The project aims to use concrete reefs to increase marine biodiversity off the Danish coast

The project aims to use concrete reefs to increase marine biodiversity off the Danish coast

The Super Rev project aims to restore 55 square kilometers of reefs off the Danish coast.

If you cycle over the Bryggebroen Bridge at Fisketorvet, you will see three concrete sculptures sticking out of the water in Copenhagen’s inner harbour. But it’s only underwater that the work of art really comes to life. Here you can see that the sculptures are filled with cracks that eventually become a habitat for algae and fish.

The project is called “Super Rev” (Super Reefs) and aims to restore 55 square kilometers of reefs off the Danish coast. Behind the work is the international artists’ group Superflex, which in an unconventional collaboration with the DTU, among others, is investigating whether it is possible to create a stone reef out of newly developed concrete.

The construction industry should be able to use the experience for future coastal protection projects, bridges, tunnels and offshore wind farms, which in practice act as stone reefs for algae and animals. The aim is to contribute to increasing marine biodiversity.

“Marine ecosystems have come under pressure from climate change, overfishing, reef degradation, and far too many nutrients from agriculture and buildings. It is therefore necessary to research innovative materials that can promote marine biodiversity.” We hope that the large infrastructure projects of the future will not only be designed to produce green energy and develop sustainable climate protection, but also to strengthen the Sea creatures contribute,” says Wolfgang Kunther, materials scientist at the DTU.

art and sustainability

Restoring Danish stone reefs is necessary as 8.3 million cubic meters of stone has been removed over the past 100 years to expand Danish harbors and to create concrete.

According to a study by DTU Aqua, this is the equivalent of removing 55 square kilometers of habitat for animals and plants – an area slightly smaller than the island of Fanø. The disappearance of the stones means there are no hiding places for sea creatures and places for algae and mussels to attach themselves. And when the algae disappear, there will be no rich life of small animals and fish either.

To focus on the lost stone reefs, Superflex set out to build some sort of underwater habitat for fish. The project functions as a home for sea creatures, a scientific experiment and a work of art. Crown Prince Frederik of Denmark inaugurated the plant in 2021. In the same year, the artists received the Culture Prize of the Crown Prince Couple for using art to put important issues such as sustainability, the life of other species and climate challenges on the agenda of debate both in Denmark and internationally.

Credit: Technical University of Denmark

“The work is not just a restoration project – we are also interested in developing new construction methods for the future, focusing on the needs of other species. It’s a huge task that we can’t do alone – but we can achieve great things through partnerships and collaborations,” says Malene Natascha Ratcliffe, CEO of Superflex.

Cement absorbs CO2

The collaboration between Superflex and DTU began when the artist group contacted Wolfgang Kunther to hear about his ideas for using concrete in artificial rock reefs. This led to a request from Udviklingsselskabet By & Havn (Copenhagen City and Port Development Corporation) if DTU would cast, analyze and test concrete elements for the artwork.

In Wolfgang Kunther’s daily work, he explores how cement and water act as binders in concrete and how they react to the environment and living organisms. So he knows there are many benefits to using concrete in the sea. One of the advantages is that concrete is an easy material to work with. You can change the composition of cement, change the texture and size, and create smooth and rough concrete surfaces that can provide ideal living conditions for fish and algae.

Another advantage is that concrete does not only release CO2 in the manufacturing process and when heated – it also absorbs CO2. Some call it a sponge effect because the concrete absorbs CO2 like a sponge. The effect is due to a slow process that occurs as the cement solidifies and reacts with the CO2 present in the atmosphere and in the sea. This results in the same limescale deposits as are known, for example, from limescale deposits in the bathroom.

The coating also has the benefit of creating a natural breeding ground for marine life, as it bears a certain resemblance to coral reefs and seashells. Not to mention the useful life.

Concrete stone reefs to increase marine biodiversity

Three concrete sculptures emerging from the surface of the water in Copenhagen’s inner harbour. Credit: Technical University of Denmark

“In the vast majority of concrete structures for maritime infrastructure projects, concrete is used in combination with steel reinforcement. But when reinforced concrete is affected by seawater, the rebar begins to rust—the question is when. With the type of concrete structure we’re testing, we don’t expect steel to have to be used. This means that the service life in sea water will be longer than traditional concrete structures. That has a great advantage for the environment,” says Wolfgang Kunther.

Pink and brown concrete

Now he has received the first concrete samples of the work of art. Together with his research group, he is investigating how seawater affects the reef structure and whether other mineral transformations take place in addition to lime. The group is also examining whether it is more cost-effective and durable to use concrete reefs instead of the imported natural stones more commonly used.

The researchers are testing three different types of concrete that they cast in the DTU’s concrete foundry. A pigment was added to a concrete that turns the artificial reef pink, which Superflex says is the favorite color of several organisms. Another type of concrete consists of cement with added recycling raw materials from brick production.

This gives a brown concrete. For the third concrete, the researchers partially replace the cement with Danish clay, which is fired at high heat, resulting in an orange color in this case. Project partners are also investigating whether concrete from other projects can be reused.

Marine researchers from DTU Aqua are working in parallel to find out how well stone reefs and artificial concrete reefs work in terms of fish – whether there will be a large increase in fish and what fish species will be found on the new concrete reef. They find answers to these questions using underwater cameras that provide a unique insight into life on the seabed before and after the creation of artificial stone reefs. In this way, the researchers can compare the measurements and document the positive effect of the artificial stone reefs.

“Ports used to be associated with polluting industries, but now the water is so clean that you can swim in many ports. It’s a fantastic development for many swimmers as well as sea creatures. That means the fish can return to our ports, but there are often different ways they can be helped with that return,” says Senior Researcher Jon C. Svendsen.

Concrete stone reefs to increase marine biodiversity

The researchers are testing three different types of concrete. Credit: Technical University of Denmark

We are pioneers

He was recently in Puerto de Vigo, Spain, to take underwater photographs before specially designed harbor walls are erected to provide plenty of hiding places for fish. The project is called Living Port and also includes concrete experts from DTU who oversee different types of concrete elements and structures. This is done together with ECOncrete – a construction company that develops new concrete and concrete elements.

A similar project is being carried out in Køge, where DTU is helping to create rich fish life on the seabed of Køge Bay through concrete reefs. The project is supported by VELUX FONDEN with DKK 9.6 million. The project is led by the local stone reef protection association Køge Bugt Stenrev and is implemented in cooperation with local stakeholders and a large number of innovative communities around Køge Bay.

DTU will share the experiences from the projects in a series of webinars that will take place together with the Danish Society of Engineers, IDA and the Danish Concrete Association. It will inspire builders, urban developers, consultants and contractors on how to include underwater biodiversity in future building projects, says Wolfgang Kunther:

“This is a whole new way of looking at concrete structures. Both in terms of biodiversity and concrete development. We do not cast standard materials. Therefore, we must ensure that the materials last a long time. We need to know what happens when we replace part of our familiar concrete with something new. Especially when it comes to large infrastructure projects that should last 100 years without major repairs. You can say that we are pioneers.”

The work of art in Copenhagen’s inner harbor can be seen until summer 2023.

Provided by the Technical University of Denmark

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