Questions & Answers – Carbon Concrete Composite e.V.

What is carbon reinforced concrete?

Carbon reinforced concrete is a composite material made of high-performance concrete and reinforcement made of carbon.

The carbon reinforcement exists in bar and mat (welded mesh) form. The carbon bars are produced in a pultrusion process, mostly with round cross-sections and different diameters.

The welded mesh (mat) reinforcement is produced in a textile process, so that it is often also called textile. The concrete reinforced with it is also often called textile reinforced concrete. In the production of carbon fabric, up to fifty thousand carbon fibres are merged to form a yarn. A carbon fibre has a diameter of around five micrometres, ten times thinner than a human hair. The yarns are in turn processed in a textile machine to produce a fabric and are given a stabilising coating. The fibres can also be aligned according to the force ratios, so that they perform optimum work in the concrete. They ensure lightweight, flexibility and strength in the concrete.

The number 1 construction material to date, steel reinforced concrete, with approx. 70 million cubic metres installed per year, is Germany’s most important construction material. Steel is covered by a thick concrete layer to protect it from corrosion. After water, concrete, with approx. 5 billion cubic metres, is the most used raw material.

Concrete is made from cement, sand, gravel and water. 1.6 billion tonnes of cement, 10 billion tonnes of aggregate (sand and gravel) and one billion tonnes of water per year are used worldwide for the new build and renovation of buildings and bridges.

Carbon is the basic building block of life on earth. It can be obtained from plants, rock and even air. Petroleum is currently still used to produce carbon – as it is inexpensive and, compared to the quantities required, its availability is unlimited.

Among other things, current research is examining carbon production from lignins, i.e. waste wood products, which remain in paper production. Industrial production should be possible in approx. 5 years time.

The change in material to carbon reinforced concrete reduces the energy requirement and CO2 emissions in the production and repair of structures by almost 50 % and uses valuable resources carefully.

What is the difference between steel reinforced and carbon reinforced concrete?

The difference between steel and carbon reinforced concrete lies in that rust-free carbon is used as reinforcement in concrete instead of rust susceptible steel.

Steel reinforced concrete is produced by encasing ribbed steel bars in concrete. Steel reinforced concrete is characterised by its high stability, as it combines the compressive strength of concrete with the tensile strength of steel. The properties of being able to carry large loads and the high bending and tensile strength, make steel reinforced concrete the ideal construction material.

Yet it has the major disadvantage of corroding. A thick concrete cover is required as a protective layer to protect the steel from corrosion. This in turn results in a further disadvantage: The production devours large amounts of energy and thus contributes to the construction industry’s high share of climate-harming CO2 emissions. In addition, corrosion causes steel reinforced concrete structures to become an increasing safety risk.

Carbon reinforced concrete is significantly more resistant than steel reinforced concrete. Carbon is four times lighter and has six times the load-bearing capacity of steel, i.e. it has 24 times the performance. Carbon reinforced concrete is far more resistant than steel reinforced concrete, as the material does not corrode. Structural members made of carbon reinforced concrete can therefore be significantly more slender, which reduces the quantity of raw materials required. Energy consumption and CO2 emissions sink by around half. The flexible formability of the carbon fibres enables very slender and intricate design of structures, whose life is also significantly longer than with the construction method with steel reinforced concrete.

From what are carbon fibres obtained?

The fibres are acquired from carbon, the basic building block of all life on earth. It can be obtained from plants, rock and even air. Petroleum is currently still used to produce carbon – as it is inexpensive and, compared to the quantities required, its availability is unlimited. Research is currently in progress on using lignins for the production of carbon fibres. Lignin is a waste product of wood, large quantities of which remain in paper processing.

Furthermore, studies are also in progress on whether and how CO2 from air can be used to produce carbon fibres.

How is a carbon textile made?

A carbon fibre has a diameter of around five micrometres, ten times thinner than a human hair. Up to fifty thousand of these fine fibres are joined together to form long a yarn. The yarns are in turn processed in a textile machine to produce a fabric and are given a stabilising coating. The fibres can be aligned according to the force ratios, so that they perform optimum work in the concrete.

Alternatively, the carbon yards can be bonded in a pultrusion process to form bars. These meshes / bars can then be transported to the construction site or precast concrete plants – in the same way as steel reinforcement.

What method exists for producing carbon reinforced concrete?

