Often, your cracked grout will need to be inspected to determine the exact cause, but there are some common causes. You don't want to blindly guess or assume you know what the problem is, but here are some things to consider:
Common Causes of Cracked Grout
Too much water or polymer additive in the grout mix is a common problem. Cracked grout is frequently the result of weak grout structure. The liquid in the grout mixture inevitably evaporates, as it was designed to do, but excessive evaporate can leave pinholes that will leave your grout vulnerable to cracking.
Insufficient thinset (the glue applied over the mortar) applied during the installation can lead to cracked grout. As the layer of thinset dries, it contracts and pulls the tile tight to the mortar. However, if it was not thick enough, it will pull away from the tile and leave gaps of air under the tile. A crunching sound can result from the broken pieces of mortar, a sign that your grout is cracking and that soon the tiles will start to crack, as well.
The timing of cement hydration, part of the curing process is critical to creating strong grout. If water is added after cement hydration has already begun this is a recipe for disaster, as the grout is likely to be weak and crumble.
As you can see, most of these problems are the result of poor installation methods. If your floor or wall tile has been recently installed, your contractor may be held liable, even if the installation isn't under warranty. In this case, you should immediately inform the contractor, preferably in writing with the date you first noticed the problem should the situation not be taken care of immediately.
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Repairing Cracked Grout
Even properly installed tile floors and walls can be susceptible to cracking grout over time. Cracking grout is something you should take steps to deal with as soon as possible. The ability and cost to repair cracked grout is directly tied to how bad the situation has become. Worse yet, if the situation continues, the cracked grout will begin to affect the tile around it, causing cracks in it and requiring you to replace the tiling installation. This isn't cheap. For manageable repair projects, a contractor can often fix the grout for a reasonable price. While you have the contractor there, you should ask him or her about the general condition of the grout to anticipate how long you can expect the repair to last.
Replacing Cracked Grout
Your grout may be in such bad shape that completely replacing all the grout is the best way to ensure your tile will continue to serve you well. This project requires stripping and reapplying the grout, letting it cure, and then applying sealant. Obviously, it's going to cost you quite a bit more than simply repairing a few areas of cracking grout, but it's still a lot cheaper than having to retile your entire wall or floor.