SPACE July 2023 (No. 668)
Concrete is the second most-consumed material in the world after water, so its environmental impact is not insignificant. However, concrete’s long history in architecture and construction means that other materials are unlikely to replace it. If so, is there anything more advanced than traditional reinforced concrete? In recent years, there has been a movement in Germany to focus on carbon fiber reinforced concrete (CFRC). Whereas CFRC was previously only used in bridges and parts of buildings, in the CUBE (2022) it will be used throughout entire buildings. SPACE asked Giovanni Betti (head of sustainability, HENN), who was involved in the design, about the future possibilities for CFRC.
View of CUBE
How Far Has Carbon Fiber Reinforced Concrete Come?: CUBE
Giovanni Betti head of sustainability, HENN × Han Garam
Han Garam (Han): Completed last year, CUBE is the world’s first CFRC building and the result of C3 (Carbon Concrete Composite) research. What is the C3 study, when did it begin, and what is its goal?
Giovanni Betti (Betti): Established in 2014 in Dresden, Germany, C3 is led by its managing director, Frank Schladitz (researcher, Institute for Concrete Structures). Its primary objective is to pioneer innovation in the field of CFRC as a new construction material to replace the traditional reinforced concrete. The association has dedicated extensive efforts to research, development,and the progressive implementationof the use of CFRC, and has engagedin communicating construction principles, life cycle assessments, and recycling processes related to CFRC across both new construction projects and renovations. With funding of over 70 million EUR and a network of 150 partners, C3 is currently the largest research project in German construction.
The CUBE, with which we were the architect partner in the concept phase, was developed in collaboration between professor Manfred Curbach and his Institute for Solid Construction at the Dresden University of Technology (TU Dresden), as well as with experts in material performance, visualisation, and model building.
Han: What is CFRC? Could you explain how this material might be thought of as an alternative to reinforced concrete in a low-carbon world?
Betti: Carbon fiber is four times lighter and six times stronger than steel. CFRC is a composite material that replaces conventional steel reinforcement with a non-metallic reinforcement consisting of carbon fiber.
A notable advantage of CFRC is its ability to minimise the quantity of material savings by 50% or more. With traditional concrete, additional layers, usually around 4cm thick, are added to protect the steel reinforcement from water to prevent corrosion. However, CFRC’s non-metallic reinforcement eliminates the need for this additional layer, resulting in reduced material usage. This also can reduce carbon dioxide emissions and the consumption of other valuable resources, like water and sand. Furthermore, carbon fiber offers higher tensile strength compared to steel, making CFRC a highly durable and resilient material. By leveraging these properties, CFRC is a significant step towards achieving sustainable and low-carbon construction practices, paving the way for a greener future.
In addition to CFRC, another type of fiber reinforced concrete is glass fiber reinforced concrete (GFRC), employing short, loose glass fibers that are simply added to the concrete mixture. In contrast, CFRC uses carbon fiber meshes or bars as structured reinforcement, enabling us to fully leverage the strength of the reinforcement material. By taking a structured approach than GFRC, CFRC achieves improved strength and performance, making it a superior choice for concrete structures.
Han: Until now, CFRC has only been used in parts of buildings, such as structural reinforcement and façade. With CUBE, it has become the main material of a building for the first time, what are the technological advances in CFRC?
Betti: Due to limited understanding of the adhesive behaviour of fully encapsulated carbon fiber, structured carbon fiber meshes were previously commonly used to reinforce and repair existing structures. However, recent advancements in material compositions and mechanical modeling have enabled carbon fiber to be used as a substitute for steel rebars. These progressions have expanded our knowledge and improved our ability to employ CFRC in various applications.
Manufacturing process of prefabricated concrete panels with carbon fiber meshes as reinforcement
Han: In the history of architecture, the appearance of a new material often affected the building’s form. I assume that you also wanted to highlight the unique property of this material in your design for CUBE.
Betti: It’s not every day that you get to design with an entirely new material, and wewere excited to explore and demonstrate CFRC’s untapped potential. One aspect that immediately captivated us was the almost textile-like quality of the carbon fiber meshes. They stand in stark contrast to conventional steel reinforcement, being thin, lightweight, and flexible. As we played with them, we couldn’t help but marvel at their strength and pliability.
Inspired by this experience, we sought to translate it into the physical structure we were creating. Our vision was to construct two interlocking shells that would elegantly twist to support one another, forming a continuous, sinuous form that seamlessly transitions between wall and roof. These shells, spanning approximately 40m in length, become monolithic elements that showcase the unique capabilities of the carbon fiber material. If built with reinforced concrete, thermal expansion over suchextensive lengths would typically result in numerous small cracks in the concrete. While these cracks may not compromise structural integrity, they create pathways for water to reach the steel rebars, leading to rusting and subsequent damage to the concrete. However, the use of carbon fiber allowed us to mitigate this issue, as it exhibits minimal thermal expansion and eliminates the risk of corrosion.
