|Q||What is the difference between the new floor flatness numbers FF and FL and the old straightedge specifications of 1/8″ in 10 ft.?
|A||2/07 – Response prepared by Cliff Craig, Vice President of Technical Operations for Dynamic Consultants, Inc. and a registered engineer in California.
The FF and FL numbers represent a statistical calculation of the flatness and levelness of a concrete slab as determined with slope measuring equipment. Testing is performed in accordance with ASTM E1155 Standard Test Method for Determining FF Floor Flatness and FL Floor Levelness Numbers. The higher the F-number the better the characteristic of the floor.
The straightedge method specifies that the gap observed under a free-standing or leveled 10 ft. long straightedge shall not exceed 1/8″. The problem with this method is that there is no standard method for taking measurements (i.e., number of tests, location, direction) or quantitative procedure for establishing compliance of a test surface.
The following excerpts from ACI help clarify some of the industry guidelines:
ACI 302 Guide for Concrete Floor and Slab Construction
Section 18.104.22.168 – The older method of using a 10-ft. straightedge can also be used to measure floor flatness, but it is much less satisfactory that the F-number system. There is no nationally accepted method for taking measurement or for establishing compliance of a test surface using the tolerance approach. This can often lead to litigation.
ACI 117 Standard Specifications for Tolerances for Concrete Construction and Materials
Section 4.5.7 – Floor finish tolerance as measured by placing a freestanding (unleveled) 10 ft. straightedge anywhere on the slab and allowing it to rest upon two high spots within 72 hr after slab concrete placement. The gap at any point between the strightedge and the floor (and between the high spots) shall not exceed:
The 1/8 inch in 10 ft specification has been a common specification. However, it is seldom measured and rarely enforced due to its unscientific and non-repeatable method. It is approximately equivalent to an FF of 50, which is a very flat floor, not normally required for typical concrete slab surfaces. Most convenventional slabs have flatness readings (FF) between 20 and 30.
The F number system is the preferred method of specifying and verifying compliance of floor finish tolerances and should be used in lieu of the archaic 1/8″ in 10 ft.
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