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A new hollow-core slab technology makes use of recycled plastic spheres to offer innovative design solutions for an open floor plan at a California college.
By Deborah R. Huso
When officials at Harvey Mudd College decided to build a new teaching and learning center, they wanted to update its 1950s look. The college leaders envisioned a sustainable structure with naturally lit open spaces that would preserve the predominant concrete architecture of the campus. The challenge was to create large open spaces in classrooms and lecture halls without the interruption of columns and beams. The project’s design-build team found the solution in BubbleDeck, a new hollow-core slab technology that allows for extensive spans of floor and ceiling without typical column supports.
Although it was the first in California and one of the first in the United States to make use of BubbleDeck technology, the product has been used successfully in Europe, Canada and Australia. The college selected the BubbleDeck technology mainly because it helped meet the school’s goals for open floor plans and sustainability with a reduced construction timeline. In addition, the BubbleDeck system offered easier construction within the tight parameters of the job site. (For a description of the technology, see the sidebar “What Is Bubbledeck?”
Building up to a new design
Founded in 1955, Harvey Mudd College (HMC) has maintained a very uniform, mid-century aesthetic and has done little building since its founding. The teaching and learning center is the college’s first major construction project in decades, and, as Josh Brandt says, “They knew they needed to step forward design-wise.”
Brandt, project architect with Boora Architects and designer of the new structure, points out that many of the school’s existing buildings are dark with low ceilings. He says one of his firm’s major goals was to “provide a building that reflected the quality of the work going on inside”– that is, HMC’s status as an engineering school. The objective was to provide as much height and daylighting as possible within the building’s classrooms.
Brandt says Boora Architects anticipated using concrete from the start. “All the other buildings on campus are concrete, so there were aesthetic reasons for using concrete,” he explains. More importantly, concrete offered the benefit of fireproofing without additional finishes. However, Brandt says typical designs that use a flat-slab concrete system would have been really limiting. The building presented a significant design challenge with its large classrooms for up to 50 people and the desire to maintain open space without columns. It was the structural engineering firm, KPFF of Los Angeles, that introduced the design-build team to BubbleDeck technology.
“A First” for precaster and contractor
Oldcastle Precast – San Diego took on production of the BubbleDeck slabs after learning about the technology from BubbleDeck North America. “We’re known for taking on unconventional projects,” explains Todd Ebbert, Oldcastle Precast – San Diego’s general manager.
“BubbleDeck has to find a precaster locally for every project in a new market,” Elan Hertzberg, general manager at Matt Construction, explains, noting that the Oldcastle plant is about three hours away from the HMC building site. For Matt Construction, the general contractor, this is its first BubbleDeck project. “The precast work we’ve done in the past has been vertical systems,” says Hertzberg. “This is the first horizontal structural deck we’ve done.”
Ebbert says the HMC project has been ideal for BubbleDeck, because it calls for an open floor plan with high ceilings. HMC is an engineering school, so the BubbleDeck slabs will be exposed, revealing “raw concrete as ceilings,” Ebbert notes, “and in most cases, plumbing and electrical will be exposed as well.” Seeing the configuration of building utility systems provides a valuable experience for budding designers.
Scheduled for completion in fall 2013, the HMC project will have an interior courtyard and feature 360 BubbleDeck panels, about 100 floor panels and 58 panels on the roof. The building’s four floors, including one subgrade level, will contain 90,000 recycled plastic bubbles, totaling 70,000 sq ft of surface area. Brandt says that unlike most projects involving precast concrete components, this one has not benefited from production repetition. In fact, he says, there is pretty much “zero repetition” in the size and shape of the slabs, which are basically a series of C and V shapes.
How the system comes together
The project team admits this first experience with a new technology comes with a steep learning curve. The diverse panel configurations “can be tricky” and consequently required careful production planning. “The underside of the deck is smooth, and it looks gorgeous,” but to achieve that smooth surface for the building’s ceilings, J boxes for light panels had to be precast into the panels, so there is significant pre-planning involved to make it work. “Everything had to be set according to a fabrication schedule.”
Hertzberg says he’d be willing to take on another BubbleDeck project but offers advice for other contractors. “The best thing to do is to actually view a project to see how simple it is, how beautiful the underside is, and how wide the column spacing is.” He emphasizes the need to be part of the planning process early on, to work with engineers before panel fabrication begins to determine the size of panels and locations of floor boxes. “BubbleDeck is a state-of-the-art technology from an aesthetic standpoint,” Hertzberg adds. “It’s amazing.”
Brandt agrees with Hertzberg in defining the major challenge as coordinating mechanical, electrical and plumbing into the slabs. “You have to make decisions a lot earlier in the process,” he says.
The BubbleDeck slabs arrive on the job site partially assembled and have a 2- to 5-in.-thick precast concrete base embedded with a reinforcing steel cage securing the hollow plastic balls with each plastic sphere precisely spaced and locked in position. The honeycomb shape of the cage adds strength to the slabs. On site, the slabs are connected with steel bats and topped with a second wire mesh (for additional strength) before concrete is poured over the balls to create the smooth finish of the building floors.
