant effort and energy. But the process takes more than a few tires from scrap tire dumps. It is estimated that a few inches of RMAC overlay can use over 1800 tires per lane mile. Not only does this process recycle a sometimes dangerous eyesore (flaming tire dumps seem to be very newsworthy) the resulting pavements have other advantages. They are quieter, reducing the need for sound wall construction along busy roadways. They are very durable and less prone to fatigue cracking which improves the pavement lifespan and reduces maintenance costs. And they can be made porous to drain water below and away from the pavement surface. For flat terrain, low lying cities that use roadways as part of the storm water management system this yields a big improvement in road safety.
Porous rubber roads. Not inflatable, but close enough. I wonder if the comedians in that old TV show realize how visionary they were? Of course despite advances in lightweight portland cement products, I haven’t seen any concrete tires yet. But being half right is still not bad.
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About the Author
John Moehring is a practicing Engineering Technologist in civil, geological, biological, and electrical engineering fields. And one of these days he may actually get it right.
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