Photo: Kontraframe

More space and light with climate-friendly concrete

Tuesday 10 Dec 13

Contact

Henrik Stang
Vicedirector, Professor, Constituent head of Section
DTU Civil Engineering
+45 45 25 17 35

U-value of high-strength concrete

A U-value is an expression of how much heat passes through a building construction. 

The Danish Building Code makes specific requirements on the U-values of different building sections. Different requirements thus apply to floors, walls, windows and ceilings. The lower the U-value, the better the insulation properties of the construction. 

A 36 cm-thick sandwich element made of high-strength concrete has a U-value of 0.088 W/m2K. A 40 cm-thick element has a U-value of 0.07 W/m2K. By way of comparison, the Danish Building Code from 1 January 2013 requires exterior walls to have a U-value of no more than 0.3 W/m2K.

Partners in the project

The new high-strength concrete construction system is the result of a successful development project that received a real boost in 2010 in the form of funding from the Danish National Advanced Technology Foundation. 

From 2010 onwards, a number of partners participated in developing the construction system, including the concrete producer Ambercon, the innovation enterprises IPU and Smith, the architects’ office Arkitema and two DTU departments: DTU Civil Engineering and DTU Management Engineering.

Minimum thickness, low weight and remarkable insulation. A new construction system featuring high-strength concrete is good news for the climate. It can be used both in new builds and in renovation projects to improve the energy properties of existing buildings.

As the name suggests, ‘high-strength concrete’ is a very strong material. In practice, this allows the construction of buildings with exterior walls just 36 cm thick, which is half the conventional thickness of 72 cm. These thinner walls free up a lot of space. For example, in a building with 100 m2 of floor space, using high-strength concrete will add an extra 10 m2—corresponding to an extra children’s room—the usable area.

Combination of excellent properties

The new construction system combines the best properties from light construction materials (steel and wood) with the benefits offered by heavy materials (concrete and brick). The system is what is known as a ‘sandwich system’, where the elements involved are built up in layers—just like in a sandwich. The front and rear sections in a high-strength concrete unit are made of slim sheets of high-strength concrete, and they encapsulate a core material of highly efficient insulation. This design ensures maximum insulation capacity with minimum wall thickness. 

The system has been developed for constructing new low-energy buildings and as for use in the renovation of existing buildings to improve their energy properties. “High-strength concrete has great potential in renovation projects as it is easy to handle, does not require scaffolding, and can be installed whatever the weather conditions,” explains Karsten Bro, Project Manager at the company Connovate. 

     Photo: Kontraframe 
Student housing on Aarhus harbourfront

Climate-friendly solution

The high-strength concrete construction system is a climate-friendly solution. Using high-strength concrete makes it possible to cut CO2-emissions appreciably, as Professor Henrik Stang, Vice-Director at DTU Civil Engineering, explains:
 “Concrete is largely composed of cement, and the production of cement is incredibly energy-intensive. In fact, cement production accounts for five per cent of global CO2-emissions. So the less cement we produce, the better it is for the climate. And if we can construct buildings whose walls are only half as thick as usual, we can eliminate appreciable volumes of CO2.” 

In addition to the fact that the production of high-strength concrete does not adversely impact the carbon accounts, the light weight of the units means that less energy is required to transport them to a construction site—and to manoeuvre them into place when they arrive. This translates into an additional reduction in CO2-emissions. It is estimated that construction using high-strength concrete will help cut carbon emissions by up to 60 per cent in relation to construction using conventional concrete elements. 

Cheap housing for poor countries

While the population of Denmark devotes a lot of energy to heating their homes, people in other parts of the world use just as much energy to cool theirs. The high-strength concrete construction system thus has appreciable potential in the hotter countries of the world, as its excellent insulation properties can help to keep buildings cool on the inside. 

The team behind the high-strength concrete have even developed a transportable factory that allows the sandwich units to be manufactured locally—even though the construction site may be located on a different continent. The solution is currently being tested in South Africa, where 92 cooperative farms near Johannesburg are scheduled for completion in early 2014.
“Our low-cost building system can be used to ensure cheap housing for poor people all over the world, with local materials and labour being used in the construction process itself,” explains Karsten Bro.

The new high-strength concrete has a great many uses, in the contexts of both new builds and energy renovation projects for existing buildings. Photos: Kontraframe

 Photo: Arkitema
Terrassed houses in the suburbs of Aarhus

 Photo: Arkitema
Low-cost houses in Johannesburg, Africa

Test building at DTU

DTU Civil Engineering has contributed to the development of high-strength concrete by researching areas such as the material properties of the concrete, its impact on the indoor climate, how it behaves in the presence of fire, its stiffness, its strength ratio and the risk of crack formation.
Photo: DTU 
Test house at DTU

In partnership with Connovate, DTU has even had a test building constructed at its Lyngby campus. This building will constitute a cornerstone of the future development of high-strength concrete. Professor Henrik Stang explains: “The test building allows us to monitor how the concrete behaves over a protracted period. We can take measurements of the house itself and trace how the materials age and how this affects the insulation capacity of the concrete. It is essential that we do this, as it will allow us to develop both the material itself and the construction elements.”

The new high-strength concrete sandwich elements were officially launched in October 2013.

Facts about high-strength concrete

  • High-strength concrete requires only 40 per cent of the cement used in conventional concrete elements.
  • A high-strength concrete element are only a third of the weight of corresponding standard sandwich elements.
  • Highly fire-retardant: can withstand 1,050 °C for 90 minutes.
  • Not affected by weather: renovation elements can be installed in all weather conditions.
  • A high-strength concrete sandwich element costs the same as a high-quality conventional concrete element with corresponding insulation properties.
  • Has the potential to serve as a low-cost building system all over the world, with the elements being made ‘on location’ using a mobile factory.
  • Can be used for both new builds and renovation projects.