Wednesday, June 05, 2013

Proper installation of large format tiles

One of the major issues facing our industry is the proper installation of large format tiles (including reduced thickness porcelain tiles). Proper substrate preparation becomes even more critical when dealing large module finishes. In order to achieve the end goal… a flat floor without lippage…. reconciliation between Division 3 (concrete specifications) and Division 9 (specifications for ceramic tile finishes) are a must. In the past, a combative sentiment existed between the trades that were contracted to perform their respective work. Cooperative efforts are continuing to improve an understanding of this issue between the two trades.

As the size of finishes keeps increasing, the construction industry has finally recognized that the trades must work together in order to produce a functionally flat floor. The 2013 TCNA Handbook now includes a section entitled “Disparity Between Concrete Flatness Tolerances Based on F-Numbers and the 10-foot Straightedge Method” (attached) in the “Substrates Requirements” section on pages 34 & 35.

In short, this section alerts the specifier than there can be discrepancies between the divisions as to how one trade views and measures the acceptable flatness tolerances versus another trade. Some of the discrepancies include how the concrete trade measures floor flatness (FF). According to ASTM E1155 – Standard Test Method to Determine Floor Flatness (FF) and Floor Levelness (FL) Numbers - FF values take into account many measurements and is an indication of the overall flatness of the concrete. In addition, the concrete industry considers a FF of 35 to be suitable for thin bed ceramic tile installations.

On the other hand Division 9 requirements apply where tile finishes are scheduled to meet a flatness tolerance that is based on localized measurements taken with a 10-foot straightedge, where the tolerance is expressed as a “gap” under the straightedge (e.g. ¼” in 10-feet, 1/8” in 10 feet, etc…).

In addition, there are other factors that further influence the disparity between the two divisions. ASTM E1155 requires that the FF measurements be taken within 72 hours of concrete placement (which is well before concrete curling and shrinkage takes place). Therefore, the concrete may actually meet the desired floor flatness requirements at the time of taking the measurements, however, the flatness tolerances can change significantly (as it cures, curls, shrinks, creeps, etc…) in effect dropping out of tolerance. In addition, no FF measurements are taken within 2 feet of the slab perimeter or at any joints (including construction, isolation, control joints, column block-outs, etc..). These are usually the areas that experience the greatest amount of flatness variation.

Therefore, it is reasonable to expect that there will be areas of a concrete substrate (that is properly placed and finished and meets the specified overall floor flatness (SOFF) requirements) that will not meet the tile setters requirements for a suitable concrete substrate.  The chart below reveals an approximate correlation between specified FF numbers and the traditional 10-foot straightedge method. As indicated, the larger the tile and the narrower the grout joint, the flatter the concrete substrate needs to be.

What’s great about this movement, is that the concrete industry also recognizes that reconciliation between the two divisions is necessary. To that end, the American Society of Concrete Contractors has issued a white paper on this matter (attached and as follows):

American Society of Concrete Contractors - Position Statement #35 - The Effects of Curling on Floor Flatness

ACI 117-06 and ACI 117-lO, "Specification for Tolerances for Concrete Construction and Materials," require F-numbers to be measured within 72 hours after slab concrete placement. This was not always the case. ACI 117-90 included no time requirement for the measurement of floor flatness, FF and the commentary stated the reason:

"Since neither deflection nor curling will significantly change a floor's FF value, there is no time limit on the measurement of this characteristic." The statement in the commentary indicating that curling will not significantly change a floor's FF value has since been shown to be incorrect by measurements published in "The Concrete Floor Tolerance/Floor Covering Conundrum," Concrete International, July 2003. This is why ACI 117-06 and ACI 117-lO now require that FF measurements be made within 72 hours. In addition to the measured field data, it's possible to calculate the effect of curling on floor flatness as shown in Chapter 8, Floor Flatness and Levelness, of Tolerances for Cast-in-Place Concrete Buildings published by the American Society of Concrete Contractors in 2009. The calculation method was also included in "The Effect of Curling on Floor Flatness," Concrete Contractor, April/May 2010. This is important because ACI 302.1R-04, "Guide for Concrete Floor and Slab Construction," states:
“Application of present technology permits only a reduction in cracking and curling, not elimination. Even with the best floor designs and proper construction, it is unrealistic to expect crack-free and curl-free floors. Consequently, every owner should be advised by both the designer and contractor that it is normal to expect some amount of cracking and curling on every project, and that such occurrence does not necessarily reflect adversely on either the adequacy of the floor's design or the quality of its construction. Design professionals should consider how curling is to be handled. This position statement from the American Society of Concrete Contractors is presented for reader interest by the editors. Curling occurs because of differential moisture loss that is a time-dependent process; thus, the initial floor flatness produced by the concrete contractor will decrease with time. The table below shows how an initial FF of 51 can decrease to 45, then 35 and finally to 22 as curling occurs and slab edges raise from 1/16, to 1/8 and 1/4 in.:

To deal with changes in floor flatness with time, design professionals can use an allowance for floor grinding and leveling as described in "Division 3 versus Division 9 Floor Flatness Tolerances," ASCC Position Statement #6, Concrete International, June 2003. An additional resource is "Responsibility for Controlling Slab Curling," ASCC Position Statement #30, Concrete International, January 20lO. ASC Concrete contractors will meet the FF specification requirements when measured within 72 hours. The effects of a decrease in floor flatness with time must be addressed by the design professional with respect to the work of follow-up trades.

The end goal of raising awareness and addressing these issues in the TCNA manual is to alert the Project Specifier (and building owner) that they will have to include a bid allowance for any necessary floor preparation to bring the slabs into the required flatness tolerance. This will level (pardon the pun) the playing field for tile setters and their competitors. This provides us with a great opportunity to provide substrate preparation / leveling solutions for these types of projects. Categories and products that can help to address these applications include; self-leveling underlayments (DRYTEK® and LATICRETE® NXT® family of leveling products), patching products (DRYTEK and LATICRETE NXT patch / skim coat products), mortar beds (LATICRETE 3701 Fortified Mortar Bed).

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