The road less worn

LOS ANGELES — Tires. Every vehicle needs them, but eventually they wear out. Roads wear out too, and they often need to be repaired. But new research by a pair of teens suggests that the rubber from worn-out tires could boost the lifetime of asphalt pavement. A second benefit: Roads made with this material might need fewer patches.

The young researchers showcased their findings here this week as finalists at the International Science and Engineering Fair, or ISEF. Created by the Society for Science & the Public and sponsored by Intel, the 64th annual Intel ISEF showcases some of the best high school science projects from around the globe. (SSP also publishes Science News for Students).

Many roads are made with asphalt. The gooey black sludge is the densest part of petroleum, or crude oil. (Lighter components of crude oil include the substances used in gasoline and home-heating oil.) But Jordan is a country that has very few oil wells or other sources of petroleum, notes Mohammad Salameh. He’s a 10th-grader at the International School in Amman, Jordan.

For Jordan, having to import most of the country’s petroleum adds greatly to its cost, the teen notes. The extra expense means that there’s less money to build and maintain roads. That has led to poorer quality roads. As a result, those roads need to be patched almost constantly. And a typical road there must be replaced every 5 years or so, says Mohammad.

So 16-year-old Mohammad and his research partner, 15-year-old classmate Adam Belaid, came up with the idea of adding rubber to asphalt to improve road quality. But that rubber doesn’t need to be new — it can come from old tires. Even a worn-out tire contains a lot of rubber, which means that the material should be plentiful, says Mohammad. Drivers in Jordan replaced 9 million tons of tires between 2006 and 2010, and only 0.3 percent of that rubber was recycled. That leaves a lot of rubber that could potentially go into road construction.

But first the teens had to show that adding rubber to asphalt resulted in better roads. To test their idea, they created several different asphalt-rubber recipes. They also tried out different temperatures at which the mix was heated. And they experimented with the size of small rocks that were blended into the mixture.

Normally, a mix of rock and asphalt is heated to 160° Celsius (320° Fahrenheit) before it is spread on a roadbed. But when the teens tested a mix of 92 percent asphalt and 8 percent powdered rubber that had been heated to that temperature, about 9 percent of the volume of this road-surfacing material consisted of air bubbles. On most road surfacing, the empty space (bubbles) will be less than half that amount, says Mohammad. So the teens tried cooking their mixes at 190 °C (374 °F). They also reduced the size of the rocky bits that they blended into the hot goop. Those changes seemed to solve the problem. They cut the volume of air bubbles in the asphalt-rubber mix to about 4.5 percent.

The teens’ tests suggested that when this surfacing material includes 8 percent rubber, it should hold up for 10 to 15 years of traffic instead of the normal 5 years. And their recipe should reduce the need for patching worn areas from once a year, now, to maybe once every 5 to 7 years in the future.

Best of all, say Mohammad and Adam, finding a new use for old tires would help solve an environmental problem. Normally, people dispose of old tires by burning them, which adds to air pollution. Some tires instead are buried, which can pollute groundwater. But when the rubber is added to asphalt, it’s largely locked away and prevented from harming the environment.

Power words

asphalt    The densest portion of crude oil, or petroleum.

pavement    The durable surface laid down to help a road, street or sidewalk stand up to vehicle or foot traffic. Pavements can be gravel or cobblestones, but most modern roads and streets are made of asphalt or concrete.

petroleum    Also known as crude oil, petroleum is a mix of hydrocarbons that often includes methane and ethane gas, many components of gasoline, and denser substances such as asphalt.