Concrete has a massive carbon footprint, with billions of tons produced each year since it’s the world’s most commonly used construction material. As you could imagine, production requires an immense amount of energy. Scientists around the world are continuously searching for new ways to make the production process more eco-friendly.
One of the ways found to improve the concrete’s strength and durability is to incorporate tensioned steel tendons before casting, which can be released to compress the material as it sets. Recently, a team of scientists at the Swiss Federal Laboratories for Materials Science and Technology (EMPA), developed an innovative version of this pre-stressing technique to produce concrete that’s lighter but with equal strength. If widely applied, this advance could reduce drastic amounts of CO2.
Pre-stressed concrete technology is often used when the material is required to endure particularly high loads, such as a bridge or a beam, with the tensioned steel tendons creating forces that compress the material from within.
There are some challenges with this technique, as steel tendons are vulnerable to corrosion. Therefore, the concrete poured around them needs to be of a specific thickness, with alternative tendons made from carbon-fiber-reinforced polymers (CFRP), which are corrosion-resistant. This method has enabled the production of concrete elements that are significantly leaner while offering the same properties.
There are some downfalls of using CFRP as the tendons, including, it’s expensive, anchoring them on the element is complicated, and they have their limits. These inconveniences are the reason why pre-stressed CFRP-reinforced concrete isn’t used as commonly as that pre-stressed with steel. “If you want to pre-stress these CFRP reinforcements in order to be able to build even thinner structures with a higher load-bearing capacity, you reach your limits,” explained Dr. Mateusz Wyrzykowski, a member of the research team.
The EMPA scientists developed a formula for CFRP-reinforced concrete that allows it to expand as it hardens. There’s no need to anchor and tension the tendons because the concrete does that on its own, resulting in the tendons staying in that state permanently by applying counter-forces on the concrete and forming compressive stress.
Dr. Wyrzykowski gave his analogy on the concrete:
If one puts a rubber band around the arms and tries to stretch them, the rubber band will be under tension, while one’s arms will experience the compression from the band. Per analogy, such a mechanism will make the expanding concrete experience compression.
The team’s test results showed that the self-stressing material could tolerate loads similar to conventional pre-stressed concrete, and roughly three times more than non-pre-stressed CFRP concrete.
Dr. Wyrzykowski said:
Not only can we build more stable structures, but we also use considerably less material. We can easily pre-stress in several directions at the same time, for example, for thin concrete slabs or filigree curved concrete shells.
This technology paves the way for leaner concrete elements that hold considerable strength, offering new possibilities in lightweight construction.
