Patches Of Loose Concrete Substrate

[JonR]

Hello,

I am looking for a bit of advice regarding patches of loose concrete substrate on a floor I am looking to tile. I have included some pictures to show the general state of the substrate whilst also showing particular areas of concern.

I have recently lifted the downstairs laminate, which was already down when I purchased the house. I am looking to tile the area with some 660mm x 400mm porcelain tiles. To my shock there are patches of loose concrete substrate under what appears to be some sort of liquid levelling compound, which is spread across the whole floor. In quite a few places where this compound hasn't quite broken up, it is hollow sounding to the tap.

So I am looking for some advice on what the best plan of action would be. Chisel off this compound from the whole floor? Break up the loose / hollow sounding parts and fill with new SLC? One thing to note, in places where the compound has broken up, the resulting concrete underneath is quite powdery and loose. So this may be the root cause of the problem.

Any help is much appreciated.


Jon

 

[Tile seller]

I think your best to try and get that black stuff up. I don't know what it is but I'm guessing it's some sort of sealer.
Hopefully it comes off easily but, I'm sure some will peal off and some will be well stuck.

 

[White Room]

As above remove any loose areas of loose, the concrete remove any loose areas from that and stabilise with a 5/1 sbr.

 

[JonR]

Thanks for the supplies so far, that all seems logical. I am looking at using Keraflex Maxi adhesive for securing the tiles. I notice Mapei do a product that may be suitable for the underlying concrete, would you recommend stabilizing the powdery concrete below with "prosfas" or a similar product, or would the SLC be suitable over the top?

 

[LM]

Im doing a job right now as it turns out that has the same problem, it's an upstairs night club of an area of about 400 mts, the original screed is failing in parts badly. Re lifting the whole screed and relaying a new one simply isn't an option with this contract. The way we are getting round it is to litterly soak the entire screed with a first coat of Bottament D11 dilutted 2:1, then a second coat of neat D11. This soaks right down through the screed and stabilises it, it ends up stronger than steel if you get me. I'll attach some pics for you.
It was so poor that when we started to investigate the composition of the screed we could literally vacuum it away. The reason the slc has fails on your job is more than likely down to the fact that that it was done over a 'Pourus and Dusty substrate'

 

[JonR]

Thanks for your help Lee, I feel a little bit better about my situation now after seeing what you have had to deal with. Pictures speak a thousands words!

 

[White Room]

The screed is kak, breaking up with no strength in it.

 

[Flintstone]

I'd get all the loose up and any that comes up easily with a scraper etc, clean it up and prime it with sbr until it's had a good soaking and it's not dusty, then level it again with some good slc, get it nice and you'll have a good surface to tile on

 

[JonR]

I have spent some of today starting to remove the old levelling compound that doesn't seem to have adhered very well. There are patches where it's a bit more bonded but I'm going to take it all up and start a fresh, prime the original screed as suggested and then level the lot. Hopefully that way I will be starting from pretty much a sound base and take it from there. I will let you all know how I get on. Should be tiling at the weekend, so we shall see how it all goes.

Thanks again.