Trays of hollow plastic spheres have supplanted tons of concrete in the floor decks of three recent projects in California and Wisconsin. These polyethylene bubbles are integral to BubbleDeck, which is a biaxial, structural-slab floor system that’s relatively new to the U.S. The technology can reduce the weight of a solid concrete slab by as much as 35 percent.
Created in Denmark, and used throughout Europe since the 1990s and Canada since the mid-2000s, BubbleDeck technology has played an integral role in the design and construction of three U.S. projects in the past two years. The system employs rows of lightweight, high-density polyethylene spheres, made mostly from recycled, post-commercial waste, to replace up to one-third of the cast-in-place concrete. The diameter of the 1/4-inch-thick spheres, or bubbles, can range in size but are typically 80 percent of the deck thickness. For instance, a 9-inch-thick deck will use 7-inch-diameter bubbles while an 11-inch-thick deck will use 9-inch-diameter bubbles.
Occupying areas of low stress in the structural concrete slab, the bubbles “are simply a plastic void,” says Dan Windorski, a structural engineer in the Madison, Wis., office of Graef USA, who consulted on the three domestic BubbleDeck projects to date. Reducing the amount of cast concrete renders the decks lighter and thinner than traditional slabs, but without compromising the floor’s strength. Because the floor slab uses less concrete and thus has less dead weight to support, architects can design with longer clear spans.
BubbleDeck’s panels are prefabricated off-site in whatever size the architect specifies. A typical panel size is 30 feet by 9 feet, says Jerry Clarke-Ames, president of BubbleDeck North America in Kirkland, Wash.—the stateside arm of BubbleDeck International. A prefabricator who is local to the project typically assembles the decks with an upper and lower layer of structural steel reinforcing mesh, with the bubbles sandwiched in between. The bottom mesh layer is embedded in a precast, 2-1/2-inch thick concrete slab.
Once transported to the construction site, the decks are crane-lifted onto temporary shoring. Workers then place additional rebar on top of the reinforcing mesh as needed and as specified by the project’s engineers. Concrete is then poured over the entire assembly to create a monolithic, two-way slab. Because the slabs are partially prefabricated, the technology can speed the pace of erecting a floor deck by 20 percent or more, according to BubbleDeck.
The system is well suited for large, institutional buildings or residential towers for several reasons. The environmental benefits of a voided-slab system were a major selling point for officials at the Teaching and Learning Building at Harvey Mudd College in Claremont, Calif., says Dan Stafford, vice president of operations for Matt Construction Corp., the project’s general contractor.
Not only did a 30 percent reduction in concrete give the project an additional LEED
As a result, the Teaching and Learning Building’s designer, Boora Architects in Portland, Ore., could incorporate expansive, open interior spaces, including a 300-seat, column-free lecture hall, says Leslie Cliffe, an associate principal at Boora who served as the project manager at Harvey Mudd. “It wasn’t your normal building,” she says. “When we looked to do a post-tension slab with those long, open runs, it didn’t work structurally.”
Cliffe notes that the surface of BubbleDeck’s precast panels was left exposed without finishing. Given its installation at an engineering, science, and mathematics college, she says, “exposing the technology seemed appropriate.”
The absence of beams and reduced slab thicknesses also freed up enough floor-to-ceiling height to turn the three-story classroom and office structure into a four-story building, Cliffe says.
Ceiling height is also an issue at the Watertown Regional Medical Center, in Watertown, Wis., which is currently under construction. The center is the third U.S. project to use BubbleDeck technology. Construction manager Tony Meyers, vice president of Maas Brothers Construction, the project’s general contractor, says that the design flexibility offered to them by the BubbleDeck system was the primary reason the design team and client chose to use it in the two-story addition.
The project’s architects, Meyers says, wanted large, open spaces in the addition, which will house an emergency room on the ground level and a women’s birthing unit on the second floor. “Any time you can open up the bay spacing,” he says, “it allows the architects to be more flexible instead of being tied to columns.”
Because the addition’s floor heights had to align with those in the original medical center building, a traditional concrete slab would have left too little clearance in the 13-foot-tall space for the host of mechanical, electrical, and plumbing systems that the hospital requires. Because the BubbleDeck-filled slabs were 7 inches more shallow, Meyers says, plenty of space was available for the equipment. “Seven inches doesn’t sound like much,” he says, “but in a hospital, it is.”
Though the reduced formwork and the use of precast planks saved his crews three weeks compared to a traditional job, Meyers said a couple factors diminished any potential cost savings. “They say you’re saving the concrete, but you’re also trucking the [prefabricated panels].” In the case of the Watertown medical center, Maas Brothers had to transport the materials from a precaster located 75 minutes away from the site. Plus, because the general contractor fast-tracked the project to pour the concrete decks before November, when Wisconsin’s frigid winter would have set in, designers were unable to specify penetration locations beforehand where the precasters could drill holes to accommodate infrastructure. As a result, Meyers and his crews had to X-ray the slabs and drill the penetrations in situ.
In contrast, when constructing the $34 million LaBahn Arena at the University of Wisconsin-Madison—the first domestic project to incorporate BubbleDeck technology—Dave Beck-Engel, president of J.H. Findorff & Son, the primary contractor, estimates that by using the hollow spheres instead of traditional slabs, they saved the project $25,000 to $30,000.
Over time, Graef USA’s Windorski says, as more designers and construction crews become familiar with BubbleDeck, projects may realize more savings both in the construction schedule and in budgets. “There’s a little bit of a learning curve, so it takes some training,” he says. Now that three U.S. buildings have used the technology, he continues, “we have a good base to work with.”