In addition, the twisting walls extend beyond the building’s envelope, providing a protective barrier for the garden and building against the nearby busy road. They are freestanding and are nearly 7m tall and a mere 5cm thick. Designing for dramatic and slender structures would have simply been impossible to build using conventional reinforced concrete.
Han: Two construction methods were used for this building. Could you elaborate on these approaches along with your reason for distinguishing between the two?
Betti: The building’s most prominent feature, the twisting shells spanning a length of 40m, is constructed using cast-in-situ techniques, using shotcrete that was sprayed onto carbon fiber meshes overlaid on a plywood formwork. These exemplify the material’s ability to facilitate highly expressive and visually captivating architectural designs.
On the one hand, the dark grey cubical volume is composed of prefabricated concrete panels. The twisting shells showcase the material’s versatility in creating complex forms, but prefabricated elements demonstrate the material’s ability to meet the requirements ofrational and efficient construction methods. By adopting two approaches, we aimed to highlight the broad range of possibilities offered by the material.
Exterior of CUBE ©Stefan Müller
Interior of CUBE. The shell structure is constructed using cast-in-situ concrete, and the cubical volume is composed of prefabricated concrete panels. ©Stefan Müller
Han: What role does CUBE play in the TU Dresden campus?Betti: The 243m2 experimental building combines a laboratory and event rooms, and sets an example for architectural and structural innovation at the TU Dresden. It is designed to accelerate the research activities related to CFRC and facilitate discussions on its application. The building acts as a meeting point where researchers, industry professionals, and stakeholders come together to discuss and promote the use of CFRC. In order to gain further insights into the method, the CUBE has been subjected to comprehensive monitoring during its actual use since its completion.Not only are the operating and life cycle costs assessed, but also the long-term suitability with regard to structural, static and building physics aspects are subjects of research.
Han: What other functions can be added by using carbon’s conductivity?
Betti: The potential of CFRC extends far beyond what has been exploredso far. With the inherent properties of carbon fiber, including high conductivity and non-magnetism, walls constructed with CFRC hold promise for a range of innovative applications.
One intriguing possibility is the use of CFRC walls as large touch-sensitive surfaces. The conductivity of carbon fiber opens up the possibility of transforming walls into interactive interfaces where touch input can be detected and responded to. This could revolutionise the way we interact with the built environment, allowing for an intuitive and immersive user experience.
Moreover, CFRC walls have the potential to serve as self-monitoring structures. The conductivity of carbon fiber enables the integration of sensors that can continuously monitor structural integrity and provide real-time data on a building’s condition. This proactive monitoring capability enhances safety and allows for timely maintenance interventions, contributing to the longevity and durability of the structure.
Beyond touch sensitivity and structural monitoring, it is also possible to integrate additional functions within CFRC structures. For instance, the reinforcement in CFRC walls could act as heating elements, providing efficient and customisable thermal control within buildings. Additionally, the high conductivity of carbon fiber offers the potential for integrating electricity storage capabilities directly into the structure, supporting decentralised and sustainable energy solutions.
Han: After having experienced CFRC via CUBE, how do you evaluate the future of CFRC? What needs to be improved for its commercialisation?
Betti: The future of CFRC is indeed promising, and there are some challenges that need to be addressed in the short term. One of the main obstacles currently faced is the lack of relevant standards and building codes specifically tailored to CFRC. In the case of the CUBE as a pioneering project, it had to be constructed as a unique exception to the strict German building code. However, there is optimism that following this pioneering project the material will receive general technical approval from the German building authorities by the end of 2023. This approval will mark a significant milestone, providing a clear pathway for widespread adoption of CFRC and serving as a blueprint for other jurisdictions to follow.
With the establishment of standards and building codes, the issues related to higher production costs and shortage of skilled workmanship are expected to be gradually resolved. As demand for CFRC increases and more projects embrace this innovative material, economies of scale will come into play, making production more cost-effective. In addition, as CFRC becomes more prevalent and accepted, the availability of skilled professionals experienced in working with this material will also improve.
Aerial view of CUBE
You can see more information on the SPACE No. 668 (July 2023).
Giovanni Betti
Giovanni Betti is an architect, the head of sustainability at HENN, and a guest professor at the Berlin University of the Arts. He has over 15 years of international experience, and his research and work have been widely published and exhibited internationally, including at the Royal Academy of Arts and the Venice Biennale. Additionally, he has developed CBE Clima, an open-source web application for designing climate-adapted buildings worldwide.
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