10 - foot straightedge method
This is the classic method for determining the flatness of concrete and other types of finished floors. When used as a specification or requirement, a maximum deviation in length under the straightedge is given, such as "no point shall exceed 1/4 inch under a 10 - foot straightedge." However, there is no standardized protocol to measure deviations in lengths less than ten feet. The same problem exists when measuring slope. There is no way to measure a localized slope that may actually be greater than the overall slope of the straightedge.
The 10 - foot straightedge method has been a standard in ACI - 117, Standard Tolerances for Concrete Construction, for many years and is optional as part of ACI 117. When the 10 - foot straightedge method is used for random traffic floors, ACI 117 sets the minimum number of samples required at 0.01 times the floor area, measured in square feet. For ramps ACI 117 refers to RMS Levelness tolerance as defined in paragraph 4.11 in ASTM E - 1486.
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