Researchers at a private university in the USA recently announced that they were developing a hemp rebar product that could replace steel rebar in construction projects. The hemp rebar, which has not yet been tested in the field, could represent a low-cost alternative to steel with lower maintenance and fewer decommissioning requirements.
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The Need for a Non Corrosive Alternative for Steel Rebar
The hemp rebar product is currently in development. Engineers at the Rensselaer Polytechnic Institute, a private university in the US, claim that it will reduce carbon emissions from building projects (over their lifecycle) due to a lower maintenance requirement than steel rebar.
The team intends to publish a research paper detailing the invention in 2023.
Currently, rust occurring in steel rebar is a significant cause of decay for steel reinforced concrete structures like bridges, roads, sea walls, and buildings. In areas with a high concentration of salt in the atmosphere (near or on the ocean), concrete structures only last for 40 to 50 years. This is due to the higher rate of corrosion caused by salt interacting with reinforcing steel.
But if it were not for corrosion in steel rebar, these structures could be expected to stay in place for three times as long, said the Rensselaer researchers.
Despite the relatively low amount of embodied energy in traditional steel rebar as a bought item, costs soon add up after installation – both environmental and financial.
In highly corrosive environments, structural designs often rely on alternatives like glass fiber reinforced polymer (GFRP) rebar instead of steel. While glass fiber and polymer do not corrode in the same way as steel, they are much more expensive and create more carbon emissions across their life cycle.
Industrial Hemp
Industrial hemp is a class of cannabis plant that contains only trace amounts of the psychoactive compound that makes recreational drug users inebriated, tetrahydrocannabinol (THC). It is prized for its fibers, which are similar to glass fiber in terms of strength relative to weight. Growing hemp is also a good carbon sequestration strategy, as the fast-growing plant takes atmospheric carbon out of the atmosphere twice as quickly as most trees.
Growing hemp was only recently legalized in the US, but the climate-resilient crop is already seen as an attractive choice for farmers to invest in. The Rensselaer team said that there are not enough high-value hemp-based products on the market yet to drive demand for industrial hemp.
The hemp rebar product that they are developing could be just such an avenue to drive up demand for hemp crops and encourage more farmers to start producing them. The researchers said that the new hemp rebar technology will become more efficient in the next few years as the industrial hemp industry matures, leading to refined extraction processes and selective plant breeding for better crops.
While none of the team’s results have been peer-reviewed yet, preliminary tests suggest that the hemp rebar will last just as long as glass fiber reinforcements in the same conditions, with a fraction of the embodied carbon cost, according to researchers.
Before research is published, the team says it will keep developing and testing the hemp rebar alongside a range of other industrial hemp applications currently in the development pipeline.
Responding to preliminary results to come out of the hemp rebar development project, a US industrial hemp start-up, Puration, has already announced plans to disrupt the steel market with future hemp products, in addition to the lumber market disrupting hemp products already scheduled for development.
What is in Hemp Rebar?
To make the hemp rebar, hemp fiber is encased in plastic. The choice of plastic material is still under consideration; polypropylene is one candidate.
The rebar is made in a pultrusion process. First, hemp is made into a rope containing natural and plastic fibers. Then, the rope is fed into a specially created machine in coils.
The pultrusion machine is large, about the size of a car. It is designed to create hemp rebar on site, already cut and bent to required dimensions.
Making Hemp an Everyday Material
The hemp rebar under development would be used in the same way as steel in concrete structures today, inserted into concrete molds before the concrete is poured and reinforcing the concrete to make the otherwise brittle ceramic material stronger and more durable.
Researchers have acknowledged the need to make the product as accessible as possible to ensure wide adoption rates in the traditionally conservative construction industry.
As a result of this, they have designed an on-site pultrusion machine – which forms hemp rebars on site from a hemp feedstock – to look like other plant machinery commonly found on building sites. It is apparent that the team think this kind of dressing is important to foster widespread adoption in the industry.
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References and Further Reading
Aouf, R.S. (2022). Hemp rebar could offer low-cost non-corroding alternative to steel. [Online] Dezeen. Available at: https://www.dezeen.com/2022/03/15/hemp-rebar-low-cost-low-carbon-alternative-steel-design/ (Accessed on 8 August 2022).
Katz, A. (2004). Environmental Impact of Steel and Fiber–Reinforced Polymer Reinforced Pavements. Journal of Composites for Construction. doi.org/10.1061/(ASCE)1090-0268(2004)8:6(481).
Puration, Inc. (2022). PURA Plans To Disrupt $200 Billion Steel Rebar Market With Hemp Alternative In Addition To Disrupting $600 Billion Lumber Market. [Online] Global Newswire. Available at: https://www.globenewswire.com/news-release/2022/03/08/2398952/0/en/PURA-Plans-To-Disrupt-200-Billion-Steel-Rebar-Market-With-Hemp-Alternative-In-Addition-To-Disrupting-600-Billion-Lumber-Market.html
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