Carbon fiber reinforcing strengthens the case for precast
Precast concrete wall technology has advanced gradually in four decades since arriving in the United States from overseas. Most of these advances have centered primarily on the concrete and the insulation board components.
Improvements in reinforcing materials for shear transfer in insulated wall panels and secondary reinforcement in the face of architectural wall panels, however, have been less noteworthy.
Nonetheless, based primarily upon the strength of concrete, precast has enjoyed a solid reputation as a durable, versatile, and reliable building system.
Recent advances in the production of carbon fiber composites have provided precast manufacturers with a stronger, non-corrosive alternative to existing reinforcement materials. The benefits derived from the use of carbon fiber grid reinforcing—such as improved strength, reduced weight, enhanced insulation, and reduced corrosion—have led to the rapid acceptance of carbon fiber reinforced precast walls on projects including schools, movie theaters, commercial offices, high-rise residential, warehouses, airport terminals, and more.
From bicycles to buildings
While many people view carbon fiber as an expensive base material for products such as aircraft, tennis racquets, bicycles, and motorcycle components, manufacturers have developed lower cost, industrial-grade carbon fiber for broader use in demanding construction applications. As a result, the precast concrete industry has been able to benefit from the same outstanding strengthto- weight ratios and durability characteristics that have made carbon fiber an ideal material to take a jet fighter to Mach 2 or a drive golf ball 300 yards.
As with steel reinforcing, a carbon fiber grid can be configured economically to meet the needs of different structural applications. Aspects of carbon fiber grids that can be varied to achieve different structural properties include:
- the type and grade of carbon fiber filament;
- the number of carbon filaments in each strand; and
- the longitudinal and transverse spacing of carbon strands.
The high-performance structural properties and non-corrosive chemistry of carbon fiber grids make it a perfect reinforcement material in precast concrete. Carbon fiber grid can support as much as 10,000 pounds per linear foot. It has tensile strength as great as 550 kips per square inch (ksi)—which is five times stronger than steel (60-80 ksi), see Table 1. It absorbs strain without yielding, displaying linear elastic behavior.
And its tight grid design provides excellent bonding to concrete. Virtually nonmagnetic, non-corrosive, and with low thermal non-conductivity values, carbon fiber mesh also withstands elevated temperatures and has been tested successfully for use in fire-rated designs.
Making a better sandwich
In 2004, AltusGroup, Inc.—a national partnership of several leading precast manufacturers—introduced the first fully composite and thermally efficient sandwich wall panel. Using carbon fiber grids as a shear reinforcing component, CarbonCast insulated wall panels have redefined the limits of performance.
Non-corrosive and virtually nonconductive, the carbon fiber epoxy grid is cut on the diagonal to act like a truss, and then cast in the panel to create composite action between the wythes reinforced with prestressing, or rebar, and wire mesh.
The use of carbon fiber grids allows for significantly improved thermal performance because the grids help prevent the inter-wythe transfer of heat or cold. As a result, the insulation between the wythes can deliver at or near 100 percent of its rated performance without the hot spots or cold spots typically found with thermally conductive metal trusses or connectors. Rvalues can reach 37; however, the typical range for carbon fiber panels is from 11 to 16, which is about 25 percent better than conventional precast sandwich wall panels. Building owners can benefit from corresponding energy savings and a smaller initial investment in HVAC equipment. In applications such as food storage or warehousing, the thermally efficient panels also help ensure a constant temperature profile and maintain the integrity of stored products.
A second important advantage of the carbon grid is that it provides continuous shear transfer. This eliminates the secondary moments that can cause cracking in panels with solid zones that concentrate shear transfer at a few places along the length of the panel.
The new face of architectural panels
Though precast concrete architectural panels (or spandrel panels) are a common choice of architects, engineers often need to accommodate load concerns—especially since most designs hang the panels from the building’s steel or concrete superstructure.
Secondary steel reinforcing in precast architectural wall panels requires significant concrete cover to protect it from corrosion that could be caused by water infiltration. Traditional precast panels are usually a minimum of 6 inches thick.
All that concrete adds weight without adding value.
Replacing conventional, secondary steel mesh reinforcement with a noncorrosive, high-strength, resin-bonded carbon fiber grid allows manufacturers to make significantly thinner or lighter precast sections using an innovative back ribbed truss design. (Note that conventional steel rebar or prestressed or post-tensioned strand still is used for primary reinforcing where needed.) This can reduce the weight of architectural panels by more than 60 percent while offering substantially improved corrosion resistance and durability. In addition, the R-value improves because the EPS foam is used as a stay-in-place form for the truss ribs.
The reduced concrete weight of carbon fiber reinforced, precast architectural panels also translate to lower transportation and erection costs. At the same time, improved insulating properties lead to more energy-efficient buildings with lower operating costs. In fact, if pursuing Leadership in Energy and Environmental Design (LEED) certification, carbon fiber reinforced precast panels can contribute 15 to 20 Materials and Resources credits out of the of the 26 necessary for certification.
The 332,000 square foot Cardinal Health office-warehouse features carbon-fiber-reinforced wall panels as high as 51 feet. The panels have a 4-inch outer wythe and a 4-inch inner wythe enclosing 4 inches of foam insulation (4- 4-4) to deliver a healthy R-16. (Other panels in the building were 3-4-3.) The high R value—provided at the same cost as conventional precast with R-12 or lower—significantly reduces Cardinal Health’s costs to heat the warehouse.
The panels feature 1/2-inch horizontal reveals throughout the wall, while the office exterior also incorporates several colors and articulations for stunning visual appeal. The load-bearing panels also support the structure’s roof.
A new way to reinforce
The development of precast wall products using carbon fiber reinforcing heralds a new era in architects’ and engineers’ quest for lower weight, highperformance panels.The high tensile strength and low weight of carbon fiber make it an ideal reinforcing material in precast concrete.With more than 5 million square feet of CarbonCast walls erected or in development just over a year after the technology’s introduction, it is evident that architects and engineers quickly are embracing the numerous benefits that this technology can deliver.
John Carson, director of commercial development at TechFab LLC, has more than 20 years of marketing and management experience in the plastics and materials industry. He holds a B.S. in textile technology and management from North Carolina State University.