Sleek Concrete Structures

Reinforced concrete components are typically bulky and heavy. That is starting to change: components reinforced with carbon fiber are enabling entirely new forms and unprecedented lightness. At the same time, the environmental footprint of these structures is improving.

Balcony slabs are usually made from reinforced concrete. On average, a typical slab measures two by five meters and is 30 centimeters thick, tipping the scales at around 7.5 metric tons—about as much as a small delivery truck. The supporting framework needs to be correspondingly robust. And that's just the dead weight: add to that another 4.5 tons of climate-damaging carbon dioxide—since every ton of concrete generates about 600 kilograms of CO₂.

If Only We Could Build Lighter!

One material now promises a breakthrough: carbon-reinforced concrete. That's right—carbon. In technical terms, carbon fiber. This glossy black synthetic fiber has already given shape and strength to airplanes, yachts, and the racing bikes of Tour de France riders. It makes structures both stiffer and lighter—and now it could transform the construction industry. Josef Kurath of Zurich University of Applied Sciences calls the union of concrete and carbon fiber "ideal." "Carbon has tremendous tensile strength, while concrete excels under compression," he explains. Unlike steel, carbon fibers do not rust, so they don't need the thick protective layers of concrete required by traditional rebar.

A balcony slab made from carbon-reinforced concrete would weigh in at less than two tons. That makes logistics easier and reduces the demands on the supporting structure, Kurath points out. On top of that, such constructions generate up to 80 percent fewer emissions.

Around 500 of these balconies, various load-bearing bridge decks, and even the first bridges have already been produced by CPC AG, a startup in which Kurath is involved. The civil engineer is the driving force behind the so-called "CPC slabs." The acronym stands for "carbon prestressed concrete" technology. The secret lies in the "prestressed" part: carbon fibers, on their own, are limp; only when tensioned do they develop their full capacity to handle enormous tensile loads—"far more than steel reinforcement ever could," says Kurath, who has spent nearly two decades researching carbon-reinforced concrete.

Building Like Carpenters

CPC slabs are standard products, manufactured in factories much like particleboard. Large CNC machines process and shape them to specification. "We're working more like carpenters—everything is prefabricated, and assembly on-site is all that's left," Kurath says. This method is not only weatherproof, but also achieves a higher level of precision.

After years of development, a real breakthrough may now be at hand. At the end of 2020, construction materials giant Holcim entered into a partnership with CPC AG and set up a dedicated factory in Essen, Lower Saxony. "We receive inquiries daily—from all over the world, which is quite unusual for our industry, from England to Australia," says Stefan Gramberg, head of precast concrete at Holcim Germany. "We expect to sell out in the medium term, even if demand in many regions is picking up more slowly than hoped due to the current construction slowdown."

Meanwhile in Switzerland, Kurath is working on plans for a larger building constructed from CPC slabs. The components have already received the strict German approvals, and fire safety issues have largely been resolved. Building with CPC slabs is more expensive, he acknowledges, but the quality is significantly higher. To showcase the material's capabilities, the Grüze Innovation Lab recently opened in Winterthur. Serving as both visitor center and multi-use community space with a café, the open pavilion is constructed from ultra-thin, recyclable CPC concrete panels.

Yet Kurath remains cautiously optimistic: "Price, not environmental protection, is what matters most in construction," he says. "We're up against building methods that have been entrenched for a hundred years."

Where Walls Become Ceilings

Roughly 700 kilometers northeast of Winterthur, Manfred Curbach is also working to bring this high-tech material to the mass market. The civil engineer and professor at TU Dresden, along with partners, inaugurated a building in 2022 that is visibly unlike any made from conventional materials: the roof of the 220-square-meter experimental structure known as the "CUBE" curves upward at a 90-degree angle and seamlessly transitions into a wall. A closer look reveals more surprises: the wall is just seven centimeters thick—and it, too, is made from carbon-reinforced concrete.

While the swooping roof-wall unit was cast in place, many other elements inside the CUBE—such as walls and stair treads—are made from prefabricated carbon-concrete components. Outside the building, benches fashioned from CPC slabs await visitors.

With this remarkable structure, Curbach and his colleagues at TU Aachen hope to demonstrate the potential of carbon-reinforced concrete. The CUBE is both a functional building for work and interaction and a showcase for a pioneering new approach to construction. "It's up to us to prove that concrete can also be used to build lightly," says Curbach. "A new architectural language is emerging—one that could make concrete a sought-after, interesting, and beautiful building material."

A New Era for Concrete

New ideas are urgently needed. Traditional concrete is increasingly problematic: on one hand, it consumes staggering amounts of resources such as sand, gravel, and water—some 12 billion tons are poured every year worldwide. But most critically, cement production, the backbone of concrete, generates billions of tons of greenhouse gases—responsible for about 8 percent of global CO₂ emissions.

Can this high-tech carbon fiber material really provide a solution? Carbon-reinforced concrete is not only half the weight of traditional reinforced concrete, it’s also more capable, and allows for more design freedom. But above all, says Curbach, it can dramatically reduce carbon emissions: "Building less simply isn’t an option, so we have to build differently—above all, more durably."

"With carbon concrete, you can save at least 50 percent of the concrete required. The ecological effect, in terms of CO₂ savings, is as high as 75 percent," Curbach explains. "That’s partly due to using less concrete, but also to using alternative binders." In that sense, the material really is an environmental ally: "On a performance basis, carbon is already cheaper than steel; and if you can replace 400 kilograms of steel with just 14 kilograms of carbon, the ecological footprint is significantly improved as well."

The material’s promise is being borne out—not only in experimental buildings and balcony slabs, but also in full-scale bridges. The world's first carbon-reinforced concrete bridge was built in Albstadt-Ebingen, Germany, in 2015. Spanning 15 meters, it is just nine centimeters thick, yet strong enough for snow plows and gritters weighing up to ten tons to cross safely. Additional carbon concrete bridges have since been completed in the United States, Canada, Germany, and Japan.