Millions of square feet of suspended concrete slabs are placed every year in the U.S., much of it on a structural steel frame and metal deck. When the designer specifies these slabs, there is sometimes a misconception that it can meet the same requirements as a slab on ground.
It can’t.
“The trouble is that architects and specifiers are using the slab-on-ground specification of FF50/FL35 on elevated decks,” says American Society of Concrete Contractors (ASCC) Technical Director Bruce Suprenant. “We need to disconnect elevated slab specs from slab-on-ground specs.”
This is especially true for suspended slabs that are to be polished. “Designers often design the slab for deflection rather than crack width,” says Chris Tull of CRT Concrete Consulting in Fishers, Ind. No one knows or cares about the cracks if floors are carpeted, but they will if the decision is made to polish.
Given these variables, always carefully review the specifications to avoid getting stuck with impossible requirements.
A Crucial Design Decisions
An important decision the owner and designer must make is whether the suspended slabs are to be an equal thickness and not level across the surface or level and of varying thickness. All construction materials deflect – bend – as dead load is applied. For suspended slabs supported by structural steel and steel decking, that deflection can be significant so that attempting to achieve a level floor can result in an overloaded structure if it’s not designed for the additional concrete
When a level floor is required, the designer must stiffen the structure with heavier steel joists spaced closer together. Scott Tarr, owner of North S.Tarr Concrete Consulting in Dover, N.H., describes the suspended slabs that are currently being specified for an online retailer’s distribution centers: “The floors need to be level and flat for the autonomous robots they use, so they’re making building frames much stiffer and using [Somero Enterprises] CopperHead laser screeds.”
Unlike most floors, the slabs must also meet the floor flatness (FF)/floor levelness (FL) requirements for the 2-foot floor perimeter and at the joints. “This frequently results in some grinding, almost always at the joints, especially if there’s some curling as the slab dries,” says Tarr.
And with the additional concrete needed to achieve levelness, the dead load further complicates placement.
“The floors are specified for surface elevation rather than thickness, so during placement someone needs to monitor the underside for deflection and the structural engineer needs to sign off on the added weight,” Tarr says. “They’re trying to completely hold cracks tight so the robots aren’t affected, and therefore are using 0.6% reinforcement plus 7.5 pounds of macrofibers and the floors aren’t jointed."
“The worst cracking situation is wide negative moment cracks directly above the steel trusses," says Chris Tull. "I try to get the structural engineer to use 18-gage metal decking and place the trusses at 6 feet apart. If they’re only designing for deflection, they’ll specify the minimum steel at around 0.15%; but for crack control, we need more like 0.6% reinforcing steel.”
Constructing Suspended Slabs
How the concrete on suspended slabs is struck off level, or of uniform thickness, varies according to the underlying structure.
“Our techniques vary widely depending on structure type,” says Chris Forster of Largo Concrete in Tustin, Calif. “We use screed bars for thickness, [Allen Engineering Corp.] Magic Screeds, laser screeds, screed pins for on-grade and some cambered slabs, and occasionally roller screeds.”
For slabs on metal deck, the goal often dictates the means and methods.
“We pin all our decks for thickness and wet screed them; we don’t use a rail system,” says David Buzzelli of Texas A&M Concrete in Houston. “We never use a CopperHead or laser because of the potential for overloading the decks; everything we do is unshored.”
While wet screeding may work for uniform-thickness slabs, metal deck slabs intended to have a level surface are often struck off with other techniques. For an Ikea store built in 2006, consulting firm Structural Services Inc., Dallas, required rigid screed guides and a vibrating handheld screed (see “Exceptional Floors”). This project used the Screed Rail invented by Allen Face and sold through LaserStrike, Wilmington, N.C. This device works with vibrating wet screeds and hand screeds.
Another successful screed rail system is from Mako Enterprises in Johnston, Iowa, an adjustable plastic device that supports a 1.5-inch steel pipe as a screed rail. To learn more about the Mako Support System, read “Screeding Specialist.”
