Concrete will crack. It is the nature of the product. And while proper curing and construction practices will negate the most serious offenders, it is unrealistic to expect the material to remain crack-free.
Cracks occur during two phases of curing. Plastic shrinkage happens soon after the concrete is placed, the result of rapid water loss either through absorption by the concrete forms or sub-grade or else by surface evaporation. Drying shrinkage occurs as the concrete is drying out while it firms up. In both cases, a loss of water means a loss of volume in the concrete. Control joints and steel reinforcing can help alleviate the problem.
A ceramic or stone floor may be at risk from cracking when a thin-bed installation is used rather than a mortar bed. All mortar-bed installations require a cleavage membrane along with reinforcing mesh (e.g. TCA detail F-111). However, this method is not only costly but requires a minimum mortar-bed thickness of 1 1/4 inches, with a maximum of 2 inches.
Flexural concrete cracking is a concern with suspended floors. Here is where the industry standard of L/360 (FCI January/February 2002, pg. 20) comes into play, to help eliminate flexural and tile cracks.
The crack-isolation committee of the Materials and Methods Standards Association (MMSA) that has been attempting to establish standards for crack-isolation materials has yet to reach a consensus. The committee will be meeting once again during the Coverings International Tile and Stone Exposition in Orlando, Fla., May 6-9.
I must point out that any crack-isolation system now on the market and those on their way are only capable of reducing or eliminating cracks in stone or tile caused by horizontal movement of the substrate. Vertical cracking of the substrate cannot be controlled.
Another important point to remember is that almost all manufacturers of crack-isolation membranes restrict their use to cracks in the substrate of 1/8-inch or less (note: There is one provider that allows up to 1/4-inch).
Why do we need a crack-isolation membrane? We never used to need one. The answer is simply that slabs are thinner and tile sizes have changed: The old 10-by-10s are not as popular, stone is growing in popularity and thin is in. Tiles are thinner than in the past and, when installed using the thin-bed method rather than the mortar method, the possibility of cracking becomes more plausible.
I recommend that you do not create your own crack-isolation system. There are many systems on the market that are effective. As a matter of fact, there are now so many crack-isolation systems being marketed that it is difficult to choose the correct fit for your situation, in terms of both cost and labor. One firm has a liquid-only (acrylic) system that, for user-friendly reasons, turns red when it is completely dry to indicate that the tile installation can begin. The product requires an initial coat that dries in 1 1/2-to-2 hours, after which a second coat is applied.
There are also liquid-mortar combinations currently in use, some of which may be applied without the need for re-enforcing mesh.
The sheet-membrane product line includes polyvinyl, polyethylene, bitumen-modified and butyl. These membranes may be self-stick or they may require a bonding material. Cork is also being used. However, cork is thicker and may be subject to corner cracking.
Whatever product you choose, remember to follow these checkpoints:
I always like to point out that you are selling your time. And as there are only so many hours in the day, be sure that, when it comes time to make the decisions, you pick smart and work smart.