Flatness and levelness are terms increasingly utilized in recent years to specify finish criteria in concrete floor slab construction.

Flatness describes the magnitude of "waves" in a floor surface. Levelness refers to the relationship of the floor surface to a horizontal plane. ASTM E-1155 "Standard Test Method for Determining Floor Flatness and Levelness Using the ‘F-Number’ System (Inch-Pound Units)" describes the ‘F-Number’ system and a recommended measurement system.

The F-Number system assigns a numerical value for both flatness and levelness. A higher number indicated a flatter or more level floor surface. FF, referred to as the flatness F-Number, defines the maximum floor curvature allowed over a 24 inch (610 mm) length. FL, referred to as the levelness F-Number, defines the relative conformity of the floor surface to a horizontal plane over a 10 foot (3 m) distance.

Care should be taken when specifying floor systems using the F-Number system. Typically, F- values range from FF25 to FF40 for hardened floor systems to over FF100 for superflat floors which are sometimes required for high-bay, narrow aisle warehouse floor systems. Concrete compressive strength, finishing techniques and equipment, and curing methods significantly affect the finished product and vary depending upon the specified F-Number.

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