Since its adoption by the American Concrete Institute (ACI) in 1990, the F-number system has become the preferred method for controlling floor surface tolerances on most construction sites.
SPECIFIED OVERALL VALUES VS. MINIMUM LOCAL VALUES
Standard practice when using the F-number system to manage floor surface tolerances is to impose two separate requirements on the finished product:
Specified overall values (SOVs), which are the combined results for flatness and for levelness that must be achieved at the completion of all concrete floor placements at a given elevation.
Minimum local values (MLVs), which are the qualities of flatness and levelness required everywhere on the floor surface in question.
COMBINING F-NUMBER RESULTS
When constructing a floor using more than one concrete placement, evaluation of the surface requires combining the results of individual placements. The procedure for combining results is described in ASTM E 1155-87, "Standard Test Method for Determining Floor Flatness and Levelness Using the F-Number System."
IDENTIFYING DEFECTIVE WORK
Using test data and visual clues, a testing agency should be able to identify areas of the slab surface where the flatness or levelness may be defective (below the specified MLV). A useful tool for identifying potential problem areas is a range of numbers known as the 90% confidence interval.