Luigis Answer: More often than not the problem is not with the tile itself but with the material below the tile and because the real cause is hidden below tile, it's difficult to investigate. The tile is likely to be showing the stresses beneath it or the result of a poor installation job.
What causes this problem?
There are seven likely causes:
Cause 1: The concrete substrate didn't cure for long enough.
Newly poured concrete is full of water. As the concrete cures, the water evaporates and the concrete shrinks. New concrete should cure for as long as possible or at least for 28 days. If your customer has a new home and the tile has cracked, it is a distinct possibility that the concrete did not cure long enough. A moisture test will clarify the issue.
Cause 2: The concrete substrate is cracked.
Almost all plain concrete basement floors, driveways and patios have a few long cracks running through them especially if it the concrete is a few years old, Even though concrete seems like the perfect substrate for tile, it too has its own problems. When concrete substrate cracks, this is transmitted to the tile above in the form of a reflective crack. If the tile cracks are long, continuous, and extending across multiple tiles, likelihood is high that the concrete below has cracked. The way to eliminate this happening is to install a crack isolation membrane during installation.
Cause 3: A hard, sharp knock.
If the crack is located in one area, extending across only a single tile, this is probably the case. Especially in kitchens, where heavy objects such as cans, pots, and pans get dropped, ceramic tiles frequently break. Standards do not regulate sharp blows to tile. Sometimes, you will also see a chip taken out of the tile where the object hit. This counts as normal wear and tear. We encourage all customers to keep spare replacement tiles at the completion of their project. It is very unlikely you will find more stock of the same batch of tile you originally received.
Cause 4: A very heavy load.
Tiles are required to be in compliance with breaking strength standards. Most tiles meet and exceed these standards. Even commercial kitchen equipment like refrigerators don't exert enough ground pressure to exceed the breaking strength of tiles. Unless there is something really heavy on site this is probably the most unlikely cause. One thing which might do it is industrial machinery or commercial sliding security filing racks. Paper gets very heavy! Tiles are simply not the right flooring to use under heavy industrial equipment.
Cause 5: Tiles installed over a concrete control joint
Control joints are expansion gaps to allow for heat expansion of concrete slabs. Since it is almost certain that concrete will crack, expansion gaps are effectively pre-planned cracks. They are intended to create a weakened area in the concrete and regulate where cracks will occur, normally in a straight line. It is not sensible or good practice to use tile to bridge a line that you know in advance will expand.
Cause 6: Tiles have lifted and cracked
This is quite common. Lifting (called tenting) will lead to cracking. Tenting is primarily caused by poor installation or inadequate grouting and expansion joints at the edges of the room and at room entrances or along very large or long rooms like passages. Of course if tiles lift they are no longer firmly based and bonded to the substrate so hollow sounding spaces start appearing underneath. This in turn means that if weight is placed on the tile it is prone to crack. The forces created by tiles expanding against each other may in themselves be enough to crack tiles. Improper use of adhesives can also be the cause e.g. Using the spot method where only five spots of adhesive are used instead of firmly bedding the entire tile.
Cause 7: Sub-standard tiles
This is unlikely but it is what many customers initially believe to be cause. Because the tile is cracked, and the tile is the only visible portion of the installation layers from substrate to tile, most homeowners assume that the tile is at fault. Nonetheless double check that the tiles you supplied are in compliance with ISO, ANSI or ASTM testing standards. It is possible that transit damaged can cause small hair line cracks in a tile that if the tiler doesnt identify can be installed. This is not considered a tile fault and a simple replacement is necessary. What is unfortunately quite common is an absorbent wall tile may have been foolishly installed as floor tiles!