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Tolerances for Suspended Concrete Slabs

ASCC Position Statement #24

Concrete floors are often placed on a suspended structural system consisting of a metal deck supported by open-web steel joists,
steel beams, or precast concrete members. 

Tolerances for the floors placed on these suspended systems are often misunderstood. ACI 117-90, “Standard Specifications for Tolerances for Concrete Construction and Materials,” gives only two tolerances for these applications:

  • Thickness tolerance: +3/8 in. and -1/4 in. (for slabs 12 in. thick or less); and
  • Flatness tolerance: FF or gap under straightedge, depending on the floor profile class.

ACI 117-90 requires no level alignment tolerance for the concrete because levelness depends on the location and placement of the supporting structure and, after the concrete is placed, deflection of the supporting structure. The steel, precast erector, or both dictate the floor elevation relative to the design elevation. ACI 117-90 states that the levelness tolerance FL shall not apply to slabs placed on a supporting structure that is unshored, cambered, or inclined. The final deflected shape of the slab influences the floor levelness and is not solely a function of the concrete floor placement. 

Some designers add ACI 117-90 to their project specifications, but also require that the floor be placed level. This specification requirement is typically not consistent with the ACI 117-90 thickness tolerance. When floors are placed on a metal deck supported by open-web steel joists, maintaining a level surface requires extra concrete in the middle of the bay due to deflection of the structural frame. The added thickness of the floor may be 3/4 in. or more, exceeding the +3/8 in. thickness tolerance.

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