FF and FL criteria have, for more than a decade now, been the accepted method for specifying and measuring the flatness and levelness for random traffic floors. The testing standards are given in ASTM-E1155-96/01. The very basic definition of FF is the "bumpiness" of the floor. To determine FF, measurements are taken at successive 1'-0" intervals and the values (curvature) are then compared over 2'-0". The very basic definition of FL is the levelness or "pitch" of the floor compared to a horizontal plane over a 10'-0" distance. To determine FL, measurements are taken at successive 1'-0" intervals but are compared as the difference in elevation between two points separated by 10'-0".

There are numerous other measurement criteria that I do not want to get into, including the minimum number of readings, minimum line lengths, minimum number of lines, etc. However, there are some criteria, which are important for everyone to understand. To begin with, these tests should be "blind." The testing layout should be done prior to ever seeing the actual placed slab. Otherwise, it would be possible to skew the results in either a favorable or non-favorable way.

Next, ACI mandates that the tests must be performed within 72 hours of placement. This seems to be one of the criteria that testing agencies, owners and contractors want to ignore, but it is one of the most important. The reason being is that these tests are supposed to test the work of the concrete contractor only. When testing is preformed after this 72-hour period, the slab is subjected to many items, which the concrete contractor has no control over such as curling, subgrade deflection, shrinkage, etc. All of these items can skew the value in a negative direction.

Another important criterion is that no values shall be taken within 2 feet of a construction joint of slab penetration. I see this criterion being ignored a lot by testing companies.

The biggest misunderstanding, in my opinion, is the difference between specifying an overall FF / FL and a local FF / FL. If a note on a drawing simply states, for example FF40/ FL30 (which many do) you would think it would be safe to assume this is referring to an overall value. This is not always the case because a lot of designers/owners don't understand the difference. The concrete contractor must protect himself by clarifying this in his proposal. The purpose of and difference between the overall and local values are simple.

The overall value is the FF / FL value that all of the individually formed placements must average. Some of the individual placements may be higher or lower than the specified overall value, but when all placements are averaged, the entire floor meets the overall value specified. The local value is the lowest FF / FL value you can produce any given day and still be acceptable to the owner. A generally accepted rule of thumb is that the local values should be set at 60 to 70 percent of the specified overall value. Given this, if the FF40/ FL30 example used earlier was actually meant to be a local value, then the overall value would be anywhere from FF57/ FL43 to FF67/ FL50. This is a big difference in not only the expectations of the owner but also the cost of producing a FF 67/ FL50 is significantly more than producing a FF 40/ FL30 surface.

All random traffic floors should have specified local and overall FF / FL values regardless of use. The FF / FL system is very fair to both owners and concrete contractors as it clearly and measurably defines the expectations of the owners. The use of this system eliminates the conflicts with the vinyl tile and ceramic tile contractors who simply say, "The floor is out of tolerance," when no tolerance was given. It also eliminates the owners/contractors calling a meeting to look at an area of the floor which, "doesn't look like I thought it would," even though there was no specified tolerance.

Lastly, it eliminates those ridiculous meetings where people pull out a 10-foot straightedge and start measuring the distance at various locations on the straightedge between the slab and the straightedge. I always know it's going to be a long meeting when I show up and the architect and contractor's superintendent or project manager have a straightedge out when I arrive.

I urge every architect to stop what they are doing and to spend 10 minutes to update their specifications to reflect FF / FL values for random traffic floors and to test in accordance with ASTM-E1155-96/01. It's amazing how architectural specifications (even from the very large architectural firms) still have that ridiculous and outdated 1/8" under a 10' straightedge specification for flatness and levelness. It's past time for all designers and contractors to familiarize themselves with the FF / FL system for random traffic floor tolerance.

The system has been the accepted system for over a decade now and for people to still not be using it or to not be familiar with it is a true detriment to the industry.