What caused the Tile Floor to Tent up / Debond?

QUESTION

Hi, I was just at my friend's home to look at her "tenting" tiles...
Tile was installed ~20 yrs ago.
For YEARS she's had a fish tank on the floor (w/stand) that weighed almost 1/2 ton.
3 weeks ago she sold it.
Soon after that the ~15" square tile IN FRONT of where the fish tank was started tenting & cracking.
I removed some of the tile & the floor under the tile was smooth, level, *very* solid & completely dry.
Could the foundation under the fish tank have cracked, and then removal of the tank caused the crack to expand (causing the tenting in front of where the tank was)? Or does anyone have any other ideas?
PS: The area of tenting/cracking is slowly expanding.

Thank you very much!

ANSWER

ANSWER - Tiles tent for several compounding reasons.

Lack of Movement Joints (also known as expansion joints) - When tiles absorb moisture or gain temperature they expand.  The more porous the tile the more it will tend to expand under those conditions.  Movement joints are installed every 8' to 25' in each direction in the field of tile, and at perimeters or restraining surfaces that the tile abuts.  In lieu of filling these grout joints with a cementitious grout, they are filled with a resilient sealant (caulking) that meets ASTM C920 standard.  Typically that is a silicone or urethane sealant.  The resilient joints mitigate the stress that the tile is subjected to from expansion.  If you don't have movement joints then tile is under compression stress.

Lack of Tile Adhesive Bond Strength - Depending on what substrate the tile is bonded to, and how well it is bonded to it, the tile may be restrained to some degree from expanding.  If the stress the tile is subjected to is greater than the tile adhesive bond strength, then the tile will debond and tent if there are no movement joints to mitigate the stress.

Substrate is Resilient - If the tile is bonded to a resilient substrate, such as a crack isolation membrane versus a non-resilient concrete substrate, then when the tile expands it can only limitedly restrain the tile.  In other words, it will allow the tile to move to some degree, which could be problematic.

Chances are that the referenced tile installation did not have movement joints and over the years was subjected to expansion/stress for one reason or the other.  So the tile had pent-up stress and was being held down by the heavy fish tank.  When the heavy fish tank was removed the tile bond was not great enough to restrain the tile, so the tile tented-up, which caused the cracking.  It is not unusual for the tenting to expand if you do not relieve the stress in some way such as removing the tented tiles.  If other tiles adjacent to the tented tiles sound hollow, it might be an indication they too have lost their bond.

The only thing to do is remove the loose tiles, install movement joints, and re-install the tiles and hope that the damage is limited.

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