“The twists perform the function of both supporting shell and weather shell,” says Manfred Curbach, director of the Institute for Concrete Structures at TU Dresden. The shell structures, just 1.5 inches thick, are designed to use as little concrete as possible. They are made with double-layered carbon reinforcements, with thin concrete webs on top of them.
“Extruded polystyrene (XPS) foam blocks are integrated between these webs, which run both lengthwise and crosswise. The pre-formed XPS blocks provide thermal insulation and reduce weight,” says Curbach. “A further layer of 4-centimeter carbon-reinforced concrete is placed over this layer and seals the entire surface. This structure forms the whole supporting shell of the twist element.”
After that comes a full-surface polyurethane foam insulation, which is also sealed. The final layer is a 1.2 to 1.5-inch shell of white concrete, installed without joints. The supporting shell and the weather shell are connected by glass fiber reinforcement bars.
Current plans are to have the box on site and the two twist shells completed by the end of February 2021. Construction of the steel and glass façade and installation of building services will follow, with an anticipated completion date of June 2021.
The Project C3 team hopes that publicity about the CUBE will help draw attention to the many advantages of carbon fiber-reinforced concrete as a building material. “Carbon-reinforced concrete is sustainable, environmentally friendly, saves material and weighs less,” Curbach says. “This offers a wider variety for architectural designs. Carbon is four times lighter and up to six times more durable than steel. It can be recycled, and the material cycle is a closed one.” In a closed material cycle, post-consumer or post-production waste can immediately be recycled back into the production cycle.
The hope is that carbon fiber-reinforced concrete will someday replace steel-reinforced concrete in structures like bridges and façades. It can also be used as a supplemental material for strengthening intact steel-reinforced concrete structures, including silos, bridges and buildings.
The initial cost of carbon fiber-reinforced concrete is a concern; 1 kilogram of carbon costs 16 euros, while 1 kilogram of steel costs 1 euro. But this equation does not take into account the many other advantages that carbon has over steel.
“Carbon is four times lighter and six times more durable than steel,” says Curbach. “When renovating silos or building ceilings, for example, the lightness of carbon means that reinforcements can be laid much faster. Because of its corrosion resistance, we need much less material and save costs here, too.” In addition, Curbach says that the predicted durability of carbon fiber-reinforced concrete is 200 years, while steel-reinforced concrete constructions last 40 to 80 years. Costs can also be reduced with applications suitable for mass production.
The Project C3 team is currently using carbon fibers derived from PAN, but Curbach says that some researchers are exploring ways to produce them from lignin, from carbon dioxide in the air and from algae oil. When lignin carbon fibers do become available, they should be about 50% cheaper than PAN carbon fibers, making carbon fiber-reinforced concrete an even more attractive option.
The light weight of carbon fiber-reinforced concrete will make it easy for contractors to handle. It is easier to transport, and fewer people are needed to perform the work, so builders will be able to reduce installation time by 50% compared to steel-reinforced concrete. Curbach says, however, that production does have to be more precise when the application calls for thin layers and components.
After the introduction of the CUBE next summer, the C3 Project network will drive a broad market introduction of different carbon fiber-reinforced concrete applications. “By 2025, a noticeable market impact and the so-called irreversible process should be initiated, so that market penetration can be achieved by 2030,” Curbach says.
Maintaining the Pressure in Minnesota
A new technique for in-place application of a carbon fiber pipe reinforcement has restored the structural integrity of a stormwater tunnel system under Interstate 35W and Interstate 94 in Minneapolis.
The tunnels, which run through the sandstone soil at depths of 50 to 130 feet, often flood during heavy storms. The volume of water is so large that it pushes up through the tunnels’ access shafts, exerting enough force to blow off the shaft lids. These cycles of pressurization when the tunnel floods and depressurization when the waters recede have caused leaks in the liner of the tunnel’s pre-cast concrete pipe.
Concerned about the impact on the tunnels’ structure, the Minnesota Department of Transportation launched a project to repair them. The general contractor was PCi Roads, and the engineering firm was Brierley. For most tunnel sections, workers would poke holes in the tunnel and inject a chemical grout that would seek out water and expand to create a moisture barrier on the outside of the pipe. That would stop the leaks.
In one 20-foot-long section of the tunnel, however, the pipe lining had eroded, and engineers feared it would not be able to withstand the internal pressures of the water and the external pressure of soil. QuakeWrap, which specializes in FRP products for infrastructure repair and renewal, and its construction arm, FRP Construction, provided the solution. “The objective was to bring the strength of that short tunnel segment up to the strength of the rest of the tunnel,” says Mo Ehsani, president of QuakeWrap.
One of QuakeWrap’s products, StifPipe®, is made from lightweight 3D core fabric with layers of carbon or glass fiber reinforcements. When wound on a mandrel, the material can be used to create freestanding pipe liners that can withstand the kind of internal and external pressures found in the Minnesota tunnels. But StifPipe would not work for this application; it simply wasn’t possible to get 12-foot-diameter liner pipes through 3 to 5-foot-wide access holes.
Lining the interior of the pipe with CFRP fabric in situ wasn’t an option. While three to four layers of CFRP could probably handle the internal pressure, they would not provide enough reinforcement for the external loads of the sandstone soil and the highway traffic above. Ehsani estimates it would take 20 to 25 layers of CFRP fabric to get the desired strength, which would be both time-consuming and expensive.
So the FRP Construction team used a recently-developed adaptation of the StifPipe technology, where the host pipe itself serves as the mold for creating the pipe insert. Because of StifPipe’s lightweight core, the team was able to get the desired strength with only eight layers of material.
This approach also provided a new liner that exactly fit the existing tunnel pipe. “Sometimes pipes may not be truly cylindrical, because they have become more oval over the years. With this approach, you don’t need to worry about taking measurements ahead of time, because you just apply it to the surface of the pipe,” says Ehsani.
After PCi Roads dried out the tunnel areas, FRP Construction brought in the required rolls of composite fabric and buckets of resin through the access shaft, transporting 3,250 feet to the repair area. Then they sealed off the area to get the desired humidity and temperature. After applying a primer, crews installed the StifPipe system, which included both glass fiber and carbon fiber, as well as the 3D core, followed by a chemical-resistant, paint-like top coat. When completed, the structural liner measured 1.47 inches thick.
The entire 20-foot tunnel length was completed in three weeks and should not require additional repairs for a very long time. “Unlike steel or concrete pipes that corrode, these materials are pretty much inert so they will never corrode,” Ehsani says. “There’s really every reason to believe that these should last 70 to 80 years.”
Ehsani believes that the in-place StifPipe application could be a solution that DOT and municipal engineers will welcome. “In terms of cost-effectiveness, this is a much more competitive solution compared to some other available techniques,” he says. “Nothing is a cure-all, but this is definitely another tool in the tool box for engineers that are designing these special challenging projects.”