What are F-Numbers?

First used on a large-scale commercial project in 1983, the F-number system for measuring floor flatness and levelness has been perhaps the most significant factor in enabling the construction of flatter and more level concrete floors.

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.

• F_F (flatness) defines the relative degree to which a surface conforms to a plane

• F_L (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. F-Numbers are linear, so an FF 20 is twice as flat as an FF 10, but only half as flat as an FF 40. FF/FL tolerances only apply to floors that support random traffic, be it vehicular or pedestrian traffic.

In the tiny percentage of floors that have defined traffic, where vehicles are restricted in their movement by wire or rail guidance, a different F-Number - F_min - is used. Most Superflat floors should use the Fmin System, since most of these slabs support defined traffic.

Why use F-Numbers?

F-Numbers replace the familiar "3mm in 3 metre" type specs that had proven unreliable, immeasurable and unrealistic. Did "3mm in 3 metres" mean ± 3mm in 3 metres (a horizontal 6mm envelope which is 3 metres long) - or - did it mean ± 1.5mm in 3metres (a horizontal 3mm envelope which is 3 metres long)

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.

Although "3mm in 3 metres" 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.

F-Numbers control both the floor's "envelope" and its bumpiness. Or, if you think of the floor profile as a wave, F-Numbers control both the wave's amplitude and its frequency. F-Numbers have shown the ability to identify and to control floor characteristics, which are critical to the floor's usefulness.

Therefore, regardless of your location around the world, the "F Number" system provides specifiers with the ability to decide on the correct tolerance, provide contractors with a meaningful tolerance to construct and owners with a facility that works.

How are 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 F_F, while the differences over 3 metres are used to determine F_L. 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 3.3 metres, 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, of which, Bess Concrete are certified operators through the Face Company of Norfolk, Virginia.

The table below details the different specifications for F-Number floor profiles.