In the HMC project, five sphere sizes are used in slabs ranging in thickness from 9 in. to 20 in. All the spheres, or bubbles, are made from recycled plastic. Brandt says 13.5 in. is the typical slab thickness. BubbleDeck’s precast architectural base serves as the finished ceiling for classrooms, offices and lecture halls.
Panel sizes are based on what can fit on a truck, so most are 10 to 12 ft wide and 40 ft long, with four panels transported per truck. Each precast concrete panel with its steel-caged bubbles weighs between 9,000 and 15,000 lbs. Slabs are trucked to the job site and installed using a 161-ft crane. Hertzberg says his construction team can install 50 panels in eight hours, adding rebar between panels. Installation of the reinforcing steel takes about two weeks plus an additional two days to pour and cure concrete around the spheres.
According to Hertzberg, the crews use a 2-in.-wide construction joint every 40 to 50 ft of panel to “allow room for error.” Crews also install floor boxes for electrical outlets in the classrooms and lecture halls, a process that often requires the removal and replacement of some of the spheres.
Three advantages of BubbleDeck systems
1. Open floor plan and finish control: The reason BubbleDeck is attractive to designers is because the slab carries its self-weight (without reliance on load-carrying columns and beams), allowing for more extensive and open floor plans. In the case of the HMC project, a traditional precast concrete (or cast in place) structure with support beams would have increased the building’s height and required closer spacing between columns, thus disrupting the available open interior space envisioned by the client. “Aesthetically, we were excited about the level of control over finishes by having it done in the factory,” said Brandt.
2. 35% less concrete, same strength: The system is designed to take the dead weight out of the center of a slab by filling it with plastic bubbles instead of concrete. One of the major advantages of BubbleDeck is that it uses 35% less concrete than traditional floor systems, yet has the same strength and more flexibility in terms of design.
3. More sustainable construction option: BubbleDeck uses less concrete than traditional concrete floor systems, offers a more sustainable construction option, contributes less CO2 to the atmosphere in the manufacturing process and also meets sustainability goals through the use of recycled plastic spheres. The spheres could be recycled yet again should the building be demolished or renovated in the future. The dead air space in the hollow spheres provides insulating value and can be injected with foam for additional energy efficiency.
What is BubbleDeck?
BubbleDeck is the invention of Jorgen Bruenig, who devised the first biaxial hollow slab (now known as BubbleDeck) in Denmark. Since then, BubbleDeck has been taking off “in a big way” in Europe, according to Jerry Clark-Ames, manager of BubbleDeck North America. The technology moved overseas in 2005 with the first projects going up in Canada. There have also been numerous successful BubbleDeck projects in Australia, Malaysia and Brazil.
“Most projects in North America have been for floor plates,” Clark-Ames says. His company does not manufacture the slabs themselves but delivers materials for making them to an approved precaster. “BubbleDeck doesn’t usually conflict with a precaster’s existing market,” he adds. “Most precasters’ stuff tends to be single-direction plate as opposed to BubbleDeck, which is a two-way plate.”
The production process for BubbleDeck begins with the assembly of cages to hold the plastic spheres that serve as the slab’s hollow core. Clark-Ames says there are two slats of steel mesh per panel. Basically, the mesh is a welded-wire fabric with an offset spring. Precast producers install a lattice girder in the longitudinal plane of the panel formwork and then add the hollow spheres. A top mesh locks everything together. The cages are then set in forms containing 2.5 to 3 in. of fresh concrete. A typical panel is about 8 ft wide by 30 ft long, about 250 sq ft.
The system uses a third less concrete than a traditional slab and does not require special concrete. “You can use standard products like 5,000 psi self-consolidating concrete,” Clark-Ames says. He says BubbleDeck can represent a substantial cost savings for large decks with a 12-in. or greater slab thickness because of the reduction in concrete and construction time. “You can put all the panels together in as little as two days,” he notes. “You’re taking labor from the job site and putting it in the factory.”
Spancrete first to use BubbleDeck in U.S.
BubbleDeck made its U.S. debut in 2011 with the construction of an underground walkway at the University of Wisconsin-Madison’s La Bahn Arena. The open-space walkway had to be able to support a road overhead that would safely carry an 80,000-lb. emergency vehicle.
Clark-Ames explains that the walkway called for large, self-supporting spans able to carry significant dead and live loads. “It was an immense amount of weight over a large area,” he says. The design-build team for the project, led by Wisconsin-based general contractor Findorff, looked to BubbleDeck to provide the needed span structural strength without traditional columns and beams.
Spancrete, the project’s precaster, made two dozen 21-in.-thick panels for the walkway’s 12,000-sq-ft ceiling, each holding the 16-in.-diameter recycled plastic spheres. “There was quite a learning curve associated with this project,” says Clint Krell, director of sales for Spancrete. “But this product is already engineered when we get it. The bulk of our cost is labor.”
For more information, visit www.bubbledeck.com.
Deborah Huso is a freelance writer who covers home design and restoration, sustainable building and design, and home construction.