Re: "Here is a question I would like to have an answer to: What is wrong with my floor? Tile floor installed over concrete subfloor.Some of the tiles when knocked on have a hollow sound and others stepped on give of a sound as when they are laying in sand. Is it a installation error or problem? Please advice and give me a tip how to solve this problem!"

In order for me to more accurately access the problem I would need more information, however for now, I will assume the floor was installed with ceramic or porcelain tiles using thinset mortar and cementuous grout.

Ideally, tile floors should not sound hollow when knocked on. Realistically, however, this is not always the case. A hollow sounding tile simply means there is no bond between the tile and concrete slab over that spot. If only part of the tile sounds hollow, there is no cause for alarm; your tile will likely remain firmly attached. If, however, the entire tile sounds hollow, than the bond has been completely broken. In this case, the culprit is an issue with a settling slab, faulty installation, or a combination of both.

In the case of a settling slab, your floor shifted slightly (horizontally and/or vertically), which separated the bond. If the movement is great enough, it can create loose tiles, regardless of how well your tile was installed. On that note, larger floors should have expansion joints consisting of a flexible caulking (rather than grout) every twenty feet or so to allow for slight expansion/contraction but again, not foolproof against significant movement.

In the case of a faulty installation, the problem could be a number of things. We already talked about expansion joints. Another issue could be that the tiles were not laid down fast enough into the bed of thinset. In this case, the mortar may have skinned-over thus preventing a bond between the mortar and tile. Another possibility is that the thinset was not initially bonded well to the slab due to surface contaminates such as paint, dust, etc. Finally, it could be due to insufficient thinset coverage of the tile and/or too thin of a layer of thinset (i.e.: too small of a notch trowel used).

So, how do you find out the cause? Unfortunately, this is nearly impossible since there are so many variables at play. My recommendation for you would be to call your tile company back out, assuming it is still under warrantee and try to get them to solve this for you. If the tiles are making strange sounds when you walk on them then most likely the thinset has become loose and you are hearing it crunch beneath the tile.

If the tiles are truly loose and you are looking for an easy self fix. I would recommend this product where you drill holes into the joints and inject a liquid adhesive to rebond the tiles. This method will not eliminate hollow sounds but it will help to rebond your tiles.

Here is the link:
www.iafsystems.com/html/floor_repa...

great info!  thanks for this post.