To ensure that F-number measurements reflect only the floor flatness and levelness resulting from the concrete contractor's efforts, these measurements must be made within 72 hours after completion of slab concrete finishing operations. Otherwise, low F-numbers are likely to be the result of curling. Thus when the floor covering contractor arrives, disputes over the floor flatness and levelness may arise. The cost for grinding or patching a floor that has curled needs to be discussed prior to construction. American Society of Concrete Contractors (ASCC) position statement #6 "Division 3 versus Division 9 Floor Flatness Tolerances" and #30 "Responsibility for Controlling Slab Curling" provide more information on the effect of curling.
More information about curling is contained in Tolerances for Cast-in-Place Concrete Buildings, a book written by the authors of this article and published by the ASCC, www.ascconline.org. The ASCC Position Statements are also available at this website.