A newer system that’s more akin to wet screeding than a fixed rail but a definite step up is Stego Industries’ Beast Screed, which earned a 2016 Industry Choice Most Innovative Product award, and the Beast Deck Foot, which earned a similar award the following year. The little yellow pad, which some call a cap, is positioned on the deck to the surface elevation of the concrete to provide an elevation guide.
“With standard wet screeding there’s no set target when you place the concrete,” says Stego's Steve Lutes. “With Beast Screed the contractor has this target. We instruct them to place concrete around the Beast Screeds and then strike off a grade strip between two of the pads to screed to. Placing the concrete around the Beast Screed as you’re placing the rest of the concrete keeps it in place and maintains the elevation."
Stego recommends placing the caps 12 to 16 feet apart in a grid pattern. “The farther apart they are the more concrete they have to move,” says Lutes, “so closer is easier on the finishers. Often they will also place a Beast Screed next to columns or other structural elements to establish the floor elevation at those locations.”
The value of this approach is crew productivity. "The thing is," says Lutes, "when I was running a concrete crew I always knew that crew efficiency was my main focus. As a contractor, I didn’t care what I paid for products, managing the labor was the way to make money on a job. I’ve brought the same thinking to product development at Stego, we’re looking to save crews time, which translates to value for the contractor."
Strike-off can be done in many ways, from a laser screed to a two-by-four. Steve Lloyd, owner of Lloyd Concrete Services in Rustburg, Virginia, uses Somero’s S-485 laser screed. “Sometimes I get some pushback from the structural engineer about the weight, but it’s only about 1,100 pounds on four tires,” he says.
Many contractors use a vibrating handheld screed (see sidebar for a list of machines).
“The safe bet is to pin the depth for thickness and either hand screed or use a combination of hand screed and vibratory walk-behind screed,” says Bryan Birdwell, senior concrete floor and paving consultant with Structural Services. “Using lasers to set elevation can be done with planning and understanding the potential impact of deflection and additional concrete weight on the structure.”
Polishing Concrete Decks
There has been an increasing desire to polish suspended slabs. “Polishing is a big issue and needs to be discussed up front,” says Birdwell. “Many architects and owners expect an aesthetic look, but it’s not discussed up front with the construction team. With constructability issues, this can be difficult to nearly impossible unless a topping or two-course floor is considered – unless an old warehouse industrial look is expected.”
“We try to use Somero’s S-840 laser screed when I know it’s to be polished and sometimes we recommend it be shored,” says Tony Lampasona of Turner Brothers in Raynham, Mass. “We also use the Stego grade-control product [Beast Screed]."
The desired level of polishing can also be an issue.
“American Concrete Institute (ACI) Committee 310, Decorative Concrete, says that to get a cream finish you need an FF70, but that’s not going to happen on an elevated floor,” says ASCC’s Suprenant. “We should tell owners and architects not to bother specifying a cream finish on an elevated slab and just go with a salt-and-pepper finish.”
Tolerances
If the suspended slab is expected to conform to ACI 117-10, Specification for Tolerances for Concrete Construction and Materials, there are only a few relevant tolerances. Section 4.4.1 indicates that deviation from the specified elevation is ±3/4 inch, but that’s only applicable on formed suspended slabs before the removal of shores. For slabs supported by structural steel or precast concrete, there is no tolerance for deviation from elevation.
That leaves two tolerances related to suspended slabs: thickness and flatness. The thickness requirement was changed in the standard’s newer version to eliminate the plus side; the only thickness requirement is -1/4 inch, to ensure the floor is thick enough to satisfy fire requirements. How much thicker the slab is than specified depends to a large degree on whether the designer expects a level floor or a floor with uniform thickness.
Flatness is specified based on the floor classification. According to ACI 117’s commentary, “The flat classification requires re-straightening after floating and is the highest feasible tolerance level for suspended slabs.” A “flat” floor in ACI 117 is defined as FF35 (or ¼-inch gap under a 10-foot straight edge). There is no requirement for levelness (FL), except for “level suspended slabs that are tested when shored.”
