Shiny steel troweled concrete can prove as problematic as presence of sealer by preventing penetration of water. Many setting material manufactures recommend a “water drop test” which is simply placing a few drops of water on the concrete to see if they are readily absorbed. A cementitious bond cannot be achieved if the slab does not absorb the water. Ideally the water would be absorbed over a few minutes time. Anything longer than that indicates the bonding will likely be compromised; how much is directly proportionate to the degree of absorbtion. Up to a certain point, using a better quality thinset may help achieving an acceptable bond. However, if after 10 minutes the water drop still looks like a bead on car wax, the risk is too great and it unlikely that any thinset will bond to such an impervious slab. Manufactures and tile industry recommendations call for a steel troweled slab followed by a broom finish to provide an enhanced mechanical bond. This is similar to the purpose of a grid or some other type of pattern you find on ceramic tile products. While highly desirable, a broom finish is increasingly rare. It would be quite challenging for a concrete contractor to predetermine the location of all the tile areas on a big pour. If the surface is not broom finished, it becomes a judgment call on the part of the tile contractor to proceed with the installation over a smooth slab surface. I can tell you from testing I have done over the last several years on smooth versus rough slabs, the difference is quite substantial. Manufactures are quite clear on their recommendations of a roughened slab. If it is a residential or commercial project where standards are referenced as part of the contractual obligation of the tile contractor, then you proceed at your own risk. If a problem occurs, you can be sure a smooth slab will be at the top of the list on reasons for the problem. It always is on mine.
Once we have the slab clean and bondable, it is time to determine if it is in compliance with the tolerances recommended or needed. In the real world it is highly unlikely that a slab will be ready to receive ceramic tile without some corrective measures. As tile grows ever larger, such as the 2’ x 4’ size which stimulated this article, floor tolerances for waviness (floor profile) become evermore critical. A tolerance of no more variation than ¼” in 10’ with no more variation than a 1/16” in 12” has long been the industry recommendation for flatness. This recommendation from the tile industry is shown in the recommendations published by the American Concrete Institute (ACI) under ACI 302. That recommendation has worked for a tile size of up to roughly 16” with a ¼” grout joint. When the tile size exceeds that dimension, the lippage (a condition where one tile is higher than the adjoining unit) standard will likely be exceeded dependent on grout joint width. Understanding the design criteria for concrete and associated tolerances needed for large tile is a difficult subject to master. To make a long and complicated issue fit in the space allowed for this article, as a general rule of thumb you can assume that anything over a 12”x12” with a grout joint of less than ¼” is going to require some floor flattening unless you are fortunate enough to have a higher than normal or customary tolerance of ¼” in 10’. Likewise, you can be virtually assured that anything over 16” in size is going to require some type of remedial measures regardless of grout joint size to bring the slab into an acceptable condition for a flat tile installation
Here is where things can get both challenging and expensive. There seems to be a long-held and practiced belief that this fractional difference in slab tolerances can be made up with a regular thinset mortar applied to the back of the tile, often by using spots of mortar as opposed to fully embedding the tile. There are several issues here. First, a standard thinset mortar, one which is not classified as a medium bed thinset mortar, is not designed to level or true a floor tile; it is a bonding adhesive, not a leveling product. It is generally agreed by all manufacturers that when a standard thinset mortar exceeds a thickness of between ¼ to 3/8” there will be both a loss of bonding value and excessive shrinkage which may even cause bond loss. A medium bed thinset mortar may be used to make up for minor inconsistencies in floor tolerances. Medium-bed mortars are defined as such by their manufacturers. They are intended to be used as bond coats 3/16” to 3/4” thick after the tile is embedded; they are designed as direct bond adhesives into which the tile is fully embedded, and also are not intended to be used in truing or leveling underlying substrates or the work of others; they are still adhesives.
Pouring and placing what are known as super flat slabs is possible. All one has to do to verify this as a true and accurate statement is to go to your local warehouse store or big box home improvement center. Because these types of facilities use high-masted forklifts, their floors must be flat to safely operate their equipment. The cost for this type of slab placement is very expensive however because this type of equipment is used through-out these types of facilities it is a necessary expense. With tile floors typically being used to cover only a portion of a floor in a given structure, it is difficult to justify the additional expense of using a super flat concrete tolerance throughout the facility when only a relatively small portion is to be covered with tile. As a result, from an overall cost perspective, the floor area to receive large module ceramic tile will quite likely require remedial measures. Whether these measures are to be grinding, floor patching, or use of a self leveling topping is very difficult to say, but it is unrealistic to bid any job using large tile and small joints thinking otherwise.
All this makes it very difficult to accurately bid a job. As tile grows ever larger, the complexity of accurately assessing what will be required to remediate the slab to accept the tile not knowing the conditions you will face is immense. The cost of floor prep can easily exceed the per square foot price of installing the tile. Floor flatness is something that should be checked and verified before starting any tile installation. Actually surveying the floor prior to installation, while not typical, may be warranted if the job is large enough. There are companies that can provide concrete profile readings using portable test equipment. Whichever means you may choose, always check the floor and make a plan using appropriate products and methods for the conditions encountered.