Q: We recently placed a concrete floor with a laser screed. The floor had an FF requirement of 35 and an FL of 30. We had no trouble meeting the levelness requirement, but on the FF we achieved numbers between 50 and 75 in a north-south direction but only 25 to 45 in an east-west direction. We ended up slightly below the requirement of 35 for the entire floor. On a succeeding floor placement, we added another finishing machine with a pan float and made first passes in a diagonal direction. This helped, and our FF number ended up being higher than the requirement for the project. But we are curious to know why FF numbers in one direction would be higher than numbers in the other direction?

A: We asked Bob Simonelli, who conducts precision flooring training seminars for contractors, about this problem. Bob represents the Allen Engineering company and presents action demonstrations all around the country. He thinks there are two possible reasons for significant differences in FF numbers in the two directions.

When using a laser screed, the machine typically makes a pass, screeding concrete in the line of travel of its boom. When the machine moves to the next location, the boom reaches over the end of the preceding pass and starts screeding the next strip. But frequently there is either a depression at this seam or a small rise in elevation. In order to make this transition smooth, finishers use a tool called a "check rod," passing it over the transition area perpendicular to the line of travel of the laser screed. A check rod is like a bull float, except that it's usually 12 or 14 feet wide.

The second possible cause for this problem may be due to the use of finishing machines. Vibratory screeds usually leave floors very flat. Each pass with a finishing machine decreases FF values. This happens especially when finishing operations begin when concrete is too soft. But the direction of the pass of the machine also could contribute to the problem. Bob recommends using a ôbump cutterö tool between each use of finishing machines to take concrete from high spots and distribute it to low spots. This becomes a critical step when the FF numbers for a floor exceed 50.