Carbon reinforced concrete can be produced in casting and in laminating processes. Liquid concrete is used. During the casting process, the fine concrete is cast in a slab, the individual layers of the carbon fabric are kept apart by so-called spacers. In the laminating process, fine concrete is placed in formwork and the carbon fabric is incorporated. The more layers are incorporated the more stable the carbon reinforced concrete.

How long does it take for carbon reinforced concrete to cure?

This depends on the concrete used and is the same as the familiar times for steel reinforced concrete. In general, it reaches 70 % of its strength after approx. 3 days. The characteristic properties are determined after 28 days. However, the curing process continues.

How durable is carbon reinforced concrete?

While steel reinforced concrete buildings are assumed to have a useful life of 40 to 80 years, in the case of carbon reinforced concrete we expect a use period of 200 years and longer.

How expensive is carbon reinforced concrete compared to steel reinforced concrete?

Carbon and steel are currently on an equal level in price terms with regard to their performance, although the kg prices do not indicate this. 1 kilogram steel costs only approx. 1 euro, 1 kilogram carbon by comparison costs approx. 16 euros. However, the density of carbon is four times lower and the strength is six times higher. Thus, 24 times the performance is obtained for 16 times the price.

How high is the resource and material saving if carbon reinforced concrete is used?

Carbon reinforced concrete reduces the energy requirement and CO₂ emissions in the production and repair of structures by almost 50 %. Façade panels or strengthening layers, for example, are only approx. two centimetres thin if made of carbon reinforced concrete instead of approx. eight centimetres thick steel reinforced concrete. Thus, 75 % less material has to be produced, transported, installed and anchored.

What effects does the use of carbon reinforced concrete have on architecture?

With carbon reinforced concrete we can build with thinner walls. In addition, the flexibility of the material opens up opportunities for architects to design building geometries, which were previously difficult to implement. Slender structural members enable a completely new use of forms in architecture.

Modules made of carbon reinforced concrete can also be equipped with additional functions such as insulating, heating or monitoring, which makes “intelligent networking” possible in buildings or bridges.

How is carbon reinforced concrete recycled?

The separation of carbon and concrete is currently being researched under laboratory conditions and is already achievable. The carbon is fed into the same recycling processes as those used in the aviation, automotive and sports goods industry. Concrete is recycled in the same way as it has been until now.

Is carbon reinforced concrete fire resistant?

Unlike the material steel, the pure carbon fibres usually used in construction are resistant to temperature up to around 650…700 °C. If it is possible to prevent oxygen accessing the carbon reinforcement, under intergas conditions the fibres remain stable up to approx. 1300°C. Impregnation is also required to ensure adequate bonding properties for use in concrete. This ultimately determines the thermal behaviour and must therefore be matched to the area of use. For most construction projects, impregnations developed for normal temperatures are sufficient.

What cost advantages result with textile and carbon reinforced concrete compared to steel reinforced concrete?

The cost advantages depend on the structure and often lie in the shorter construction period and smaller material consumption, as well as the reduced cost of transport and fixing. In several cases, the creation or retention of a large usable structure volume is decisive.

In the remediation of bridges, 2 cm thick layers can be achieved if carbon reinforced concrete is used. 7-8 cm would be necessary for remediation with steel reinforced concrete. With carbon reinforced concrete, up to 80% concrete can be saved, which does not have to be transported and installed. Due to the thin carbon reinforced concrete layer, the weight of the bridge is not increased by a noteworthy amount, so that carbon reinforced concrete can also be used to repair bridges for which remediation with steel reinforced concrete is not possible. Both the material saving, the reduced construction period and thus any reduced shutdown time as well as the increase in life of the existing building fabric lead to cost advantages.

Carbon reinforcement used for the remediation of silos can be laid significantly faster than steel reinforcement. There is no need to fix the reinforcement with anchors. Instead of a 7-8 cm thick layer with steel reinforced concrete variants, only 1-2 cm layer thickness is required for carbon reinforced concrete. In total, up to 80 % concrete is saved, the construction period and closure time is reduced and the usable silo volume is largely retained. All this leads to a cost advantage.

In the renovation of listed structures, whose appearance after the renovation should be similar to that before the renovation, the strengthening layers may not lead to a visible change in the structure. The thickness of the strengthening layer to be applied must be removed from the existing structure first. If 7-8 cm steel reinforced concrete is to be applied as a strengthening layer, 7-8 cm concrete must be removed beforehand. With carbon reinforced concrete it is only 1-2 cm. Up to 80 % less concrete must be removed and recycled and reapplied. The cost advantages lie in the reduced material consumption and the shorter construction period.

