Exceeding Specs on a Structural Slab-on-deck

A recent project for the University of Alabama's Department of Science and Engineering exemplifies Birdwell & Associates' confidence, ambition and talents in concrete construction. The floor was small, only about 3,000 square feet, but it was a formed and shored 3-foot-deep structural deck with high floor levelness and flatness requirements spec'd in. "Although it was small, it was one of the hardest slabs I have ever placed and finished," says Bryan Birdwell, one of the owners of Birdwell & Associates, Lakeland, Fla.

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Bryan was on site during the project and directly oversaw each step of the placing and finishing process. Only the second of its kind in the United States, the 6,000 psi slab is located in the testing lab for the University of Alabama's engineering school. Students will perform various concrete-related testing in the lab, and the slab is serviced by a 15-ton crane. Because of the high tolerance needs for this type of use, the floor levelness and flatness requirements were spec'd at FF 50 and FL 30, normal for a slab-on-grade but incredibly high for a slab on an elevated deck.

The general contractor on the project, Gary C. Wyatt, Birmingham, Ala., approached several concrete contractors throughout the country who declined the job and its requirements before finally striking a deal with Birdwell & Associates. Bryan says of the spec, "I would never normally agree to an FL slab-on-deck on this type of floor, but I felt a little gutsy, gambled it and we won."

The floor had No. 9 rebar 6 inches top and bottom with No. 5 galloping stirrups every 12 inches. In addition, 348 steel sleeves 3 foot on oenter ran each way on the X and Y coordinates with a 1/8± tolerance. Birdwell & Associates crews placed the floor by hand. Not only did the company meet the flatness and levelness requirements, they greatly exceeded them with an overall FF of 122.93 and an FL of 50.21. "Trying to place a slab-on-deck and control the shrinkage of the concrete as well as setting elevation by hand-held laser receiver and wet screed was a huge accomplishment, which I look forward to repeating again," Bryan says.

The project and its spec's left no room for mistakes. Because it was a 3-foot-deep structural slab, there was no option to rip it out and do it over if something went wrong. And because of the steel pipes, going back to grind and level the slab for FF and FL inaccuracies was not an option.

"To me it is satisfying to have done so well on a slab that very few others would attempt to try," Bryan says. "It's also really neat that college kids will learn about concrete on a floor that's such high quality. You never know who is going to come out of the University of Alabama to achieve greater things than are happening today."