Concrete Floors: Flatness vs. smoothness

Application of Overlay on Prepared Floor Surfaces
Application of overlay on prepared floor surfaces. Photo courtesy Mapei

The functional tolerance
Functional tolerances for resilient floor coverings on concrete floor substrates are established by ASTM F710, Preparing Concrete Floors to Receive Resilient Flooring. This standard requires the concrete to be smooth enough to prevent irregularities, roughness, or other defects from telegraphing through resilient flooring products. It also requires a flatness to within an equivalent of a 4-mm (5/32 in.) gap measured along a 3-m (10-ft) straightedge with no gap measurement greater than 0.8 mm (1/32 in.) within 300 mm (12 in.). This requirement fits within the expectations of FF20 to FF25, and the esthetic tolerances expected by many flooring products manufacturers.

The primary difference between ASTM F710 and ASTM E1155 is the functional tolerance is measuring the relative waviness of the floor. Using the omnidirectional short-span measurement addresses the typical concerns of using FF20 or FF25 where the installed gap tolerance is greater than the 3-mm (1/8-in.) required by flooring manufacturers—a floor that is acceptable by the statistical structural tolerance may not meet the esthetic or functional tolerance required by the flooring installers.

Hard-surface flooring materials require a higher degree of flatness, often approaching true planarity to avoid installation problems such as lippage between large tile products. The larger the hard surface material is, the more restrictive the F number becomes. The Terrazzo, Tile, and Marble Association of Canada (TTMAC) has found stone and ceramic tile products with any dimension larger than 400 mm (16 in.), or any tile installation requiring joints 3 mm (1/8 in.) and less, are particularly problematic when installed on conventionally achieved FF20 and FF25 floor substrates, and recommend FF50 to FF60 as a more appropriate tolerance.

In addition to the floor flatness criteria, there is also a structural deflection concern wrongly associated with floor levelness. Suspended floors deflect and all slabs can change shape significantly in the first few days of placement, which is the reason flatness is measured within 72 hours and before shoring is removed from suspended slabs.

The deflection occurring after shoring is removed can be significant enough to produce a sharp drop-off at the structural lines of support and the field area of the floor space. This drop-off can lead to significant lippage concerns with large-bodied tile installations as the tiles are placed across the crest of the deflection. To counteract this concern, specifiers often set requirements for L/600 to L/720 slab deflection restrictions that are not economically feasible to attain from a structural costing point of view. Changes to the reinforcing can help limit the sharpness of the drop-off; however, in reality, the flooring installers will be dealing with L/360 and tiling installation details need to account for the change in floor slopes caused by deflections.

Identification of the problem
The root of the problem is the differences between the three types of tolerances; we use the structural tolerance to define the esthetic tolerance, but are looking for a different measurable functional tolerance result. A smooth and even concrete finish satisfies the requirement for the esthetic concern of not telegraphing concrete imperfections through flexible floorcoverings. This is achievable using the FF20 and FF25 floor substrates.

Too often, defects attributable to the concrete floor finisher through (and appropriate references to) patching and levelling within the Work Results specified in Division 03−Concrete for work done by the concrete floor finishers are not being addressed. Corrections and repairs made by the concrete floor finishers address only the structural tolerances, and not the esthetic or functional tolerances. Defects can also occur as a result of weather conditions—for example, rainwater dilution of the surface paste can lead to powdery residues, exposed aggregates, pitted surfaces, or loosely bonded surfaces.

Additionally, there are consequences of multi-floor construction that leave remnants of previously installed temporary construction such as crane block-outs and screed points. As a result, flooring installers are often made responsible for repairing or correcting these concrete fossils, which is different than the spot patching and smoothing they are required to do as a part of their Work Result covered by Division 09−Finishes.

Finding a solution
In the historical sense to which this author previously alluded, floor preparation on suspended slabs often entailed installation of a topping material. This was called a two-course pour and covered a multitude of floor problems including crane block-outs, overworked surfaces, rain damage, and other types of imperfections leading to flooring installation issues.

CFCA addresses the requirements for limitations of floor flatness established by CSA A23.1, and states tolerances greater than FF35 are only practical through the use of improved mix design and reinforcing, by the use of deferred application of cementitious floor toppings, or incorporating advanced (i.e. non-standard) methods of construction. Installation of cementitious overlays is not the responsibility of the concrete floor finisher. The logical conclusion is the flooring installers have this responsibility since the increased tolerances are a requirement associated with the products they install.

As a practical solution to most of the improved floor flatness problems, toppings are the ideal solution. They are also a good idea for most suspended slabs in multi-storey buildings to maintain a consistent, smooth, and even finish as required by most flooring manufacturers.

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