The construction of suspended slabs that exhibit superior flatness and levelness can't happen without vision, planning, and execution. The slab-on-metaldeck floors at the new Brooklyn, N.Y., IKEA store were beneficiaries of all three elements—taking the placing and finishing of concrete on steel decks to a new level.
IKEA purchased the 48-acre property that formally was the Todd Shipyard and took strong measures to maintain the maritime atmosphere. Four of the original gantry sweeping cranes were restored and incorporated into the graving dock that was transformed into a long, open area (known as an esplanade—a feature typically found near a body of water). Tools found during the demolition of the original buildings and dredging operations also are on display.
The completed 350,000-square-foot retail space has a one-story parking garage constructed below the building at grade. The structure features steel framing with the elevated warehouse and retail space built using slabs on metal deck. The loading dock also is elevated and attached.
Exposed concrete floors were a key component of the IKEA store. It was necessary for the design/construction team to develop a warehouse floor slightly less than 150,000 square feet on metal deck. The surface required levelness and flatness characteristics similar to those normally restricted to slabs on ground. The many displays commonly found in IKEA stores made superior levelness desirable for floors in that area, but without the aesthetic distraction of supplemental leveling materials.
J&A Concrete, Bohemia, N.Y., first built a concrete retaining wall/dam at the entrance to the graving dock. The wall footing was 30 feet 4 inches wide by 116 feet long by 2 feet 10 inches thick. The wall itself was 39 feet 6 inches high by 116 feet long by 2 feet thick with five battered counterforts. The graving dock then was filled with dredged sand material, which was located, approved for use, dredged onto barges, barged to the dock, and transported to the site, using conveyers for placement. The steel gate entrance to the graving dock was removed and placed on a cradle, just askew of its original location. In this way, the concrete retaining wall had sand fill on one side and the Erie Basin on the other. This 'reclaimed' land then was developed partially for the esplanade and partially paved for parking.
Barry Levin & Associates (BLA), Somerset, N.J., in consultation with Structural Services Inc. (SSI), Richardson, Texas, developed contract documents that provided Aurora Contractors, Ronkonkoma, N.Y., and J&A Concrete with the tools necessary to produce successful floors. The structural steel framing in the warehouse portion of the project was designed to support the anticipated load with minimal deflection. The floor framing in the retail portions of the store were lighter and cambered to offset anticipated movement during concrete placement using a strategy suggested by SSI. A significant amount of deformed reinforcing steel rebar was used in the slabs on metal deck to control the width of cracks.
The project specifications imposed a number of requirements on the contractor—all geared toward producing deflected floors that are level. They included the following:
- Slabs on metal deck were required to exhibit a combined overall floor flatness F-number of 30 and minimum local value of 20.
- All slab elevation measurements were to be tied to a common benchmark. This allowed the combination of the overall levelness of multiple placements in order to evaluate the general levelness of the deflected floors.
- Deflected floors evaluated using ASTM E-1155 were required to have 80% of their elevation samples within a ¾-inch-deep envelope. This levelness criterion for deflected floors has been used successfully by SSI for nearly 20 years. Without planning, most deflected floors only have between 40% and 60% of the deflected surface within a ¾-inchdeep envelope.
- The mean elevation of the deflected floors was to be within 3/8 inches of design grade. Requiring the mean elevation of a deflected floor to match exactly the elevation shown on the contract documents rarely is critical. It's of more importance the mean elevation be within a fraction of an inch of design grade and the mean be used to evaluate the overall levelness of the floor.
The contractor was required to hire a firm to assist in planning and executing successful construction of the slabs on metal deck.
The contractor was required to verify preplacement elevation of the erected structural steel frame by survey. The only elevation tolerance contained in the AISC Code of Standard Practice is for base plate elevation. The erected steel frame is seldom level, so surveying the steel prior to concrete placement alerts the contractor to any levelness issues for the supporting steel in a timely manner and also allows the concrete subcontractor to respond to any severe levelness issues.
It was required that the contractor coordinate a preconstruction conference to discuss the specifics of concrete placement for the suspended slabs on metal deck. Agenda included the following:
- Discuss documentation of erection tolerances and preplacement quality assurance.
- Discuss proposed placing and finishing procedures for slabs on metal deck.
- Rigid screed guides were specified in lieu of wet screeds.
- The use of straightedges during the finishing process was specified.
- Discuss testing and inspection requirements.
- Discuss specifics of data collection and evaluation.
- Develop a layout of construction and contraction joints. The location of construction joints in composite slabs on metal deck like those at this project can influence deflection of the structural steel.
Discuss construction options for dealing with these issues.
Discuss options for dealing with behavior that differs from the expected.
Executing the work
J&A Concrete began work at the project in September 2006 and finished in June 2008 (with substantial completion in October 2007). Prior to beginning work on the slabs on metal deck, pre-and post-placement surveys of the structural steel were done. During the course of construction, these post-placement survey results were compared to the preplacement data to confirm movement of the steel frame during concrete placement. This information was used to fine-tune placement procedures on subsequent placements with similar structural steel support framing.
Accurate strike off of the concrete is a critical component to producing deflected fl oors that are level. SSI recommended, and BLA dictated, that rigid screed guides be used to control the strike off of concrete. The use of wet screed guides placed in fresh concrete as a means to establish concrete levelness was inaccurate.
By using moveable rigid screed guides, there is significantly more control over the process. With brackets on each end of the guide, the depth of strike off over supporting steel can be very precise. Once deflection patterns have been established—only part of one placement is necessary for this purpose—any adjustments to thickness can be implemented with accuracy.
Rigid screed guides were selected for use on this project. J&A was not familiar with the use of these, so initial placements were scheduled for the loading dock portion of the work. Representatives of Allen Face & Co., Wilmington, N.C., manufacturers of the Screed Rail, visited the jobsite to assist J&A in learning the differences between wet screeds and the rigid screed guide . The multiple placements involved in that part of the work allowed J&A to develop a high level of confidence in the process. When work moved to the warehouse floors, J&A proceeded without any problems.
Once work began on the warehouse, the strike off and finishing procedures were well established. The rigid screed guide was used on the 'fresh' side of the placement strip. A handheld vibratory screed was used to strike off the concrete—one end riding on the rigid screed guide and the other overlapping the previous placement strip. A wide bullfloat was used at an angle to the placement strip to smooth any distortion at the joint between placement strips. After evaporation of the bleed water, workers continued the finishing process using a walk-behind power float with float shoes, restraightening with a highway straightedge, running multiple passes with a riding machine with float dishes attached, and finally operating a riding machine with trowel blades.
The testing procedures identified in the preconstruction meeting were implemented. It turned out that the local testing agency retained for the work was not capable of collecting and processing data as required, so J&A purchased a walking floor profiler and, after training by SSI, collected data on each of the concrete placements. Data were sent to SSI for processing and the results are shown in the table.
The overall flatness and levelness numbers for the floors at this IKEA store take placing and finishing concrete on steel decks to a new level. It didn't involve higher job costs, just careful attention to the details and the use of a rigid screed guide to achieve better strike off work.