Concrete Floors: Flatness vs. smoothness

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A successful finished tile project requires an understanding of who is responsible for what when it comes to tolerances in concrete slabs and floor finishes.

CFCA supports the tolerance requirements of CSA A23.1 within its guide specifications. However, do specifiers and flooring installers understand what constitutes an FF20 or FF25 floor finish? As explanation, FF indicates how well the concrete floor finisher worked the surface—more effort usually results in better overall flatness. The FL metric indicates how skillfully the side forms were set by the contractor and where the concrete was struck-off. The FL number has nothing to do with the concrete floor finisher’s workmanship or skill level in producing the specified FF number—in other words, placing concrete and finishing concrete are different Work Results.

There is one more sticking point: FF and FL are measured within days of concrete placement, and before removal of shoring for suspended slabs, and concrete is a natural material that continues to change as it cures. Most changes occur within the first 30 days, although they continue for four to six months or longer depending on humidity and temperature conditions during the curing period.

Drying shrinkage and curling modifies the surface profile to a much greater extent than the concrete floor finishers can account for within their Work Result. Curling has become more common with the use of high perm rating under slab moisture mitigation membranes where the concrete mix design and reinforcing have not been modified to account for the improved floor flatness requirements for finish materials. The esthetic and functional tolerances listed in this article are not integral to the structural design, and as such will not be accounted for unless specifically identified by the architect or interior designer.

The esthetic tolerance
Floor finishing materials manufacturers establish tolerances for flatness based in part on what makes their products look good. A typical manufacturer’s requirement will state a required tolerance of a 3-mm (1/8-in.) gap measured under a 3-m (10-ft) long straightedge.

The tolerance does not have a direct correlation to the FF and FL tolerances established to meet structural requirements, with the additional concern the FL does not (and should not) apply to the finishes. The materials are not affected by the levelness component of the structural tolerance. Essentially, the straightedge measurement is not a practical tolerance guide for concrete floor construction.

When it comes to thin and flexible floor materials, the concern is more about the actual waviness of the substrate—more peaks means a less esthetic appearance for the applied floor finishes. The flooring manufacturers are describing the smoothness and evenness of the floor, not the flatness or levelness.

The tolerances for flatness and levelness (i.e. FF and FL) are used improperly by specifiers in an attempt to describe an expectation for the concrete floor finisher to which they can relate; specifiers unintentionally create unrealistic tolerances for conventional concrete floor finishes. As an example, based on the manufacturers’ 3-mm straightedge gap measurement criteria, it is possible to achieve an FF20, FF25, FF50, or even an FF150.

The floor flatness number is a statistical measurement that takes into account the relative waviness of the concrete floor. Waviness in concrete is omnidirectional, meaning the peaks and valleys do not align in parallel rows as they would if you were watching waves crashing against a beach. Based on the statistical model supported by the FF approach; the 3-mm gap measurement counts the numbers of peaks and troughs measured along the 3-m straightedge, and it is the number of observed gaps that count toward the floor flatness rating along several repeatable lines of measurement established by ASTM 1155.

Fewer valleys mean a higher FF measurement. Four troughs under the 3-m straightedge is equivalent to FF20 whereas one trough provides FF150. By the same statistical measurement, a floor having two 8-mm (5/16 in.) troughs under the 3-m straightedge would also achieve the same FF20 rating, but probably would not be acceptable as an esthetic tolerance.

The FF result is important, but does not tell the whole story without the understanding of the 3-mm gap limitation. Specifying a higher FF number results in a more esthetically pleasing expectation. The closer to true planar perfection the specifier sets as a project requirement, the higher the cost will be to achieve that level of perfection. If true perfection is specified, the expectation may not be practically achievable using concrete placing methods only.

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