(Inside Science) – Construction and maintenance of roads and buildings use up lots of money and resources. Asphalt roads wear out over time, and acquiring concrete for buildings means digging deeper and deeper into quarries for material. But new recycling technologies may take some of the pressure off the world's resources while keeping roads paved and buildings safe.
Asphalt is a paving material made of gravel and other materials bound together with a thick petroleum. Over time, exposure to the elements causes asphalt to age, become brittle and crack. This means that roads paved with asphalt must be repaved periodically. As the old asphalt is pulled up, only a tiny fraction can be reused: most of it heads to a landfill or gets stacked up for later use.
A few years ago, when chemist John Warner heard about this problem with asphalt, he got excited.
"If we could reverse it, rejuvenate it, it would be good from a save-the-world perspective, but it would also be great for the pavers, because the single biggest cost to make more asphalt is the petroleum," said Warner.
One idea was to add oil to the old asphalt, making it supple again – but it would be too soft to perform well on roads after the process. Instead, Warner and his colleagues at the Warner Babcock Institute for Green Chemistry, located in Wilmington, Massachusetts, created a special, non-toxic molecule called Delta S that reverses oxidation in the asphalt, by pulling off the old petroleum from the pieces of stone aggregate, then binding the whole substance together.
By restoring the binder in recycled asphalt to its original performance, the molecule allows as much as 50 percent of the asphalt to be reused with the same performance and lifespan as new asphalt.
"There are some other products out there that people claim do similar things, and they're really toxic," said Warner. "In contrast, this stuff is edible. That's the goal of green chemistry: products that have superior performance, cost and are better for the environment."
They spun the molecule off and started a company, called Collaborative Aggregates, also located in Wilmington, Massachusetts.
There's more work to be done in mending streets sustainably, said Warner.
His ideal solution would be a kind of Zamboni for the streets: a simple truck that can recycle and relay asphalt as it moves across the roads.
"Why bring the asphalt to the plant, if you can just create it in place?" asked Warner.
To make buildings embrace more recycled elements, some researchers have turned to recycling concrete. About half the volume of concrete is made up of gravel or limestone aggregate, which usually comes from a quarry or pit.
Engineers at the University of Notre Dame in South Bend, Indiana wanted to see if this aggregate could come from cement in recycling yards. They went to 17 different places around the Midwest and collected recycled concrete aggregate. The chopped-up rocks are typically used as fill for a building's foundation, but engineer Yahna Kumara and his doctoral student Adam Knaack wanted to know if it could be used as a component of new concrete.
The recycled concrete, which is chopped-up pieces of old concrete up to an inch in size, usually has much higher quality than what is being used for, said Kumara. "So, if the quality is so good, why isn’t it being used? Uncertainty. There's a lot of variability in region and recycling yards."
The researchers found that the materials were just as strong as concrete made without recycled components, though they created concrete that was stiffer than expected – a factor that builders need to take into consideration in deciding where to use the material. The results of their study were published September 1 in the American Concrete Institute Structural Journal.
Knaack pointed out that the timing is perfect for using recycled concrete in new structures, like bridges.
"We have increasing demand for new structures, and we have to take down the old ones to replace them," he said. "It could be a self-sustaining cycle to use recycled aggregate in new infrastructure."
Other more unusual recycled materials also make it into buildings. Coal fly ash, bottom ash, slag, and spent foundry sand can all be used as structural fill. Charles Kibert, director of the Center for Construction and Environment at the University of Florida in Gainesville, warned that recycling shouldn't be confused with down-cycling, where materials can't be reused for their original purpose, but instead go to a lower-end use – like concrete becoming structural fill.
"At the end of the day, we're trying to work through that point where we have a closed-loop and keep things in productive use as long as possible," said Kibert.
Bradley Guy, an architect who teaches sustainable design at The Catholic University of America in Washington, D.C. said that reusable materials aren't really as much a question of technology as much as a question of infrastructure and economics – and as materials from around the globe become more scarce, people will begin looking closer to home.
"If we have restraints on carbon, and issues with trade, then people will look more to local resource security, and we'll reuse more," he said.