Carbon is chemically resistant. It suffers no fatigue before it reaches its fracture strength. In comparison to other textile fibres, carbon is very stiff and exhibits no longitudinal thermal expansion. However, the disadvantage of the material lies in its high price. Although the price of the basic material, carbon fibre rovings, has come down by a factor of 40 over recent decades, carbon is still much too expensive compared to steel reinforcement for the equivalent structural performance. Processing rovings into fabric further increases the price. At the moment, carbon cannot generally be used as a substitute for steel reinforcement. Carbon’s corrosion resistance plays no role in conventional building construction. Our fire tests at Zurich University of Applied Sciences (ZHAW) in Winterthur have shown that concrete cover to reinforcement and therefore the thickness of CPC panels cannot be reduced from that of steel-reinforced concrete panels. The present methods of producing carbon fibre rovings offer no sustainability advantage compared to steel reinforcement. Because the stiffness of a carbon rod made from conventional carbon fibres is less than that of the same cross-sectional area of steel, and no significant increase in stiffness is achieved even with high-stiffness fibres, the very high stiffness of carbon cannot be exploited if the rods are unstressed when the concrete is poured. Our calculations show that the price for carbon reinforcement would have to be between 1/60th and 1/20th of what it is now in order to be competitive with steel reinforcement. This does not appear likely in view of today’s technology for the manufacture of carbon fibre rovings.
Competitiveness by prestressing
To compensate for the price disadvantage of carbon, new forms of construction or methods of building must be found. We have been pursuing such an approach for the last 14 years in our research. The result is a completely novel way of constructing with concrete. With the newly developed CPC panels, 1– 2 mm thick carbon strands at 15 mm centres are prestressed and cast into thin concrete panels; the strands are arranged parallel to one another in the X and Y directions to form an orthogonal reinforcement mesh. The concrete cover to the strands is between 6 and 8 mm. The result is a concrete panel prestressed in the two principal directions. This semi-finished product can then be further processed. Openings cut out within the panels or at their edges have no adverse effect on the prestressing: measurements have shown that full force transfer between carbon and concrete still takes place even with a cut edge passing within one centimetre of a strand. 2 cm thick panels are reinforced with 2 layers of strand; thicker panels to resist higher loads have correspondingly more layers. In the near future, panel thicknesses of up to 8 cm with eight reinforcement layers will be available. The length and width of the panels are determined by logistics requirements. For highway applications, the semi-finished panels are cut to a maximum of 13 m ≈ 3.50 m out of considerably larger production panels. In the same way as timber construction, the concrete quality is tailored for the specific areas of use: fire protection, lightweight, thermal insulation or resistance to salt. Once a decision about the method of construction has been made, the semi-finished products can be manufactured in great numbers in a factory environment. Thanks to their low self-weight, they can be inexpensively transported over large distances and are simply stored by stacking. Building with CPC would be a particularly attractive option for areas such as the Middle East or in underdeveloped countries where resources are sparse.
At first glance, one of the great advantages of in situ concrete construction, the high flexibility and the ability to cast concrete to complex geometric profiles, would appear to be lost. However, most components for load-bearing structures in the mass market for buildings and in bridge construction are almost all straight, planar items. In the open air, where no fire protection requirements apply, CPC panels can be designed as steps or platforms, spanning as simply-supported slabs on a substructure or as balcony slabs in prefabricated steel frames. The possibility of adding these panels into complex load-bearing structures also makes semi-finished precast components an attractive option in bridge construction. Digital design software and automated CNC machines in weather-protected production halls allow the components to be manufactured to precise dimensions and quickly assembled on site as dry construction. We based the idea of creating individual building components from semi-finished products on the practices adopted in modern timber construction and its use of glued boards. Over the last 200 years or so, cast components have largely been superseded by semi-finished products in structural steelwork too.
The lightest concrete bridge in the world
The capabilities of the new method of construction was proved in practice for the first time in 2016: the 7.85 m long, 2.44 m wide pedestrian and cycleway bridge over the River Eulach in Winterthur, Switzerland. The existing bridge had to be replaced because the main steel beams were no longer capable of safely carrying the applied loads. Over the years, rainwater had flowed through gaps in the 120 mm thick concrete planks comprising the bridge surfacing and over the main beams, which led to serious corrosion damage. The steel beams were retained in the new bridge only to act as supports for the existing pipework under the deck.
The new bridge deck consists of a single large-format, 40 mm thick CPC panel with a continuous milled-out drip rebate on the underside near its edge. Smaller, 40 mm thick panels glued to one another back-to-back in pairs form a continuous supporting edge beam frame, which is glued to the underside of the large panel to form a combined structural unit. The depth of the frame in the abutment area is 320 mm, which is derived directly from the level of the top of the existing bearing and the carriageway surface on the approach to the bridge. The transverse beams of the new deck had cut outs made in them to accommodate the existing pipes. The top surface of the longitudinal beams were cut to a radius of 218 m to create an effective structural depth at mid-span of 354 mm, where the greatest bending moment occurs. The falls thus created ensured water would drain off the bridge deck surface. A pocket was created along the whole length of the underside of the longitudinal frame beams by milling out the surface of one of the smaller panels to accept a 10 cm wide carbon strip, which was glued into place as additional reinforcement.
The top layer of concrete of the deck panel forms the running surface and is designed to be the wearing course. There is no corrosion risk. The concrete resists freeze-thaw action and the carbon reinforcement cannot corrode. The prestressing ensures the deck is crack free and therefore relatively watertight.
Life cycle analysis and economic efficiency
The life cycle analysis of this bridge sets new standards. The CPC panels, being prestressed, require only small quantities of carbon. The weight of carbon is well under the limits for organic contamination of the concrete. This allows old CPC components to be directly processed into recycled concrete or sent to landfill, without having to separate the reinforcement from the concrete.
At 190 kg/m² of usable deck surface, the construction is four times lighter than a comparable RC deck: the equivalent of a light steel bridge. In terms of life cycle analysis, the CPC bridge is five times better than its conventional concrete rival.
An alternative solution for the deck replacement in which two additional steel beams were inserted would have cost an estimated CHF 22,000; one with additional in situ concrete beams CHF 27,000. Another alternative was a replacement new bridge in concrete to a similar design as the existing. The estimated cost for this was CHF 77,000. The constructed design with CPC panels was completed for CHF 39,500 and in terms of quality is as good as a new build.
Use in building structures as lost formwork
An interesting application in building structures would be the use of a composite slab as a floor slab. In this, the CPC panels would act as lost formwork and would contain the bottom layer of reinforcement. These composite floors are considerably stiffer than reinforced concrete floors. Instead of 100 kg of steel reinforcement, we would need only 2.5 kg carbon to keep deflection below the specified limiting value. A master’s degree study of fatigue behaviour, bending and shear stresses with various surface roughnesses between the CPC formwork and the applied in situ concrete showed that this type of composite deck complied with all the applicable standards and could be designed in accordance with Eurocode 2. CPC panels placed on both sides of in situ concrete wall elements replace both reinforcement and formwork. However, the costs for these applications are still too high but, if enough of them were constructed as part of a project, the solution would become increasingly economic.
