Researchers discover potential use of recycled plastic in concrete

Millions of tons of plastic are thrown away every day, and for most of it, there are few traditional recycling options
.
But because Montana State University scientists are harnessing microbes, the material could soon find new beneficial uses
.

In a recent study, researchers at the Norm Asbjornson School of Engineering at the University of Michigan found that plastic treated with a certain type of bacteria can be added to concrete in large quantities without affecting the strength of the structural material
.
The research was published in the journal Materials
.

"This is really exciting," said study co-author Cecily Ryan, assistant professor in the Department of Mechanical and Industrial Engineering.
"These initial results are very encouraging as we consider potential applications
.
"

Typically, the addition of plastic or other filler materials disrupts the mix of sand, aggregate and cement, allowing concrete, the world's most widely used building material, to hold together and withstand heavy loads
.
But the Michigan State University team found that using bacteria to coat the surface of plastic with a thin layer of minerals could allow the plastic to bond better with cement
.
Studies have shown that concrete samples containing up to 5 percent bacteria-treated plastic have nearly the same strength as conventional concrete
.

Chelsea Heveran, assistant professor of mechanical and industrial engineering, said: "That 5 percent is really big compared to the current growth rate
.
We were surprised by the magnitude of the impact
.
"

Heveran points out that because concrete is so widely used and used in such large quantities, replacing even 5 percent of the concrete would allow the plastic to be reused in large quantities
.
She also said that since the manufacture of concrete requires a lot of energy, the plastic filler could significantly reduce carbon dioxide emissions
.
Concrete production is one of the largest industrial sources of climate change gases in the United States, according to the U.
S.
Environmental Protection Agency
.

At Michigan State University's Center for Biofilm Engineering, researchers soaked plastic in a water-based solution containing the harmless bacteria Sporosarcina pasteurii, which grow on surfaces to form so-called biofilms
.
The microbes stay in the solution for 24-48 hours, consuming the added calcium and urea (a nitrogen-based substance widely used in fertilizers), coating the plastic surface with a thin layer of white calcite, the hard mineral that makes up the limestone
.
The plastic is then mixed into small concrete cylinders, which are crushed with specialized equipment to measure its strength
.

Although the researchers started with pieces of plastic No.
1 commonly found in single-use water bottles, after initial success, they achieved similar results using a mix of plastic Nos.
3-7
.
Plastics 3-7 are used in various containers but are not accepted by most recycling facilities
.

"It's really exciting that we got this result with a plastic mixture that's not usually 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 in leaking oil and gas wells underground
.

The next step, Phillips said, is to study the long-term durability of the material and how to scale up processing so the material can be mass-produced
.
Working with Frank Kerins, associate professor at the Jake Jabs School of Business, the researchers began exploring its commercial applications
.

The research grew out of a summer 2019 study in which two high school teachers, Kendra Lunday of Helena Capital High School and Hakan Armagan of Omaha, Nebraska, visited Michigan through the National Science Foundation's Teacher Research Experience Program State University
.
The duo tested various concrete fill materials, including straw and other agricultural biomass
.

Armagan and Lunday were major contributors to the study, which was also "largely driven by talented undergraduate students," Heveran said
.
In addition to the two high school teachers, paper co-authors include McNair scholar Michael Espinal, a senior majoring in mechanical engineering; Seth Kane, a doctoral student in engineering; and Abby, laboratory manager at the Center for Biofilm Engineering Thane
.

"What's cool about this project is that we use microbes to make small changes to a common material, but it could have huge societal benefits," Heveran said
.