Jet Fighters and Golf Clubs: Similar to Parking Garages?
For years, architects, engineers and parking consultants have looked to precast concrete as a viable technology for parking structures. After all, precast concrete's consistency, erection speed, long-term durability, aesthetic flexibility and fire resistance are desirable qualities for most projects.
However, precast double-T's have two challenges not uncommon to large concrete components. First, although often lighter than poured-in-place options, they're still heavy, requiring substantial structural support. Second, their steel reinforcing is sometimes prone to corrosion, especially when vehicles carry in chlorine-laden ice, snow and water, or when salt is used to melt ice on the deck. When the steel corrodes, it doubles in volume and can cause cracking or spalling, as well as unsightly staining -- all problems the building or garage owner wants to avoid.
A new approach to reinforcing involves the use of carbon-fiber grid as a replacement for conventional steel-mesh reinforcing in the double tee double-T flange. Carbon fiber is noncorrosive, which virtually
eliminates the problems noted above, as well as the cost of admixtures and barriers used to inhibit corrosion. At the same time, carbon-fiber reinforcing can reduce flange depth -- leading to a 12 percent reduction in double tee weight -- without compromising performance.
Jet fighters, golf clubs, double-T's
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 strength-to-weight ratios and durability characteristics that have made carbon fiber an ideal material to take a jet fighter to Mach 2 or drive golf ball 300 yards.
As a lightweight, noncorrosive "enabling technology," carbon-fiber grid reinforcing allows double-T's to be lighter, more durable and often less costly in the long run than conventional precast products that rely on steel reinforcing.
Exceptionally strong, nonmagnetic C-Grid carbon-fiber grid reinforcing can support up to 6,000 lbs/lf and has tensile strength more than four times that of steel. C-Grid also absorbs strain without yielding and displays linear elastic behavior. Its epoxy coating chemistry and small-aperture grid design provide excellent bonding to concrete and superior crack control.
Carbon fiber's cover story
Steel's tendency to corrode forces double-T manufacturers to add concrete cover to the flange with the sole purpose of protecting the steel. In fact, most double-T's have a minimum flange depth of 4 inches to provide ample cover for the steel mesh. That results in extra weight just to safeguard the reinforcing.
Carbon fiber's highly efficient structure, corrosion-resistance and strength allow precast manufacturers to reduce the amount of concrete cover in each double tee flange by up to 3/4 inch. (Conventional prestressed strands are still used in the stems because they are extremely well-protected by more than 12 inches of concrete as measured from the top of the flange.)
Strong and durable
Protecting conventional concrete decks on parking structures requires constant vigilance. A litany of chemical treatments are typically used during double-T fabrication and after installation to inhibit and delay the corrosion of steel-mesh reinforcing in the flange.
With the noncorrosive carbon-fiber reinforcing, you can forget about sacrificial barrier coatings on the steel, and eliminate the need to add costly corrosion inhibiting admixtures to the concrete or to apply sealants to the precast deck surface. And best of all, you can eliminate the cost and hassle of re-applying these sealants every five years.
Used to enhance durability
A 270,000-square-foot, four-level parking structure in suburban Milwaukee has carbon-fiber reinforced double-T's on its top deck. The owner - a Fortune 500 company - opted for carbon-fiber reinforced double-T's because of their long-term durability and resistance to corrosion, especially important considering the deck is exposed to Milwaukee's diverse weather - everything from ice storms and severe snow to summer heat waves. The 15-foot-wide double-T's have a two-hour fire rating to match the balance of the garage.
New technology, familiar names
The introduction of carbon-fiber reinforcing heralds the first major advance in precast technology in decades -- perhaps since the introduction of precast itself. Several of the nation's leading precast companies -- such as Oldcastle, High Concrete Group, Metromont and Gate Precast -- and its suppliers have collaborated as the AltusGroup to develop, test and market carbon-fiber reinforced precast components under the CarbonCast brand name. Their partnership has enabled the first-ever national rollout of a novel precast brand and establishes a new model for the introduction of technologies to the precast concrete industry.
From an initial cost standpoint, carbon fiber tends to be more expensive than steel mesh. However, when one considers the skyrocketing cost of steel, the cost for corrosion inhibitors, potential reductions in substructure and site costs, and increased life space, carbon-fiber reinforcing lowers life cycle costs dramatically, especially after the first decade of operation.
As an important innovation, the use of carbon-fiber reinforcing promises unparalleled corrosion resistance by eliminating the use of steel reinforcing double-T flanges. The resulting weight reduction and life cycle improvements usually provide benefits over conventional precast concrete, poured-in-place and other options. It is clear that the use of carbon-fiber reinforcing will grow in popularity as its effectiveness is validated by a variety of installations.
Ken Baur, P.E., who works in the Architectural Panels and Bridge Decks unit of High Concrete Structures, can be contacted at firstname.lastname@example.org. Gregg Blasak, P.E., is C-Grid Technical Marketing Manager for TechFab and can be contacted at email@example.com. Both are members of the AltusGroup technical committee