The concept is actually quite old. Engineers and manufacturers of forming systems have decreased the weight of floor slabs by creating voids through a variety of techniques for a very long time. But newer systems now make it possible to increase the efficiency of cast-in-place (CIP) concrete construction and reduce overall cost.

Traditional voided-slab forming systems that have been used include waffle slabs, single- and double-direction pan forming systems, and hollow-core precast slabs. The guiding principle, according to Dan Windorski, a project manager for GRAEF Engineers, Madison, Wis., is that building owners and designers can get the advantages of a thin two-way-spanning flat-plate structure while the structural engineer can lighten the floor by more efficiently using the concrete.

Voided slab advantages

Voided slabs can save money on a project but there are other important reasons to consider using them. The key advantages of voided slabs start with spans 25 feet and longer:

  • Voided slabs eliminate concrete where it isn’t needed and the subsequent weight reduction makes longer spans possible—up to 55 feet between columns without beams. As a result, some long-span structural steel floors can be converted to voided-slab floors.
  • The reduced weight of the floor slab using voided slabs allows a reduction in concrete and steel in floors, columns, and footings, saving money and reducing the total building weight, allowing lighter foundations.
  • Some voided-slab systems can reduce construction time, especially precast systems or those placed on flat-plate forming systems.
  • Flat-plate construction eliminates beams and drops, resulting in reduced floor-to-floor heights.
  • Projects specifying reduced floor vibration can benefit from voided-slab systems over metal deck floors.
  • Voided slabs are beneficial in seismic design, since the reduced dead weight of floors results in lower seismic forces applied to structures.
  • Voided slabs can take advantage of post-tensioned reinforcement benefits to provide a thin slab with greater span.
  • Voided slabs increase sustainability by saving materials and all materials are recyclable.

Plastic void systems

Two European companies now manufacture and sell systems in the U.S. for constructing voided slabs: Kirkland, Wash.-based GRAEF’s BubbleDeck, and Cobiax, based in Switzerland, which is represented by Barker Steel (a Harris Rebar Co.), Milford, Mass. Each company uses recycled plastic molded into a variety of shapes that trap air inside and each product shares the advantages of voided slab construction. According to the Concrete Reinforcing Steel Institute’s (CRSI) Atlantic regional manager, Mike Mota, both suppliers started many years ago from the same European company.

When you first see a deck formed with one of these systems, the immediate impression is that the voids are there to use less concrete and save money. Although this is one of the results, it is not the primary reason to use plastic molded void units.

By using these voided systems, Mota says it’s possible to lighten the weight of slabs as much as 35%. “A 21-inch-thick slab can be reduced by the equivalent of 4 to 5 inches in thickness of concrete or 30 to 40 pounds per square foot of dead load,” he says. He adds that a typical slab with 40-foot spans is 16 inches thick based on ACI 318 Table 9.5a, but a voided slab with these spans can reduce the equivalent solid thickness to about 12 inches. This reduced weight of building floors also permits engineers to reduce columns, walls, and foundations as much as 40%, although concrete can’t be removed from all locations in a floor slab; voids are omitted near columns to maintain slab punching-shear capacity.

BubbleDeck. This system is most often supplied as a filigree system. Dan Windorski, project manager for GRAEF, explains that the process starts by assembling bubble cages of steel and plastic and then precasting the bubble cages in panels that are typically 8x30 feet. The panels, or filigree, include most of the required reinforcing. These panels are trucked to the jobsite and positioned by crane, saving time by eliminating the deck forming operation. Afterward, workers place concrete on top of the precast filigree to complete the floor, with the fresh concrete covering the rest of the plastic voids and wire trusses that protrude from the precast section to ensure bond between the precast and cast-in-place concrete.

Cobiax. Using this system, plastic voids in a variety of shapes and sizes are installed on the jobsite. First, deck forms are set up and the bottom layers of reinforcing steel are installed. To facilitate the installation of the plastic voids, the system’s bubbles come bundled to a jobsite in 1/4-inch-diameter steel wire cages 8 feet long with six or seven voids per bundle (depending on void sizes). Workers can easily cut the cages down in size to fit the required spaces then use tie wire to secure them to the first layer of reinforcement (which typically is installed in both directions) before proceeding to place the top layers of rebar. Concrete is placed in two lifts: the first covers the bottom layer of reinforcement and a portion of the plastic voids. This concrete holds the voids in place so they can’t move or float upward when the second lift is placed. When the first concrete lift is stiff but still fresh, the second lift is placed to the finished elevation.

A third method of forming voids is used for projects that only require one-way voided systems. To form the voids, contractors position cylindrical concrete forming tubes made from paper fiber, the same material used to form columns and footings. The tubes are sealed at each end and secured to the deck forms with special hold-down hardware to prevent them from floating to the surface during concrete placement.

A primary difference between pan or waffle forming systems and the plastic void systems is that recycled plastic voids are lightweight and sacrificial so they remain in place, permanently encased in concrete while pan and waffle forms must be removed and cleaned for reuse.

Increasing demand

Two-way flat-plate slabs with plastic voids have been successfully used in Europe for many years. Their recent introduction to the U.S. is causing significant interest because of their many benefits, including reduced construction costs. Owners, engineers, and contractors are leveraging these advantages to seize the opportunity to convert projects to voided slabs, even converting some projects originally specified as having floors supported by structural steel.

The new Miami Art Museum features elevated rooms and a large outside space protected by the roof.

Cobiax voids are caged in long wire baskets. On average, one semitruck load of voids replaces six ready-mix truckloads of concrete.

When completed in 2013, the new $220 million Miami Art Museum (MAM) will provide a total of 200,000 square feet of space—120,000 square feet inside and 80,000 square feet outside. It’s an unusual structure featuring hanging gardens, plazas, shaded verandas, and an arrangement of suspended rooms covered by an overall roof structure. One of the architect’s goals for the open-air structure was to provide panoramic views by reducing the number of columns. Mike Russillo, senior manager special products for Barker Steel, says the Cobiax product offered the solution the design team sought. The plastic voids are being used in approximately 105,000 square feet of the building, eliminating the need for 955 cubic yards of concrete and reducing the dead load by 3.9 million pounds. “One of the difficulties, in terms of reducing slab weight, was the owner’s specification requiring 2 1/2 inches of concrete cover to protect reinforcing steel from salt spray coming from the Miami harbor. The required increase in the depth of the slab was offset by the designers through the use of Cobiax,” adds Russillo.