For over a century, steel rebar has been the go-to material to reinforce concrete but a new approach promises to make the common building material stronger and more environmentally friendly. Scientists have leveraged 3D printing to produce a polymer lattice structure that can act as the backbone for low-carbon concrete that also boasts great strength and durability.
The research was conducted at the University of California, Berkeley and builds on previous efforts to reinforce concrete using polymer fibers. This emerged around half a century ago as a promising alternative to steel rebar reinforcements, which offer great strength but are heavy, expensive and degrade over time.
Polymer fibers, on the other hand, are lightweight, cheap to produce and are resistant to corrosion. Current approaches involve mixing these fibers into the concrete before it is poured, but this can lead to an uneven distribution meaning some parts of the final structure are stronger and others are susceptible to cracks.
The engineers behind this new study sought to address this shortcoming with a 3D-printed octet polymer lattice, a structure favored for its unique combination of lightness and strength, and one hoped to prevent cracks from forming through a dense arrangement of trusses. The team found success using acrylonitrile butadiene styrene (ABS) polymers to produce the lattice, with the gaps then filled with ultra-high-performance concrete, which is four times stronger than regular concrete in terms of compression.
The researchers experimented with variations of this recipe, using different versions of the polymer lattice so that they ranged from 19.2 percent of the overall volume of the concrete to as much as 33.7 percent. While these tweaks brought about small changes in terms of compressive strength and peak loads, the overall mechanical properties of the concrete remained largely the same.
“When a material is brittle, it can hold up to a certain peak load and then it fails,” says study co-author Claudia Ostertag, professor of civil and environmental engineering. “In this case, we did not observe that failure. It got stronger and stronger. For us interested in concrete, this is amazing. You are rendering something very brittle into something very ductile.”
All of the samples tested scored high in strain density values and are therefore able to absorb a lot of energy, while those with thinner lattice structures were just as tough as those with the thicker ones. This part is key to one of the overarching aims of the research project; using higher concentrations of alternative materials to reduce the carbon footprint of concrete manufacturing, which accounts for eight percent of the world’s CO2 emissions.
“The reaction that produces cement inherently produces CO2,” says study co-author Hayden Taylor. “In contrast, there is a conceivable route toward polymers that are net carbon-neutral or even potentially carbon-negative through the use of biopolymers, recycling and renewable energy sources.”
From here, the team plans to experiment with different lattice shapes to see if different geometries can serve different uses.
“Going forward, my biggest question is how to choose the best lattice structure for a particular application,” says lead author Brian Salazar. “There could be even more optimal geometries waiting to be found.”
The research was published in the journal Materials and Design.