As I've written previously, all concrete slabs are not created equal. They all may look and feel the same, but they are not necessarily identical. On-grade slabs are never truly dry. Even a seemingly bone-dry slab that's been coated with a curing compound can be deceiving. In actuality, that curing compound can dramatically inhibit evaporation of water from the concrete.
In any event, if a curing compound has been used to slow down water loss as a means of enhancing the curing process, it is incumbent upon the flooring professional involved to remove the compound before he proceeds with a ceramic tile installation. Failure to remove the curing compound virtually ensures a loss of mortar bond at some point in the future.
Oxidized curing compounds can be removed from a slab surface by applying a common alkaline cleaner that contains surfactants. Surfactants promote an action that "lifts" the oxidizing compound from the concrete pores. Scarifying, via methods such as bead blasting, should be used to remove other types of curing compounds.
Through ANSI A.108, the ceramic industry warns that liquid curing compounds or other coatings should not be used on concrete slabs because they prevent bonding. To check the slab for the presence of such a coating, drip a few drops of water on its surface. If the water beads, it's highly likely that a coating was applied.
Shrinkage cracks in the slab may also present a floor prep problem. In this case, the concern is that any continuation of the shrinkage and further opening of the crack(s) may transmit stresses through the mortar and cause cracking in the finished tile installation. In essence, the tile will reflect the conditions in the slab beneath it.
Such a condition may require that a crack-isolation membrane be used. Many such membranes are available and they come in either a liquid or sheet-membrane form. Unfortunately, an official industry standard is still under consideration by members of the Materials and Methods Association. However, a number of crack-isolation products on the market have established histories of success.
It's important to discuss with your supplier the slab problem you're facing so that the both of you can evaluate the membrane warranties, particularly with regard to the width of the crack(s). I also must emphasize that you should use only the type of products that have been specifically designed for crack isolation. Do not use building paper or any other material that's subject to degradation from exposure to a current or future exterior source of water/vapor that penetrates the slab.
Remember to ascertain the age of the slab, and determine whether the shrinkage has stopped. There have been cases in which tile floors buckled solely due to slab shrinkage. Another important aspect of floor preparation is to make sure that the expansion and control joints are placed correctly.
An additional factor you need to consider is the levelness of the concrete slab. With increased use of larger-format tiles and marble, a level floor is very important. Lack of levelness may promote cracking or lippage. If the concrete slab has a layer of self-leveling cement on it, make sure you perform a sounding test to determine whether any hollow spots exist. These hollow areas will eventually cause the self-leveler to delaminate from the slab. Remember, the floor is only as good as what it is stuck to.
When embarking on ceramic tile and marble installations, you should always keep copies of the Tile Council Handbook and ANSI A1.08 on hand for instant reference. Page 11 of the Handbook features a floor tiling installation guide that will aid you in job prepping. And within the body of the book, you'll find detailed references to almost every type of installation.
When used in combination with a proper membrane, cement backerboard can provide a waterproof floor. Coated glass mat backerboard, on the other hand, may be used only in dry areas and with tiles that are 2 inches square or larger.
Whichever type you use, make sure that the ceramic tile is set on the positive moisture barrier side of the backer unit. It's also important to comply with the 1/360 deflection requirement. Joints between the cement backerboard units should be filled with latex-Portland cement mortar. I prefer to use a latex-Portland cement mortar in concert with nails to bond the units to the subfloor. The subfloor should be a 5/8-inch exterior-grade plywood on joists with 16 inch o.c. If you are installing tile on cement backerboard with organic mastics (Type I), make sure that the backerboard is not wet or a failure may occur.
Let's again turn our attention to concrete slabs on grade for a brief discussion of installations that use organic or epoxy adhesives. This is addressed in TCA F116-2K.
It's important to note that organic mastics for floors are restricted to residential uses only. In such uses, it's also important to confirm that the slab has cured and that no moisture problem is present from external sources. If you cannot confirm this, problems due to moisture and alkali attack are likely to crop up.
And as I noted earlier, it's also important to determine whether a curing compound has been applied to the concrete. If so, it must be removed for two reasons: to achieve a suitable bond, and to obtain an accurate reading of any moisture transmission. It's also vital that you clean the slab, as alkali dust may be present as the result of free water evaporation from the concrete. That alkali dust can have a corrosive effect on the adhesive you use.
In conclusion, I very strongly suggest that you create your own job-prep checklist so that you can be sure that you've considered all potential reasons for any installation-related problems that may arise. Don't forget the old bromide: forewarned is forearmed!