“Construction sand is a very limited resource, and good quality construction sand is actually becoming scarce,” Olsen said. “Here, we’re reducing the use of a natural resource and also avoiding waste ending up in a landfill by taking plastic and using it for aggregate replacement.”

To find the best ratio of plastic materials without compromising the concrete’s properties, the researchers conducted several controlled laboratory studies. They evaluated how small the plastic particles should be broken down into and how well plastic particles adhered to other components of the cement. They also studied the impacts of environmental factors, like extreme temperature changes, on the durability and strength of the pavers. 

“Anytime you put a new material out there, you have to verify that the pavers will work, not just in terms of strength, but also for other short-term and long-term effects so it lasts,” Head said. “To do that, we looked at the percentage by volume of how much plastic waste we could add without compromising the overall strength and ran a series of material tests to examine the effects.”

After two years of research, the team found that the sustainable concrete pavers could be made of up to 15% unrecyclable plastic chips that were sourced from EcoPlastics Delaware. 

“I think these pavers are working so well because of our processing technique, where we are trying to replicate the properties of sand into our plastics,” Olsen said. “There are a few things that can go wrong with using plastics in concrete, but we are coming up with different materials to make up those properties so we can have a fully viable product that can be used outside.” 

In the future, the team will continue to conduct fundamental research on these pavers, with one goal being to understand the mechanism of adhesion between the plastics and the rest of the cement. 

“We want to understand what adhesion depends on, how it varies with different temperatures or environmental conditions, and if different kinds or sizes of plastics behave differently,” Advani said. “If we understand those factors, we can better understand how to create sustainable concrete that has the biggest enhancement in its properties.”

Real-world impact

In May, UD’s Facilities, Real Estate and Auxiliary Services installed the sustainable concrete pavers into the center of the sidewalk immediately in front of Du Pont Hall. 

Far from being just a nice addition to UD’s campus, these bricks will continue to provide the researchers with insights on their capabilities under real-world stressors. Thanks to sensors installed inside several of the pavers, the researchers can now collect data on weather conditions and foot traffic patterns to see how the pavers perform when subjected to stressors such as deicing agents, extreme freeze-thaw situations and campus events.

“When we work with the pavers in the lab, we’re careful as we load them into our testing set-ups. But out in the real world, they’re being scraped with snow plows and driven on by campus vehicles,” Olsen said. “Because they are exposed to a lot more extreme conditions, we can really test them to their limits.”

Head noted that an added benefit of the pavers being installed by Du Pont is that it’s a great way to showcase the College of Engineering’s collaborative, innovative and impactful research to visitors and prospective students. 

“This project is a demonstration of engineering in action while also showcasing engineering innovation and sustainability initiatives on campus,” she said. 

“Being able to actually put our work into practice, to see how it works here on The Green, is really timely and exciting,” Advani added.

“The Office of Sustainability was excited to award $5,000 to the research team during our annual Green Grants competition to promote sustainability research and initiatives on campus,” said Chris Williams, associate provost and academic director for the Office of Sustainability. “This proposal was a perfect blend of innovative research to promote a more sustainable campus and planet as well as support engaged student scholarship.” 

In the future, as the researchers continue collecting data both inside the lab and outside of Du Pont, one of their goals is to be able to completely replace construction sand with plastics in their pavers. 

“That is a very hard thing to do and requires some specialized things that we're working towards. But we’re almost to that point, and that’s the overall goal of our next phase of research,” said Olsen, whose master’s thesis will reflect this research.

With their continued research on these pavers, it’s possible that this sustainable concrete could make its way onto other parts of UD’s campus in the future — and maybe even someday into sidewalks and paved pathways across the world.