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onthejob 03-25-2012 10:34 AM

Concrete prep for outdoor tile
I plan on installing ceramic outdoor tiles on my terrace. I'm in Rochester, NY and I have researched which specific outdoor tile I will use. My problem is the concrete I want to put the tile on.
I recently purchased a 1920 Tudor with a concrete ground level terrace that is about 365 sq ft which I would like to install tile on. Below the terrace is a partially heated garage and part of the basement. Half the terrace has a roof over it and the other open half has two floor drains. When I try to flood the terrace the water does flow off the surface so there is some pitch in all the right directions. There was outdoor green carpet glued to the concrete which I have removed. I would estimate this carpet was installed about 20 years ago with some incredibly strong industrial strength adhesive. A majority of this adhesive remains on the concrete surface and beneath this I can see two shades of green paint I believe to be oil based.
Knowing the best way to set tiles over concrete would be to get the surface of the concrete down to bare concrete to accept the thin-set. I first tried scrapping with an 8 inch razor floor scrapper and this did not budge any of the adhesive or paint that wasnít already loose. I was only able to bring up a very small amount and could not get down to bare concrete. Then I tried a few different chemicals and was unsuccessful. Next a grinder with a carbide scrapping head and that didnít work at all. Finally I used a 3500 psi pressure washer and that took up just a small amount of the loose stuff that I missed with the scrapper. Probably 80-90% of the adhesive remains secured to the concrete surface which is porous because it was troweled on in a circular fashion.
My question is with this adhesive so well anchored to the concrete surface will it accept and properly secure the thin-set? Should I try a bonding adhesive first? Or use resurfacing concrete over it with a boomed surface? Or should I rent ($200+) a scarifier and remove/rough off 3/8Ē of the surface along with all the paint and adhesive before thin-set? I thought of Ditra but will add that adds more than $600 to the project and not really sure that's the answer.
I apologize in advance if this topic has been covered before but I couldn't find the answer I was looking for.

Rochester, NY

oh'mike 03-25-2012 02:34 PM

Did you try a commercial floor buffer with a set of carborundum stones? that's a pretty good method for cleaning up a concrete surface.

I can't guide you on setting over the old adhesive---I've only done a few outdoor tile jobs and the surface was good and clean already.

onthejob 03-25-2012 04:12 PM

Thanks, actually that was one of the suggestions United Rental came up with but warned the adhesive may heat up into gumball sized balls and just clog up the what he called number 8 grit disk quickly. Not sure if this is comparable to carborundum stones but I don't remember him saying carborundum stones were option for the sanding disk. Anyway to test this theory I got some 40 grit sandpaper for my palm sander and tried it out. Chunks of adhesive began to stick to the sandpaper making it ineffective in a short period of time. Reluctantly, that's why I was considering the concrete grinder to scarify the surface if I positively need to get down to bare concrete to set tile.

oh'mike 03-25-2012 06:48 PM

This disk I speak of is a wood disk with actual blocks of carborundum attached---Great for knocking off old thinset.

It might also end up a gummed up mess---Never tried that----a sand blaster would be great but the cost would be high.

onthejob 03-25-2012 08:43 PM

Never heard of or seen it but I like how it sounds. Agreed with the sand blasting idea. My next thought was a shot blaster just quick way to rough up the surface but still in the $200+ price range.

mnp13 03-27-2012 12:37 PM

If there is paint under the adhesive, then I would say that the easiest way to get the adhesive off is to get the paint off.

Have you tried working stripper under the adhesive to get at the actual paint?

rjniles 03-27-2012 02:05 PM

Do not use ceramic tiles outside, they are porous and will absorb water, freeze and crack. Us porcelain tiles.

onthejob 03-29-2012 10:14 AM

Iím trying to avoid using chemicals because I have to use thin-set to set tiles and I donít want to take the chance the chemicals wonít let the thin-set bond to the concrete. The rental company said the concrete grinder/scarifier or floor buffer will gum up with the adhesive and be useless. They reccommneded a shotblaster. Of course no one in my area rents shotblasters which sounds like my best option. Someone suggested a hammer drill with the chisel tip to scrap it up. Might have to give that a try.

mnp13 03-29-2012 12:34 PM

After you neutralize the stripper there shouldn't be an issue. I'm not really sure what would be in stripper that would stop thin set from adhering though.

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