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While carbon fibre is universally acclaimed for its strength as a primary material – witness its use in aerospace, automotive and wind energy applications – carbon fibre also has a remarkable capacity to make other materials stronger. The same high-strength characteristics that make it desirable for demanding applications render it ideal as a reinforcing material. Nowhere is this property more evident than with concrete.
(Published on January - February 2008 – JEC Magazine #38)
JOHN M. CARSON DIRECTOR COMMERCIAL DEVELOPMENT TECHFAB, LLC
As a replacement for conventional steel reinforcing, carbon fibre grid can dramatically improve many aspects of concrete performance. It had been used primarily in concrete for rehabilitation applications, everything from bridge decking to balcony repairs to industrial floor retopping to roof renovation. It delivered outstanding resistance to corrosion, adding years of durability to the projects.
Several forward-thinking companies have explored the initial use of carbon fibre grid in precast concrete for large concrete elements such as facade panels, walls and parking garage components that are manufactured in a factory and then shipped to a job site for erection.
In 2005, five precasters along with C-GRID™ manufacturer TechFab LLC formed AltusGroup, a partnership dedicated to expediting the development and commercialization of precast concrete using C-GRID carbon fibre grid reinforcing. Because precasters’ market areas are limited by the cost effectiveness of transporting the heavy concrete pieces, their willingness to share the technology with other AltusGroup members enabled the development of the building industry’s first-ever national brand of precast concrete in the United States: CarbonCast.
Since introducing the technology nearly four years ago, AltusGroup members have realized significant progress in promoting the acceptance of carbon fibre grid reinforced precast concrete. It has been used on nearly 200 projects ranging from 30-storey condominiums to 75,000- seat sports arenas.
Why precast concrete needs reinforcing
Concrete is strong in compression but weak in tension. Reinforcing steel compensates for this weakness, but it requires extra concrete covering it to develop its strength and to help protect it from the elements that cause corrosion. There are two types of reinforcing: primary and secondary. Primary reinforcing resists tensile stresses induced by external loads, such as gravity, seismic events, and wind loads. It typically takes the form of rebar, prestressing strand, or post-tensioning cables. Secondary reinforcing – usually a mesh or grid, or occasionally loose fibres placed in the mix – controls cracks caused by shrinkage or thermal loading on the surface.
In terms of rebar and mesh, common carbon steel is the least expensive material, though its price is rising. Carbon steel is strong and generally easy to work, but it still rusts, even when galvanized or epoxy coated. When steel corrodes, it doubles in volume, causing cracking or spalling. When wet, it may also cause staining. Naturally, any cracks in the concrete will accelerate the ingress of moisture, causing even more havoc. As a result, a structure’s performance and aesthetics suffer over time.
Nonetheless, for decades steel has been the reinforcing material of choice for precast concrete. Until recently, no other material made major inroads. Advancements in industrial-grade carbon fibre production have made it a costeffective alternative to steel mesh reinforcing for precast concrete structures.
Reinforced precast products manufactured with epoxy-coated carbon fibre grid are extremely competitive from a cost perspective with conventional precast products that employ steel mesh for secondary reinforcement. Carbon fibre effectively eliminates concrete cover requirements, enabling new precast product designs that reduce the materials used in fabrication. After factoring in savings on raw materials, reduced transportation costs, reduced superstructure demands, time saved on-site, improved insulation qualities, speed of erection, and other system benefits, using epoxy-coated carbon fibre grid reinforced precast products will likely result in long-term cost reduction.
Precast concrete can be cast into hundreds of forms and shapes. To realize the benefits of carbon fibre on a significant scale, the precasters in AltusGroup have centred their efforts on large structural and architectural components for buildings and parking structures, namely high-thermalperformance wall systems and pretopped double tees. The use of carbon fibre grid reinforced precast has also been explored for multi-family residential applications, including highrise condominiums.
Architectural cladding panels
Ranging from 152.4 to 304.8 mm thick, architectural cladding panels feature a carbon fibre grid reinforcing in the face, or the side that comprises a building’s exterior. Because carbon fibre will not corrode like steel mesh, precasters can use less concrete to protect the reinforcing – up to 66% less. Less concrete means less weight, leading to savings in picking and lifting costs related to construction cranes and building substructure. And their design can provide R-values up to steady-state R-20.
