The most polluting part of a building is hiding underfoot. Floors account for more than 40% of most multi-story buildings’ total mass, and many of the surfaces that hold up office floors and apartments are made of concrete. They represent a significant chunk of the emissions caused by constructing and operating a building over its lifetime. With the production of concrete being responsible for an estimated 8% of global carbon emissions, lowly floors are a serious climate challenge.
A new method of building concrete floors could change that. By rethinking the design and structural requirements of building floors, a team of architects and engineers at ETH Zurich have developed a floor slab that requires only 30% of the concrete and 10% of the reinforcing steel of a typical floor. With multi-story buildings rising along with growing populations in China, India, and Africa, these material reductions could represent a major decrease in the carbon emissions caused by urban development.
“Spanning space is such a material-intensive aspect in construction, and hence also the most polluting,” says Philippe Block, an architecture professor at ETH Zurich who led the research behind the new floor project.
After more than a decade in development, the lighter concrete slab has just gotten its first use in an actual building. Dubbed HiLo, the building is a two-story module built into the Next Evolution in Sustainable Building Technologies research building platform in Switzerland. With curved concrete roofs soaring over the space like wings, and large walls of windows, HiLo is a bold architectural prototype.
“It’s there to shout so people pay attention,” Block says. “It’s our neon billboard, but the message is in the floor.”
Inside, the concrete floors use just 3 centimeters of concrete (that’s less than 1.25 inches) arching over a skeleton-like framework of thin supporting steel bars, and look a bit like the face of a waffle iron, with thicker lines of concrete only where they’ve been calculated to be necessary.
This could soon be part of building projects around the world. Block’s research group has partnered with the global construction-materials company Holcim to turn this approach into a mass-market product. Holcim recently announced plans to have this system available for industrialized construction by 2023, and estimates the system can reduce concrete flooring’s embodied carbon emissions by up to 80%.
The material reductions in these floor slabs could have wide-reaching impacts. By 2060, the total floor area of buildings around the world is expected to double, to the tune of an additional 2.4 trillion square feet. Much of this growth will be concentrated in cities, and most of the urban buildings to come will be mid- and high-rise towers. Concrete, says Block, is “the only material that’s available at the scale of the rapid urbanization and population growth underway.” Cutting down the amount of concrete that would be used to build the floors in those buildings could greatly reduce the environmental impact of accommodating that growth.
In a hypothetical 25-story building, Block says the reduction of concrete would translate to 1,200 fewer truckloads of concrete being driven to the construction site, and a saving of 14 miles worth of steel rebar on each floor.
The floor slabs that Block’s team developed have been optimized over years to reduce material requirements while maintaining their strength—but the principles behind the design actually date back to Gothic cathedral construction. Masonry building techniques—relying on arches and the compressive strength of stones carved and stacked upon each other—have been used to build structures that can stand for centuries. It’s an old form of building that’s finding new relevance in the age of 3D printing, and was recently used by Block and designers from Zaha Hadid Architects to build an arching pedestrian bridge in Venice, Italy, using 3D-printed parts that stack into place without the need for any mortar.
Block’s team applied that same concept to the floor slabs, designing them to distribute the force of supporting weight to the corners—the places in buildings where floors would be held up by beams. That’s reduced the need for extraneous reinforcement within the concrete slabs, allowing them to be built like individual prefabricated components instead of being poured on site over cages of rebar.
Though many have argued for the need to transition away from concrete in construction toward sustainably sourced materials like cross-laminated timber, Block says it’s also important to improve the common construction materials that much of the world relies on. “Sustainability should not be a message about materials, but about how you use the materials,” he says.
Block says the new floor-slab system has already been proposed for inclusion in a large project that’s currently seeking building permits, and he’s hoping to see it used in other projects in the coming years. The partnership with Holcim and its goal of marketing the process by 2023 will likely help. But Block is also on the lookout for projects that want to incorporate this new system even sooner. Making an impact, he says, requires this new type of floor construction to start making its way into buildings around the world.
“The materials are available, and the system is entirely engineered, plus we already demonstrated it in one building,” says Block. “This is not the future. This is already ready today.”