Sustainable construction

Building tomorrow’s cities with yesterday’s concrete

The award-winning start-up circrete has developed a digital platform that will ensure greater reuse of concrete elements in buildings through matchmaking, quality testing, and knowledge sharing.

Two women are standing in front of a building by a pile of discarded concrete building supplies. Foto: Bax Lindhardt — Annika Bang Thomsen (left) and Luise Dennin have come up with an award-winning idea, which they hope will lead to more recycling in the construction industry. Photo: Bax Lindhardt

Thursday 27 March 2025

Magnus Stenaa Jensen

The norm in the construction industry should be to reuse old concrete elements in new buildings. This is the mission of the start-up circrete, which was established by two MSc students at DTU. The start-up hopes that their matchmaking platform can provide an important tool that can help the construction industry reduce CO₂ emissions in new buildings.

The reality is that the limit for the maximum permitted emissions per square metre in new buildings will gradually decrease towards 2029, when the permitted limit will be 7.5 kg CO₂ per square metre. This is a big change from the current permitted emissions of 12 kg CO₂ per square metre in new buildings of more than 1,000 square metres.

Therefore, the start-up will test the quality of the discarded concrete elements and help to facilitate the process so that reusable concrete elements from, e.g., demolished buildings can be incorporated into new buildings that can use them, thus limiting the necessity to produce new concrete.

On the matchmaking platform, circrete will act as a link between, for example, a demolition company that needs to dispose of concrete elements and a developer that needs recycled elements for their new construction. On the platform, circrete will—through testing of the elements—describe the concrete’s robustness and usability, so that a developer can get the right concrete match.

Closeup of discarded concrete building material. Photo: Bax Lindhardt — Concrete used by the construction industry is produced using cement, which contains a large amount of limestone. Burning limestone requires heat, and limestone itself releases CO2. Photo: Bax Lindhardt

Concrete LEGO bricks

One way to test a concrete element is to subject it to axial loading. This is done by clamping the element to a tall metal structure, which presses down against the concrete with a large block at the top to see how much the concrete can handle.

“If you dismantle a building, you only have a skeleton of walls, columns, and floors left. The skeleton is mainly made of prefabricated elements. These elements have hollow cores, and therefore we got the idea that—just like with LEGO bricks—you could use the elements in new ways by cutting them to size,” says Luise Dennin, co-founder and CTO of circrete and MSc Eng student at DTU.

Ever since she can remember, she has been interested in construction, and there was therefore no doubt that she would work to develop new sustainable solutions in precisely the construction industry.

“Since I was a child, I’ve been building small wooden houses with my grandfather. There is something fascinating about being able to create things from different elements. It’s actually one of my hobbies to go out and look at construction sites,” Luise Dennin says with a smile.

From study project to start-up

Luise Dennin met her partner Annika Bang Thomsen, CEO and co-founder of circrete, for the first time on the Intro Day for MSc students at DTU. Annika Bang Thomsen is doing an MSc in Design and Innovation and did not share the same enthusiasm for construction. Still, it didn’t take much persuasion from Luise Dennin to convince her that they should join forces on a study project on recycling in the construction industry.

“This whole world of construction is very new to me. But I’m devoted to sustainability, and I think that if you’re interested in sustainability, there is no way around the construction industry, because it has such a big impact on our CO₂ footprint in Denmark,” Annika Bang Thomsen says.

According to the Circularity Gap Report Denmark, the construction sector accounts for approx. 17 per cent of Denmark’s CO₂ footprint.

This marked the start of a thorough research process, where the two DTU students visited demolition companies, interviewed experts, and established contacts in the industry. However, the way to a start-up was paved through DTU’s annual Green Challenge conference, which gives students the opportunity to present and compete for the best sustainable solutions.

“We received a lot of positive feedback, and we even won an award. Afterwards, we were contacted by start-up coaches from DTU Skylab (DTU’s innovation hub, which brings together students, researchers, and businesses, ed.). They asked us if it wouldn’t be a good idea for us to start our own business. And so, we did,” Annika Bang Thomsen says.

Woman standing in front of a blackboard filled with calculations. Photo: Bax LIndhardt — Annika Bang Thomsen (left) and Luise Dennin have done a lot of tests and calculations so that they can provide potential entrepreneurs with information on the circrete platform about the history, robustness and usability of concrete elements. This information is intended to ensure the right concrete match. Photo: Bax Lindhardt

A bright idea

In the autumn of 2024, the start-up received the Otto Moensted Foundation’s The Bright Idea Award 2024 in the ‘early stage’ category. The prize of DKK 250,000 is a testament to the start-up’s huge potential.

“It’s great to get funding to take circrete forward, but I think it’s even more valuable to get the recognition. I think this will help us to be taken more seriously and strengthen our network,” Luise Dennin says.

With thorough sparring from one of DTU’s offerings in the university’s large innovation ecosystem, DTU Skylab’s Ignite programme, where budding DTU start-ups are trained in an intensive 10-week course, Luise Dennin and Annika Bang Thomsen have made a plan for the future.

“We need to test whether there is actually a market for our platform. It costs a lot of money for us to test the elements, and therefore it has to be financially viable if we’re to make a living from the start-up in the future,” Luise Dennin says.

Therefore, the plan is to launch a pilot project, where the start-up will collaborate with a demolition company and a developer or a firm of architects to test their digital tool. The two entrepreneurs are very optimistic, and their ambitions to create a greener future in the construction industry are even greater.

Facts

About circrete

Founded in 2024 by the two DTU students Luise Dennin, CTO, and Annika Bang Thomsen, CEO.

With the slogan ‘We build tomorrow’s cities with yesterday’s concrete’, the business tries to reduce CO2 emissions and concrete waste in the construction industry through its matchmaking platform, which makes it easier to reuse concrete structures in new construction projects, while also testing and disseminating knowledge about reuse in large building projects.

Sustainable construction

Around 22 per cent of Denmark's carbon footprint comes from the building and construction sector. By building more sustainably, we can reduce energy consumption and thus greenhouse gas emissions.