Sharing Experiences
Achieving a flat and level elevated slab is a challenge, but one that has been met by many concrete contractors. To move the industry forward and prevent disagreements with the general contractor, we would like to collect ideas from concrete contractors on how they have been able to get flat and level elevated slabs.
There is no question that obtaining floor levelness F-numbers (FL) of 20 or higher on elevated slabs placed on cambered steel framing is a complicated task. There is an old saying, “the impossible is hard until someone does it.”
As a fully integrated builder, Erdman Co. (a wholly owned subsidiary of Cogdell Spencer Inc.) has been a design/builder of healthcare facilities for more than 50 years. We, like many in the industry, have struggled to get consistently level slabs and reduce the amount of money spent on floor leveling products. From that experience, our teams of architects, engineers, and construction managers have developed the following steps to improve slab levelness results:
- Design the steel framing to be capable of supporting the additional concrete needed to obtain a flat floor, on slightly undercambered framing.
- Ensure that the specifications state average and minimum FL values, as well as lanuage to enforce them.
- Determine industry best practices and ensure that subcontractors understand how the framing will react as concrete is placed.
- Incentivize the subcontractors to achieve results better than the specification.
Levelness and flatness
Concrete slabs can be described in terms of flatness and levelness. Flatness can be thought of as smoothness—how bumpy or smooth the slab is. Levelness, on the other hand, is the average change in elevation over a longer distance, for example, ½ inch in 12 feet. See Figure 1.
The industry standard for measuring these two criteria is the F-number system. FF numbers are flatness criteria and FL numbers are levelness criteria. Although we typically achieve average flatness (smoothness) values in the 30s, the levelness values measured on unshored elevated slabs have been as low as 10. Note that flatness is a result of good finishing, while levelness is a result of good screeding, followed by finishing processes that don't undermine the screeding.
The core of the problem is that cambered steel moves, or deflects vertically down, when loaded with concrete. For our floor systems, with the most economical steel framing on 30-foot-square bays, the anticipated deflection under wet concrete ranges from 0 near the columns, 1±¼ inch at midspan of the beams along column lines, and 2±½ inch at the center of the bay.
Increasing the beam sizes to significantly reduce the wet concrete deflections is not economical. When Erdman first began using composite framing in the mid-1980s, we designed for shored construction, but found the cost and time to set and remove shores could be avoided with unshored construction. Further, the shoring did not necessarily provide the expected levelness values, because the slab system moved down when the shores were removed.
Designing for additional concrete
The ultimate goal is a flat, level slab. We know the steel will deflect under the weight of the wet concrete, and because the slabs are often UL rated, the design slab thickness is a minimum thickness to achieve the necessary fire resistance. To ensure this minimum thickness is achieved, the steel beams are undercambered intentionally. So if the calculated deflection under the weight of the wet concrete is 1 inch, we might specify a camber of 5/8 inch. Then the steel fabricator is given a camber tolerance range of -0 / +3/8 inch, so the floor system is not overcambered.
This combination of under specifying camber, and limiting overcamber, prevents issues with minimum slab thickness. Then the potential overcamber can be neglected and the design can focus on the entire structure, including the deck, beams, columns, and footings, to support more concrete than the theoretical slab thickness. Using this process, the cambered steel framing will change from an inverted bowl shape prior to concrete placement to a dished shape after concrete placement. The concrete will be thicker in the middle of the bay. We calculate the additional amount of concrete to be placed and call that out on the construction documents—typically 10% to 15% of the theoretical slab volume.
This steel framing design method is an essential step. If the steel is not designed to support additional concrete, then uniform thickness is the only option available to the concrete subcontractor. Because both the calculation for camber and the cambering process have tolerances, the finished floor can be off by the sum of those tolerances plus minor thickness variations.
Specifications
Erdman's specification for concrete slabs—whether it's an interior slab on ground or an unshored slab on steel deck—includes provisions intended to result in a flat and level floor slab.
