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The F-number system provides greater precision in defining floor finish requirements to match project needs. The system consists of two values defined by ACI 302 as follows.
· FF (flatness) defines the relative degree to which a surface conforms to a plane.
· FL (levelness) defines the relative degree to which a surface is horizontal
Flatness relates to the bumpiness of the floor, while levelness describes the tilt or pitch of the slab. The higher the F-Number, the better that characteristic of the floor.
How F Numbers are measured
F-Numbers are derived from a statistical analysis of the floor's elevation measured at 300mm intervals. The elevation differences over 600mm are used to determine FF , while the differences over 3 metres are used to determine FL.
Basically, measurement lines are laid out on the floor, and elevation measurements are taken every 300mm down the line. Each measurement line should be at least 3.3metres, and at least 34 individual elevation measurements should be taken for each 95m2 of floor area. Detailed rules for performing F-Number tests are set forth in ASTM E 1155.
After collection, the elevation readings are put into standard mathematical formulae to calculate the floor's F-Numbers. Several devices are approved by ASTM for F-Number measurement, including the Dipstick Floor Profiler.
Straightedge Equivalents
F-Numbers replace the familiar "3mm in 3 metre" type specs that had proven unreliable, immeasurable and unrealistic.
There was disagreement as to what they meant.
Did "3mm in 3 metre" mean ± 3mm in 3 metre (a horizontal 6mm envelope which is 3metres long) - or - did it mean ± 1.5mm in 3m (a horizontal 3mm envelope which is 3 metres long)
(measuring of F numbers, using the dipstick floor profiler)
Straightedge measurements were unscientific and non-repeatable.
Before F-Numbers, floors were only "measured" long after the fact, when someone didn't like the floors' general appearance. That's when the straightedge was finally hauled out by the aggrieved party in an effort to prove his case. Of course, no two people got the same results, since there was no standard for either the test method or for interpreting the results.
They were unrealistic.
Although "3mm in 3metres" has been used to specify millions of square metres of concrete, it was seldom, if ever, achieved. The typical industrial floor, for example, is closer to a horizontal 15mm by 3 metre envelope.
For more information please see the 40 most asked questions about F numbers |
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