We're bidding on a concrete floor job that requires a minimum FF of 20 and a minimum FL of 17. What do these numbers mean and how hard is it going to be to build to these specs?
The F-numbers refer to flatness and levelness. FF is an indicator of floor flatness or smoothness. FL indicates floor levelness. The higher these numbers, the flatter or more level the floor. An FF 20 isn't a particularly flat floor, but the floor probably won't be flat enough if you use bull floats for finishing. You'll need to use an 8- or 10-foot straightedge to cut down high spots and fill in low spots during the finishing operations. For a review of F-numbers and factors influencing floor flatness and levelness, see the article by Bill Phelan on page 5 of the January 1989 issue of Concrete Construction.