Stone on stone, fungus, or wood?

Cities are growing all over the world, and climate change is exacerbating the respective conditions for human life. Researchers are developing materials, methods, and concepts to facilitate safe and healthy living arrangements in the future as well as climate-friendly construction.

Urban dwellers experience some of the consequences of climate change with particular severity. Because water seeps away poorly through sealed soils, heavy rainfall can lead to devastating floods with far-reaching consequences for people and infrastructure. Many building materials absorb solar radiation. During summer heat waves, they heat up like ovens. In densely built-up cities, the heat island effect occurs on hot days: Here, it does not cool down sufficiently even at night. Elderly people in particular suffer from such heat accumulation. Meanwhile, construction continues worldwide, from single-family homes with home offices in the countryside to new parking garages and urban skyscrapers. According to United Nations forecasts, hundreds of millions of people will move to cities in the coming decades.

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Classic concrete construction: Could CO₂ emissions be a source of reusable materials?

In this vein, research on materials and methods for the construction of tomorrow is pursuing two objectives: On the one hand, the materials should be produced in as climate-friendly and resource-friendly a manner as possible. On the other hand, new buildings and infrastructures should be as resilient as possible against the effects of climate change.

The No. 1 construction material in use today is concrete. There is no other man-made material used more commonly throughout the world. Its components – sand, gravel, and water – are bonded together with cement. The production of concrete accounts for about six to eight percent of global CO₂ emissions. At the Karlsruhe Institute of Technology (KIT), non-university research institutions and a team of scientists led by Professor Frank Dehn are investigating the possibility of sorting and activating the smallest particles from old concrete and reusing them as bonding agents for new construction material. “That would be an impressive example of upcycling,” says the Head of the Institute of Reinforced Structures and Building Materials. In this way, major cuts in CO₂ emissions could be achieved in cement production. At present, downcycling is the best thing taking place: Old concrete is partially reused in road construction or civil engineering.

“Doing without cement and concrete won’t be an option in the future,” says Arne Stecher, who is responsible for the German decarbonization strategy at the cement manufacturer Holcim. As one of the largest manufacturers of building materials, Holcim has annual CO₂ emissions in excess of 100 million tons worldwide. “Our task is to remove CO₂ from the manufacturing process and to further use it as a raw material,” Stecher explained at the 2021 dialogue conference of the Helmholtz Climate Initiative. At the Helmholtz Centre for Materials and Coastal Research (Hereon), researchers led by Dr. Torsten Brinkmann have developed a membrane separation process to separate CO₂ from flue gas. Holcim will be testing such a carbon capture technology in a prototype from the beginning of 2022. This could make it possible in the future to further process the collected and processed CO₂, for example as a raw material for synthetic fuels.

Generally, concrete is used in conjunction with steel. The production of steel generates just as much CO₂ worldwide as cement production. Obtained exclusively via renewable energy, green hydrogen is intended to make the process of melting steel more climate-friendly. “If steel plants have supplies of hydrogen, it is, technically speaking, not a problem to produce high-quality steel,” says Professor Olena Volkova from the Freiberg University of Mining and Technology. Up until now, however, the production capacities for green hydrogen and the infrastructure for transporting it have been lacking. That said, the steel manufacturer Thyssen Krupp has set itself the goal of achieving carbon-neutral production by 2050, with the help of hydrogen and carbon capture.

Wood: on top and all around

Right now, more and more attention is turning to locally sourced construction materials: Clay, which regulates humidity and thus ensures a comfortable climate in interior spaces; or sheep's wool, which is as an agricultural by-product and can be used for thermal insulation. Wood is another highly praised building material. Given that trees store CO₂ and do not release it for a long time during the installation process, the building material is sometimes referred to as a CO₂ storage unit. This is somewhat misleading because the crucial CO₂ reservoirs are living forests. And they suffer from events such as fires or infestations of insects, which are in turn intensified by climate change. Excessive logging on top of that would be far from climate-friendly. Not to mention: the wood reserves available are limited.

Furthermore, as a material, it is sensitive to moisture, which must be taken into account when opting to construct buildings out of wood. And also if there are construction methods with load-bearing elements made of wood: “We still have little experience with tall buildings,” says Dr. Carmen Sandhaas, a KIT specialist for timber construction and building structures. “Planning, designing, and constructing such buildings is currently even more complex than for concrete buildings. In this respect, we need more practice.” Given the rapid rate of growth currently being witnessed in urban environments, tall structures are simply inevitable in order to avoid covering large surfaces with concrete and paving long roads. “Of course we’re not likely to build a Burj Dubai out of wood,” says Sandhaas. Nonetheless, wood does have its own advantages as a building material: “Thanks to its light weight, it’s possible, for example, to create additional living space,” Sandhaas explains. This can be done by simply adding on to the top of old buildings, where the static design would not allow for the weight of other floors made of stone or concrete.

Combining and separating materials

At KIT, a particular concern of the materials researchers is combining different materials so as to optimally bring to bear their respective advantages. Sometimes, most poignantly so: For example, the KIT specialists are experimenting with wood as sheathing for steel beams, to make them more stable against pressure from above. “There is still much research to be done when it comes to connecting wood and concrete elements,” says Carmen Sandhaas.

