More On Concrete Floor Flatness

SpecPress, the newsletter for subscribers to the ARCOM MasterSpec system, has more to say about the contradictions faced in the specification of flatness of concrete slabs as previously discussed on this site in articles Concrete Floor Flatness and Concrete Slab/Floor Covering Issues. In The Concrete Floor Tolerance/Floor Covering Conundrum (Volume 9, Issue No. 4, Fourth Quarter 2003), Bruce Suprenant identifies the following basic problems:

• Concrete placement in Division 3 relies on the F-number system to specify floor flatness. Finish floor coverings specified in Division 9 rely on the straightedge method. There is no direct correlation between the two methods.
• The criteria for measuring F-numbers specified in Division 3 do not satisfy the requirements for Division 9 finishes. For example, F-number measurements may not be taken across construction joints, they may not be taken within 2 feet of a slab penetration, and they must be taken within 72 hours of the initial slab pour. However, floor coverings typically are applied across construction joints, close to slab penetrations, and after the slab is fully cured.
• Different floor coverings applied to different sections of the same slab may require different slab finishes (such as hard trowel, broom, etc.), even though it is often not practical to specify different finishes across a single pour.

Suprenant recommends the following possible approaches to dealing with the contradictions:

The specifier can require an arbitrarily higher initial floor flatness hoping that the final, cured slab will meet minimum requirements. The difficulty with this approach is that there is no method to predict final flatness based on flatness measured within the intial 72-hour measurement window.

Additional reinforcing can be added to the slab in order to reduce curling that occurs during curing. Suprenant claims that with additional reinforcing, control joints are not effective, and thereby money can be saved by "not cutting and filling joints".

Assign responsibility to grind or patch the floor as required to the floor covering contractor(s). The advantage to this method is that responsibility for achieving the required floor flatness is put into the hands of the floor covering contractor whose product dictates the flatness requirement. Suprenant suggests specifying an allowance amount in the specification to be applied toward anticipated grinding and patching.