Holes in concrete subfloor found when removing old tile.

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Old 03-14-17, 10:59 AM
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Holes in concrete subfloor found when removing old tile.

So in the process of removing my old tile there seems to be some holes/cracks in the seem between two slabs that run the length of my condo from one end to the other through the dining room kitchen and foyer. The worst parts seem to be in the dining room where there about three or four big holes about half inch wide by 3 to 4 inches long.

The holes go all the way through the subfloor and I'm pretty sure there's only empty space and metal studs/drywall separating our floor to the ceiling of the unit below so I'm trying to figure out how to fill that in without having any filler concrete/cement seep through to the ceiling of the neighbor below?

In doing some searching I found a foam backer tube that is used for concrete expansion joints that I think would do the trick to fill in the holes by stuffing it in to fill the bottom of the holes which I think would allow me to fill in the top half with cement.

That actually brings me to my next question. Would I need something more flexible to fill in the gaps/holes, or would I need a cement/concrete patch type of filler to do the trick to avoid future cracks?

Could I just use the mortar I'm planning to use for heavy tiles which I'm told is a little more flexible? Or some form of cement patching compound is need before the mortar is applied with the tiles?Name:  IMAG2605.jpg
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Old 03-14-17, 01:34 PM
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Welcome back. This appears to be a failed cold joint. Were any of the tile above it cracked or separated due to these cracks opening up? If not, you may be able to use backer rod stuffed down in it, but I would use a flexible product to fill it up. It may be best to see if the remainder of the seam is easily removable and do it all at one time.
 
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Old 03-14-17, 01:49 PM
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Thanks, and no, all those old tiles were on that floor solid with no cracks anywhere in that area. And I'm thinking they were the original tiles from the 70's when the condo was built. Aside from a few bad tiles in the kitchen and foyer, the rest were on there pretty good which is why I used a demo hammer which I think may have helped with some of the breaking up of the joint?

There is a slight hairline crack that runs through the kitchen to the foyer and coat closet but it's no where near as bad as the dining room. What product would you recommend to patch?
 
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Old 03-14-17, 02:48 PM
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IF it hasn't cracked tile in 40 years, you may could go the concrete patch route and just trowel it in (after backer rod) and smooth it out on the surface. Our church lobby was slab on grade and the tile guys tiled right over a movable expansion joint. Tiles didn't like it too much when the first winter hit.
 
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Old 03-14-17, 02:53 PM
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Got it, thanks for all your help and input.
 
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Old 03-14-17, 03:01 PM
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I does appear that they installed the tile so the holes were pretty much lined up with a grout line which would help them not crack with movement. You say there are cracks in other rooms. Interesting to know where the cracks were in relation to the holes under them. You may need a crack isolation membrane prior to installing your new tile.
 
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Old 03-14-17, 03:22 PM
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You could be right, unfortunately I can't remember how the other tiles were laid in relation to the cold joint but it's a possibility that they were laid the same way. Being that the mortar I'm to use has some flexibility built in, would it be enough to use that to fill in the gaps? I'll also look into that membrane. It won't add too much thickness though to the floor would it?
 
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Old 03-14-17, 03:36 PM
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You should be able to make out the outlines of the tiles on the concrete as a halo effect.

You can probably give yourself some piece of mind by applying Redgard to the concrete before installing your tile. RedGardŽ Crack Prevention and Waterproofing Membrane . Your fancy flex mortar is really designed for vertical flex on a framed wooden subfloor. Won't help much on a slab. In fact, you really don't have to use any super duper mortar on a slab. You could save some money and use a simple modified thinset and spend your money on Redgard crack isolation membrane.

If the movement was excessive you would step up to something like Ditra.
 
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Old 03-14-17, 03:37 PM
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The membrane is 1/8" thick, so it won't add a bunch to the flooring height. What type "mortar" are you thinking of using? LFT thinset or Versabond?
 
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Old 03-14-17, 03:47 PM
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Mapei ultraflex LFT. So are you saying to use the membrane on top of the concrete patch with the backer rod, or instead of?
 
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Old 03-14-17, 03:52 PM
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And just checked/measured from one of the faint tile lines in the kitchen and it looks like they tiled over the joint line by about 3" of the end of the tile.
 
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Old 03-14-17, 03:54 PM
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If the slab moves it will move regardless of what you do inside the cracks. Your tiles will span the gap so that means nothing. It is determining if you have movement that is of question. My guess is NO as you have a previous tiled floor with little issues, it is a poured slab that acts as a ceiling for the room below and it has fully cured since the build and is a steel reinforced slab.
 
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Old 03-14-17, 04:05 PM
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So that being said, I'd be just as good filling the gaps with whatever, as in a little extra mortar with the backer rod so that it doesn't seep through and just tile over it.
 
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