Tiling over old concrete slab


Old 12-30-19, 10:14 AM
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Tiling over old concrete slab

I'm planning to install tiles for my garage/workshop floor, which is currently an old 1950s concrete slab. Being that old, it has a bunch of stains, etc. and it also looks like it was sealed at some point, but I have no idea with what.

My original idea was:
1. Strip away the concrete sealer to get a nice bonding surface.
2. Lay a mortar bed.
3. Install tiles.

For #1, I was trying different kinds of concrete sealer strippers, and to test if it worked, I would pour some muriatic acid and look for fizzling. But so far, I've tried different paint strippers and they haven't really worked.

And then upon further research, I read that mortar doesn't really like to stick to old concrete, even if it's not sealed. Is that true?

All that is getting me very confused, so my questions are:

1. Should I try a mechanical approach to prepare the concrete surface?
2. If a chemical stripper is still the best approach, do you have a good recommendation?
3. Does the above not matter because mortar won't stick to the old concrete anyway?
4. What kind of mortar should I use?

Thanks for any advice, and happy new year everyone!
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Old 12-30-19, 11:22 AM
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I would pressure wash it, then spray muriatic acid over the entire flooring rinse it off with clean water, allow it to dry then, look for any remaining high spots of residue from over the years, grind this spot down to the bare concrete.. You don't want to put mortar over the concrete, you may add a bonding agent to the thin set for the tile. But, don't put a mortar bed in unless you can achieve 2"-3" of mortar. As an architect I would not recommend the mortar.

Also, what size tile are you installing?
Old 12-30-19, 11:55 AM
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Thanks! The tiles are 3"x3".

Most of the slab is pretty flat, but there is one area with a substantial crack and may not be even. Can I put a self-leveling layer before laying the thinset? (I read that thinset shouldn't be more than 3/16" or so and I was worried that might not be thick enough to level everything in some spots?)
Old 12-30-19, 12:14 PM
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I read that mortar doesn't really like to stick to old concrete
Mortar on concrete is actually very good but as you have noted you've go how many decades of crap on the concrete?

Are you planning to park cars on this?

If you really want to clean the old crud off, you will need to mechanically grind it off, sandblast, or a combination of both.

Acid is not going to be enough to remove!
Old 12-30-19, 12:25 PM
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I would rent a surface grinder and then pressure wash. Surface grinders are available at most tool rental places.
Old 12-30-19, 12:34 PM
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Thanks--kinda what I'd feared given how old the concrete is, but perhaps good that I don't have to deal with nasty chemicals.

Are you planning to park cars on this?
Yes, but maybe once in a blue moon (1-2 times a year). Most of the time it's just my workshop. But yes, the floor would still have to be strong enough to support a standard car.
Old 01-01-20, 09:01 AM
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best bet's grinding the conc & repair the crks,,, then resurface w/polymer-modified cement overlay mtls
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