Next Gen Eco Materials 3 - Hemp Rebar

Hemp Rebar - Growing stronger than steel?

Within the concrete foundations, walls, and floors of the vast majority of buildings is a critical but risk-prone building material: steel reinforcing bar or rebar. In the aftermath of the collapse of some buildings in recent years, investigators found the steel rebar to have extensive corrosion—enough to cause a massive building to simply fall to pieces.

Steel reinforcing bar, or rebar, is a key part of the way buildings are built, and it is critical to their structural integrity. But steel rebar is also susceptible to the moisture that can wind its way through the concrete, causing mostly invisible corrosion that can lead to massive structures collapsing almost without warning.

A team of researchers at Rensselaer Polytechnic Institute in the USA has been researching an alternative. Instead of using the traditional but fallible steel to reinforce the concrete, they’ve developed a composite reinforcing material made of surprisingly strong hemp fibre. Their hemp-based rebar uses the hearty fibres of the hemp stalk in combination with a bioplastic to form bars that can safely replace steel rebar in concrete construction in future buildings.

“There are some natural fibres that have a similar strength to steel for the same weight,” says Alexandros Tsamis, the professor of architecture at RPI. Synthetic materials like carbon fibre and fibreglass have such strengths, but so does hemp, natural, cheap and very fast growing. When combined with plant-based bioplastic, hemp rebar could be an all-natural alternative to steel rebar.

“Instead of extracting it from the earth, you grow it,”

The potential is massive.

Steel rebar is everywhere. “It’s pervasive in any concrete structure. They’re just full of rebar,” says Dan Walczyk, a professor of mechanical engineering at RPI. “And it’s millions, if not billions, of dollars of construction material annually.”

Replacing steel in all concrete construction could eliminate the corrosion risk that recently brought down the condo in Florida as seen in the international news. With less corrosion, the concrete in buildings and bridges could last decades longer.  And, importantly, switching the material used from steel to a composite of fibres, you can drastically reduce the carbon footprint of the building industry, because you extend the lifetime of structures.

Hemp rebar and the machinery to make it are being developed at RPI, alongside architecture and mechanical engineering students. They equates the technology to 3D printing, which requires both a filament and the machine to form it. The filament for hemp rebar is a combination of extracted hemp fibers wrapped in a thermoplastic that is then wound into a rope-like coil. The machine, which the team is currently building into a proof of concept, is a car-sized device that pulls in that coiled material, heats it, and consolidates it into hardened bars. The machine is being designed to work on-site during construction projects, allowing for the hemp rebar to be made as needed.

It may sound advanced, but the material is not particularly complicated to manufacture, according to Tsamis, who notes that the project started during the pandemic when the university’s labs were closed. “The first experiments that we did with consolidating happened in kitchen ovens,” Tsamis says. “And the first ropes we were making by hand. I had my son pull the other side of the rope I was making at my desk. My student’s cat was chewing on the rope while he was making his own version. So the beginnings of it were pretty modest in terms of the technology we had available.”

Once the material is widely available (hemp has only recently been made legal to produce in some countries and bioplastics are still in relative infancy), it wouldn’t be very complicated for the building industry to swap out its steel rebar for the hemp-based variety.

The researchers say hemp rebar is probably still a few years away from getting into a building, but they’re already in talks with several large construction companies about how the material could be integrated into new projects.

With climate change spurring the building industry to grapple with its high carbon footprint, reducing the use of heavily polluting materials like steel will become increasingly important. Hemp rebar’s wide applicability makes it the kind of natural material the industry could adopt to cut down its massive impact.

image credit - engineering.com