Carbon reinforcement used for the remediation of building ceilings/floors can be laid significantly faster than steel reinforcement. There is no need to fix the reinforcement with anchors. Instead of a 7-8 cm thick layer with steel reinforced concrete variants, only 1-2 cm layer thickness is required for carbon reinforced concrete. The weight of the existing building fabric is only slightly increased by the thin carbon reinforced concrete layer, so that it is largely not necessary to strengthen the adjacent load-bearing structural members (columns, walls and foundations). In addition, the usable room height is largely retained. The concrete saving of up to 80 %, the construction period reduction as well as the absence of a need to reinforce the adjacent structural members lead to a cost advantage.

In the new build of bridges, above all the significantly longer life is taken into account. The remediation cost is reduced significantly due to the non-rusting reinforcement and a new build replacement is not necessary until significantly later. This leads to cost advantages over the life.

New curtain walling made of carbon reinforced concrete has structural member thicknesses of approx. 3 cm. The steel reinforced concrete variants are at least 7 cm thick. More than 50 % of the expensive, partly dyed high-quality concrete is saved. More façade panels can be transported on one truck. The fixings for fixing the panels on the structure can be dimensioned smaller. The reduced wall thickness leads to a larger saleable or lettable area. The reduced costs for concrete, fixing and transport as well as the larger usable space lead to advantages.

Lightweight railway platform systems made of carbon reinforced concrete can be laid quickly and thus lead to reduced track possession times for the railway line. If reinforcement made of glass (electrically non conductive) is used, it is also possible to omit the earthing. Both lead to cost advantages.

What is the future of textile and carbon reinforced concrete?

Concrete is the construction material most installed worldwide. It is essentially made from cement, sand/gravel and water. 1.6 billion tonnes of cement, 10 billion tonnes of aggregate (sand and gravel) and one billion tonnes of water per year are used worldwide for the new build and renovation of buildings and bridges. The cement production alone is responsible for approx. 5% of worldwide CO2 emissions.

Against the background of increasingly scarce resources and the targeted CO2 reduction, in future only products that can be produced with far fewer resources and much lower CO2 emissions will be available on the worldwide market.

This is precisely where carbon reinforced concrete comes in, as an alternative to steel reinforced concrete. An increasing number of structures are erected from carbon reinforced concrete and are remediated or renovated with carbon reinforced concrete. The development of carbon reinforced concrete is linked to a long-term growth in numerous industries. Due to the enormously broad effect, apart from the construction industry, above all the chemical, mechanical engineering and textile industries are major winners.

In which areas can carbon reinforced concrete be used?

Reinforcement made of carbon is corrosion resistant, lightweight and freely formable. The concrete cover, which is intended to protect the reinforcement from corrosion is approx. 5 mm for carbon reinforced concrete and is thus significantly smaller than for steel reinforced concrete where the concrete cover is up to 6 cm thick. Therefore, extremely thin structural members and remediation layers can be achieved. Carbon reinforced concrete can be used in structural members with a high standard for a long life. In addition, carbon is electrically conductive, which means that multifunctional structural members, e.g. with integrated heating, can be implemented.

Thus, the innovative construction material can be used wherever resources and energy-efficient, long-lasting, space-saving and multifunctional construction methods are required. This particularly concerns the whole bandwidth of construction and in addition to buildings, also includes civil and structural engineering.

In new build projects, carbon reinforced concrete is primarily used where a reduction in weight or structural member thicknesses as well as an increase in durability are advantageous. For example, bridge structures, bridge edge beams, prefabricated garages, façades and walls in office and residential buildings, as well as decking slabs in multi-storey car parks.

In remediation and renovation, carbon reinforced concrete is used if the load-bearing capacity of structural members is to be increased and at the same time small layer thicknesses, the smallest possible increase in member weight or the retention of structure volume is required. Carbon reinforced concrete is also used to increase the load-bearing capacity in listed buildings in which the appearance of the structure must be retained. Examples include the remediation of bridges, and the renovation of industrial plants and buildings. The silo walls, sewers, ceiling slabs, columns and roofs of listed buildings have already been remediated or renovated.