The required materials should be installed in such a way that they are as completely recyclable as possible – in a manner that is modular and unmixed. In an optimal scenario, an entire (concrete) wall can be transplanted from an old building to a new one. The basic idea is to use materials in as unmixed a fashion as possible – i.e. to design, for example, load-bearing structures separately from thermal insulation. This calls for optimized and innovative techniques for joining various materials. Professor Dirk Hebel, Dean of the KIT Faculty for Architecture, refers to this as “a technological challenge.” As for the parties performing the construction, Hebel says: “We always have to ask ourselves: Can what I’m designing be dismantled and reused for new tasks? If it will end up as waste instead, then we haven’t designed it the right way!” Or will it simply end up in the compost heap?

In addition to wood, Hebel and his collaborators are experimenting with other natural building materials that might seem exotic at first glance: They are growing construction elements from fine, spongy fungal cells – i.e. the mycelium. By combining these with grasses such as bamboo, bending forces can be absorbed in a manner similar to the combination of steel and concrete. “In a manner of speaking, our vision is to grow houses in the future,” says Dirk Hebel. The fungi's fine cell network quickly assumes any conceivable shape that is imposed on it. Together with other experts, Hebel and his team are developing novel structures that can absorb compressive and tensile forces and thus constitute an alternative to established systems.

Durable and convertible

From a technical point of view, sustainable and climate-friendly construction means combining different materials as intelligently as possible in order to conserve the individual resources. It also means constructing buildings to be durable and using the required materials in a recyclable manner. The German Federal Ministry of the Interior has made available a tool for planning sustainable buildings. Building planners can consult the ÖKOBAUDAT database to draw up an ecological assessment of a planned building. Materials are taken into account, in addition to construction, transport, energy, and disposal processes.
Already in the planning phase, it is important to design the buildings for the longest possible service life. In addition to the selection of materials, a convertible – modular – design helps: For example, KIT researchers, in cooperation with researchers from the Technical University of Kaiserslautern, are designing a parking garage that can be converted into a residential building if necessary. It would also be possible to convert detached houses that are no longer needed into smaller apartments by simply moving the walls – and back again if necessary.

Designing and using surfaces

Existing buildings offer many opportunities for optimization in terms of climate protection and resilience. Adding plants to facades and roofs, for example, brings several advantages at once. Plants on sunlit facades not only provide shade on hot summer days, but also lower temperatures through evaporation. In winter, on the other hand, the plants reduce the heat loss on shady facades. Both of these effects help to reduce the buildings’ operating energy. This additional greenery also helps to improve urban air quality. It absorbs CO₂ and releases oxygen. In addition, plants can bind certain air pollutants such as fine particulates. Rooftops with plants have another benefit: They restrain and store water, and help regulate runoff after heavy rain.

But why do we see so few green buildings in Germany? “This is probably due to how the topic is perceived by the relevant actors,” says Steffen Bender. The adjunct professor at the Helmholtz Centre Hereon provides consultation to property developers, planners, and building owners at the Climate Service Center Germany (GERICS). “In their considerations, they often focus on the ongoing effort required and the associated costs,” he explains. The plants would require care, and the stonework would also have to be adequately protected. Sometimes, the greenery conflicts with monument protection rules or long-term development plans. In contrast, their numerous benefits receive less attention. “So far, green buildings simply aren’t being properly lobbied.” He sees opportunities for action, especially at the municipal level. “If there are local heroes, role models, or influencers who are enthusiastic about the topic, they can motivate many homeowners,” notes Bender. Large cities often have large property developers, as well as construction and management companies. “If these actors were on board,” says Bender, “this could impact a large number of buildings.”

Removing legal hurdles

Solar cells are another way to make facades and roofs useful and climate-friendly. A study conducted as part of the EU-funded BIPVboost project produced the following results: Building-integrated photovoltaic systems (BIPV) could cover up to a quarter of private electricity demand in Germany. Facades with modern solar modules can be designed in many ways. They can take on a variety of colors and completely different forms. In the future, there will also be an interesting option of shading windows and glass facades with partially transparent – e.g. organic – solar cells. Such modules are also compatible with rooftop plants. In their synthesis report on green buildings, Bender and his colleagues describe how the cooling effect can even increase their efficiency.

Dr. Björn Rau from the Helmholtz-Zentrum Berlin für Materialien und Energie (HZB), for example, sees obstacles in tax law to the widespread use of solar cells on building facades. Suitable facades can often be found on public buildings like schools, administrations, or hospitals. Buildings with such facades may not easily become commercially active by feeding the generated electricity into the grid. “At the Helmholtz-Zentrum, we also had to be very careful in defining the legal framework for the use of our solar facade in order to comply with our donors’ rules,” reports the deputy head of the BIPV advisory center at the HZB.

Often, there is a further challenge of designing photovoltaic facades in accordance with the fire protection requirements of the applicable building law. “One thing, however, is clear: The solar laws are calling for more and more photovoltaics,” says Rau. “This means that public entities must also be allowed to operate photovoltaic systems. For this reason, it is important to adjust the current rules and guidelines accordingly.”

Ulrike Schneeweiß