This simplifies production and requires less labor. “The use of carbon fiber and a casting machine may reduce the cycle time by 60 to 90 minutes per tee,” Baur says. “This lets the whole curing process start an hour earlier. It's like McDonald's putting in two windows to speed up their drive-thru process. Our goal is to cast two tees in the same mold in a 24-hour period. We're getting pretty close.”
In addition, the new manufacturing process is permitting manufacturers to place the carbon fiber at optimal depths in the flange based on its proximity to each stem. “In the cantilever, you want the steel near the top,” says Gleich. “So we bring the carbon fiber within ¾-inch of the top surface, then lower the profile between the stems, where any cracking would be on the underside.”
“The depth of the carbon fiber emulates the moment curve,” says Baur. “We put the grid where the stress is highest. We weren't able to do that with steel. It wouldn't flex that way, and we didn't have a machine to place it.”Changes on the horizon?
Carbon fiber represents the only significant change to manufacturing precast double-tees that delivers reduced weight. However, changes may be around the corner to revise the fire-rating codes that force parking garage designers to mirror transmission requirements for residential and commercial structures. Fire codes typically mandate 3¼-inch flanges for a one-hour rating and 4 5/8-inch flanges for a two-hour rating.
“There's nothing combustible in a parking deck, compared to a building,” Gleich says. “That part of the code is based on an apartment where you don't want the heat from a fire to ignite the materials on the next floor. In fact, if the garage is classified as an open structure, the codes don't require the joints between the tees to be fire-rated, meaning flames could go right through. There are inconsistencies in the code.”
In addition, heat would not collect in a garage. “Most garages are open, so the heat would naturally escape,” Baur says.
Lehigh University is performing modeling studies to assess the fire rating issues, and Precast/Prestressed Concrete Association committees have been addressing this issue. Still, Gleich explains it may take 20 years to make changes the code.Longer lasting
Of course, thinning a double-tee is only acceptable if the design does not compromise performance or long-term durability. Both men are confident in carbon fiber's ability to meet and exceed the life spans of conventional double-tees.
“We're giving owners a more durable, longer-lasting structure,” Gleich says. “Other engineers might not understand everything about carbon fiber, but they understand it makes concrete more durable. If the cost of carbon fiber-reinforced precast equals pre-cast with conventional steel reinforcement, owners get the added durability for free.”
Baur adds, “When bridges were designed in the 1960s and ‘70s, departments of transportation were talking about them having a 25-year life. Today, DOTs are talking about a 100-year life. With carbon fiber and stainless steel connectors, we could make double tees with a 100-year life with very little maintenance.”
The author is a Pennsylvania-based business writer who has covered the construction industry for 15 years.