High-performance insulated wall panels
Vertically installed insulated sandwich wall panels can be used for both nonstructural and load-bearing applications. Manufactured in thicknesses from 152.4 to 304.8 mm, widths to 4.57 m and heights of 15.24 m or more, CFRP composite grid is used for shear transfer between the inner and outer wythes. With its relatively low thermal conductivity, epoxy-coated carbon grid maintains essentially 100% of the insulated wall panel’s core R-value, generates an even insulation profile without hot spots or cold spots on the interior wall surface, and provides a truly structural composite panel for added strength and reliability.
Pretopped double tees for parking structures
In pretopped double tees, carbon fibre grid replaces conventional steel mesh reinforcing in the flange, or deck. Because carbon fibre does not corrode, parking garages do not face long-term flange degradation from cracking and spalling. No corrosion in the flange means precasters require less concrete cover to protect the reinforcing – reducing weight up to 12%. They also eliminate the need for sacrificial barrier coatings on the steel, corrosioninhibiting admixtures, and sealants on the deck surface. Sealant re-application every five years is avoided, too, saving money in long-term maintenance costs.
Multi-unit residential applications
Load-bearing and non-load-bearing CarbonCast ribbed residential wall panels weigh up to 60% less than conventional precast panels and offer value-added insulation performance of up to R-13. They are specially designed to make drywall furring with metal studs quick and easy. They can be formed or finished to simulate stucco, brick, stone and even clapboard siding.
The promise of CFRP technology has been borne out by nearly 200 applications in the past three years. Several projects are highlighted below to illustrate how architects and building professionals have begun to embrace the concept.
The 31-storey Symphony House, a breathtaking $125 million, 163-unit condominium in Philadelphia, features CarbonCast architectural wall panels that deliver a traditional aesthetic sensibility and a remarkable 60% weight reduction of precast members.
The weight reduction provided two benefits to the owner. First, the restrictive building site necessitated a tower crane to lift the 770 exterior panels into place. The CarbonCast panels were easily accommodated by the crane, even at the more distant corners of the building.
In addition, the lightweight panels reduced load on the floor slab where they were mounted and on the rest of the reinforced concrete structure, all the way down to the sizing of the foundation. Further, the thinness of the panels and the reduction of the structure provided more interior floor space and better apartment layouts. To accommodate the architect’s colour selection, precast panels were fabricated with a red-pigmented, sandblasted finish meant to evoke the feeling of brick, but without the expense or limited colour range of brick or other systems.
Constructed from CarbonCast highperformance insulated wall panels, Georgia State University’s 2,000-bed four-building student housing complex is the largest privately funded university housing complex in the nation. The aesthetically pleasing wall panels greatly reduce mildew risk because concrete inhibits water penetration. In addition, using the precast wall panels eliminated the drywall, eradicating any potential food source for mildew and mould.
The panels deliver R-12, attained by 101.6 mm of expanded polystyrene sandwiched between two 63.5 mm thick concrete wythes. The inner and outer wythes were connected using thermal-efficient C-GRID carbon fibre trusses. Compared with the original design, the increased R-value enabled the university to save $700,000 in HVAC costs. And cost savings will only increase over time as the annual utility and maintenance costs will be significantly less.
The new Lucas Oil Stadium, Indianapolis, Ind., will feature an insulated, brick-inlay architectural facade with CarbonCast highperformance insulated wall panels. The project will require over 37160 m2 of limestone-finished architectural precast featuring blasted accents and embedded brick.
With C-GRID shear grid reinforcement connecting the 76 mm wide interior and exterior concrete wythes, the panels will maximize energy efficiency and reduce the possibility of condensation and mould. As the system’s rapid installation does not compromise its clean lines and exacting detail, construction time and cost savings are anticipated. Additionally, the prestressed panels allow longer spans – another costsaving aspect of the system.
The construction industry has quickly been learning what other markets have known for years: carbon fibre is a material that can impart numerous benefits to end-products. The use of carbon fibre reinforcing in precast concrete signals a shift toward its use in new construction after years of success in rehabilitation applications. And considering a new focus on energy efficiency, the insulation benefits of carbon fibre reinforced precast is more attractive than ever.
Each new structure stands as a testament to carbon fibre’s viability as a reinforcing material. With about 1 million m2 of carbon fibre reinforced precast erected or in development just over four years after the technology’s introduction, it is evident that architects and engineers are quickly embracing the numerous benefits that this technology can deliver.