This new rebar could allow buildings to last longer and pollute less
When a 12-story condominium building suddenly collapsed last June in Surfside, Florida, one of the reasons was hidden deep within the structure. Within the concrete foundations, walls and floors was an essential but risky building material: steel rebar. Following the collapse, investigators found that this material exhibited significant corrosion, sufficient to cause the massive building to collapse.
Steel rebar, or tie rods, are a key component in the construction of buildings such as this condo tower, and are critical to their structural integrity. But steel rebar is also susceptible to moisture which can work its way into the concrete, causing largely invisible corrosion that can cause structures to collapse almost without warning.
A team of researchers from the Rensselaer Polytechnic Institute (RPI) offers an alternative. Instead of using steel to reinforce concrete, they have developed a composite reinforcement material made of surprisingly strong hemp fibers. Their hemp-based rebar uses the strong fibers of hemp stem in combination with a resin or bioplastic to form bars that can safely replace steel rebar in concrete constructions.
The magnitude of the potential of this material is considerable. Steel rebar is everywhere, from sidewalks to skyscrapers. “They are ubiquitous in all concrete structures. They're just full of rebar,” says Dan Walczyk, professor of mechanical engineering at RPI. "And that's millions, if not billions, of dollars worth of building materials every year."
Eliminating the steel in the concrete construction could eliminate the risk of corrosion that caused the Florida apartment to crumble. With less corrosion, concrete in buildings and bridges could last decades longer. "By replacing the material used in an object with a fiber composite instead of steel, you can significantly reduce the carbon footprint of the construction industry, as you extend the life of structures," says Mr. Tsamis.
Hemp rebar and the machines to make it are being developed by Tsamis and Walczyk, along with architecture student Daniel Cohen and mechanical engineering student Sharmad Joshi. Tsamis likens this technology to 3D printing, which requires both a filament and a machine to form it. Hemp rebar filament is a combination of hemp fibers that have been extracted and wrapped in a thermoplastic which is then wound into a rope-like coil. The machine, which the team is building as a proof of concept, is a car-sized device that sucks in coiled material, heats it and consolidates it into hardened bars. Mr. Tsamis explains that the machine is designed to operate on-site during construction projects, making it possible to manufacture hemp rebar as needed. A specially adapted die can shape the rebar to suit the needs of the construction project, and even bend it into complex shapes that would otherwise be made on site by construction workers.
It may sound fancy, but the material isn't particularly complicated to make, according to Tsamis, who says the project started during the pandemic, when the university's labs were closed. “The first experiences we had with consolidation were in kitchen ovens,” says Tsamis. “And the first strings, we made them by hand. I would ask my son to pull the other side of the rope that I was making at my desk. My student's cat chewed on the rope while he made his own version. The beginnings were therefore quite modest in terms of available technology. »
However, some challenges remain. According to Walczyk, the United States lags behind other countries in the production of industrial hemp, which was only legalized in 2018. Bioplastics are also relatively new. “There is a lot of uncertainty about where these materials came from,” Walczyk says. “If we were to develop this technology in the next two weeks, we don't necessarily have the material supply chain for it. »
But once the material is available, Tsamis says it wouldn't be very complicated for the construction industry to replace its steel rebar with the hemp-based variety. “People already know how to work with this material. All the construction methods and all the know-how are there,” explains Mr. Tsamis.
Researchers affirm Hemp rebar probably won't enter a building for a few years, but they're already in talks with several major construction companies about how the material could be incorporated into new projects. State or federal funding would help accelerate technology development, Walczyk says, and Tsamis says partnering with industry, such as rope manufacturers, could help shift production.
With climate change pushing the building industry to tackle its high carbon footprint, reducing the use of highly polluting materials like steel will become increasingly important. According to Tsamis, the wide applicability of hemp rebar makes it the type of natural material the industry could adopt to reduce its massive impact. “It will require a high volume of production, and we think it will force certain elements of the supply chain to fall into place,” he explains.