That question doesn't concern only concrete contractors and owners. It's an issue involving all the trades that provide finished surfaces for floors. More to the point, it's an issue that starts with how flatness is measured.
For the concrete contractor, flatness is specified as an FF number and levelness as an FL number. This applies to floors that will receive finishes such as wood, quarry tile, carpet, or sheet vinyl products. The measurement typically is determined with a “dipstick” tool the morning after concrete is placed and isn't taken across control or construction joints.
As such it is intended to reflect the contractors' ability to place and finish concrete within the specified tolerances and not to reflect how a slab curls over time. Concrete contractors want to be evaluated on their workmanship and not by the properties of concrete between the time that it is placed and when finished floor work starts.
For the trades who install flooring over concrete, the problems and issues are different. They must install their products over surfaces that meet specified standards for flatness. Most of their specifications require that flatness be measured by a 10-foot-long straight-edge with no more than a ±1/8-inch deviation. A problem for this system of measurement is that the results can't be duplicated. By rotating the straightedge in different directions over the same location, different results are observed. So it's possible to grind down perceived high spots from a floor, making it less flat as a result.
What to do?
Developing an industrywide standard for measurement is the first step. All the trade associations involved with floor construction should agree on a system of measurement that will provide relevant information related to their requirements as well as everyone else's. In terms of accuracy and reliability, the straightedge test should be retired in favor of more technical instruments such as dipsticks, pro-filographs, and 3-D laser scanners (see “3-D Laser Scanning” on page 38 of this issue) to provide accurate information.
A concrete contractor's workmanship should be honored—certainly in terms of being paid for his work. The amount of curling that a slab produces may be due to factors outside the contractor's control. Yet flatness at the time of special finish installations is an issue. Specified floor tolerances at this point in time are important also.
In 2003 Howard Kanare, senior principal scientist at the CTL Group, Skokie, Ill., convened a conference titled “The Inter-Industry Working Group on Concrete Issues.” Attended by professionals representing almost all the flooring trades, the conference's purpose was to discuss industry issues. As a next step, a group similar to that should meet again to agree on a system for measuring flatness that works for everyone and creates standards for flatness that will serve everyone's interests.