In a recent study, researchers in Montana State University’s (MSU’s) Norm Asbjornson College of Engineering found that plastic treated with certain bacteria could be added to concrete in significant quantities without compromising the structural material's strength.
"This is really exciting," said Cecily Ryan, assistant professor in the Department of Mechanical and Industrial Engineering and co-author of the study. "These initial results are very encouraging as we consider potential applications."
Typically, adding plastic or other filler material disrupts the mix of sand, aggregate and cement that gives concrete its ability to bind together and support heavy loads. However, according to MSU, the research team found that using bacteria to coat the plastic with a thin mineral layer allowed it to bind better with the cement.
Concrete samples containing up to 5 percent of the bacteria-treated plastic had virtually the same strength as traditional concrete, according to the study.
"That 5 percent is really a big increase from what's been allowable so far," said Chelsea Heveran, assistant professor of mechanical and industrial engineering. "We were surprised by how much of an effect there was."
Because concrete is used so widely and in such high volumes, replacing even 5 percent of it could result in massive reuse of plastic, Heveran noted. And because concrete is so energy-intensive to make, the plastic filler could significantly reduce carbon dioxide emissions, she said.
In MSU's Center for Biofilm Engineering, the researchers immersed the plastic in a water-based solution containing Sporosarcina pasteurii bacteria, which grows on surfaces to form biofilm. The microbes, left in the solution for 24-48 hours, consumed added calcium and urea—a nitrogen-based substance widely used in fertilizers—to give the plastic a thin, white coating of calcite, the hard mineral that constitutes limestone. The plastic was then mixed into small concrete cylinders that were crushed with specialized equipment to measure their strength.
Although the researchers started with chipped-up No. 1 plastic commonly found in disposable water bottles, after initial success they achieved a similar result with a mix of No. 3-7 plastic, which is used in a variety of containers but isn't accepted at most recycling facilities.
"It's really exciting that we got this result with the mixture of plastics that typically aren't recyclable," said Adrienne Phillips, associate professor in the Department of Civil Engineering, who has used the same mineral-forming bacteria to seal tiny, hard-to-reach cracks deep underground in leaking oil and gas wells.
The next step is to study the material's long-term durability as well as how the process could be scaled up so that the material could be manufactured in useable quantities, Phillips said. The researchers have partnered with Frank Kerins, associate professor in the Jake Jabs College of Business and Entrepreneurship, to begin exploring commercial applications.