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Responsibility for Slab Curling

ASCC Position Statement #30

Curling is the distortion of a slab into a curved shape by bending of the edges due to differences in moisture between the top and bottom of
a concrete slab. It is caused primarily by loss of moisture from the concrete surface as the concrete dries. The moisture loss occurs in all environments
at less than 100% relative humidity and is typically more severe in areas of low relative humidity.

The effect of curling on floor serviceability varies depending on the intended use. A curled slab may be serviceable when covered by carpet
but not by vinyl tile. A curled slab may also experience joint deterioration when subjected to forklift traffic. “A Checklist for Industrial Floor Design” published in Concrete International, November 2001 includes a “crack and curl management” checklist to aid the designer in minimizing curling to suit the Owner’s intended use.

Concrete contractors are responsible for initial floor flatness and levelness that is measured within 72 hours of placement, as required by ACI 117-06 and ACI 301-05. But flatness can change with time due to curl. “The Concrete Floor Tolerance/Floor Covering Conundrum” Concrete International, July 2003, includes two examples of measured floor flatness and levelness that decrease by as much as 50% in a year due to curling. Both the Canadian Standards Association (CSA) and the American Concrete Institute (ACI) recognize that control of curling is a designer responsibility.

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