Last year, I installed 18" porcelain tile in two bathrooms and a laundry room. Now, the grout has cracked along several joints and there is a crunching sound when you step on the affected tiles. I installed the tile on concrete. I thought I did a pretty good job of getting up all the glue(from vinyl) and paint off the concrete. There was one hair-line crack in each of the bathrooms. The crackes were very small and I did not use a crack isolation membrane. Also, I used a modified thinset. Does anybody have any guesses? I'm thinking either the concrete wasn't clean enough or the small cracks have become larger. What can I expect down the road? Any solutions would be greatly appreciated!

 

There could be several causes to this, one being the cleanliness of the slab causing a shearing of the thinset from the slab. Another could be the thinset skinned over before you set the tile causing the tile to shear from the thinset. Another could be there was insufficient coverage of thinset to the tile. Porcelain is much harder to bond and benifits greatly from burning the thinset into the back of the tile before setting into your combed trowel lines. Another could be the wrong sized trowel. The absolute minimum for that size would be 1/4x3/8 square notch but larger tiles generally have larger lugs on the backs to offer greater contact area with the thinset, but without burning (flat troweling the backs to get a good layer of thinset contact and filled lugs) can cause bonding issues. On that sized tile, I'd have gone with a 1/2" square notch. All of these issues will be exasperated if you tiled tight to the wall or filled the periter around the walls with grout. The pressure of the walls as they expand seasonally will create a tremendous force along the shear plane. Even with all the proper techniques, not allowing expansion will cause the floor to fail from stresses exerted by the walls. First step is to cut out the grout around one of the affected tile and judge if you have 90% contact to the tile or if the shear occurred from the slab or from the tile. If your perimeter walls are grouted, remove all the perimeter grout, loose tiles (Some may be able to be salvaged and some might need to be replaced) Clean up the slab and tile and reset, and regrout, leaving the perimeter clear of anything rigid. You can use caulk to fill the gap around the walls or leave it open and cover with trim.