polished concrete floor effect on an actual concrete floor

[Barbicanite]

Short version:

I'm planning to pour Mapei 3240 SLC onto the rather rough concrete screed (which is damaged in some places) in my flat, in which are laid underfloor heating cables. 3240 has a compression strength of about 25N/mm^2 and it's flexible so I hope it might be suitable as a finished floor, but if not how should I finish it? I'm looking for a smooth finish suitable for a domestic flat that's easy to clean, doesn't show up dust or debris too much, and looks "neutral" or "industrial". I'm going to do two small experimental areas (the kitchen and hallway) before committing to the whole flat if they work out. Previously I tried simply painting these areas, but the paint shows the roughness and imperfections of the concrete screed, and the paint collects marks that are hard to clean off.

Any advice very welcome!

More details in the long version:
My flat in the Barbican is built of solid concrete, with electric
underfloor heating cables laid under about 40mm of concrete screed.
The flat is mostly carpeted, with gripper rods glued to the screed
back in 1973 when it was built, so they are hard to remove. Bashing
the carpet gripper rods with a scraper and a mallet tends to either
leave some of the adhesive behind on the screed, or to bring up some
chunks of crumbly screed, so something would be needed to smooth or
fill the edges of the screed once I've removed all the gripper rods.
Also there's an expansion gap, which could perhaps be filled with a
sufficiently flexible material. The screed itself is very flat but
it's got quite a grainy surface.

I experimented with "Rustoleum chalky finish floor paint" about 5
years ago in a couple of small rooms. In the entrance hall I pulled up
the gripper rods and filled and smoothed the damaged screed edges with
Ardex feather finish, which I found quite satisfying so I ended up
smoothing a thin layer across the whole surface of the screed. I
painted directly onto this. The paint looked nice but shows my poor
skimming skills: you can see trowel marks if the light is right, and
you can see brush marks in the paint where I was lazy. After 5 years
the floor is smooth but quite grubby. It cleans up with soap and water
and elbow grease but it's not a low-maintenance surface.

In the kitchen there was vinyl stuck directly to the screed. No
gripper rods, so I didn't skim the floor, I just pulled up the vinyl,
scraped off the worst of the adhesive and painted the Rustoleum
straight on. It looked OK, though you could see the traces of the
notched-troweled vinyl adhesive showing through. The paint surface is
rougher than the entrance, because I painted directly onto the screed
and so all the little pits in the surface fill with kitchen muck. It's
hard to clean, and honestly I'd just paint on another coat if King
Charles was coming round for tea, rather than try to clean it up with
a mop.

OK, that's the story so far! I want to do the whole flat (65 sqm), but
I don't want to do it all at once, as I live here and would need to
move furniture around to do a section at a time. I would consider
doing a single room as an experiment to see how I like it, as I have
previously (described above). Also, I don't mind taking a bit of a
risk as long as I'm not spending too much time or money on it. For
comparison, my neighbour recently had a resin floor professionally
done. It looks amazing but cost £15k and of course he had to empty
everything out of his flat and vacate for a month, so I want to DIY something
that's >50% as good for <10% of the cost!

 

[Deleted member 174758]

Also, I don't mind taking a bit of a
risk as long as I'm not spending too much time or money on it...

...so I want to DIY something
that's >50% as good for <10% of the cost!

Yes. Right. My experience of commercial resin flooring is that the end result comes from a combination of flawless prep work and the skill of the installer(s). The material costs in my area of the UK, however, are a lot more than 10% of the total cost as far as I can see. So whilst I think a month duration is slightly ridiculous (it should possibly be more like 2 to 3 days a floor, after the prep work has been done), I think your ideas are still somewhat wrong headed.

SLCs are invariably laid before installing a finished floor (e.g. LVT, linoleum, carpet, etc) - left in place on a sub-floor for any time without the finished floor being installed on top and they all seem prone to chipping if stuff is dropped on them, wear and they do generate dust (if used for any volume of traffic). Hence floorers getting the hump if we damage their SLC. So, does the manufacturer state anywhere that this product is suitable for use as the finished floor? Because I can't see any such recommendation in their spec sheets

 

[Barbicanite]

Thank you Job. You’re absolutely right—Mapei do not suggest 3240 can be used without a finishing layer. And that why I’m asking for your help. How can I finish it? A company called Resincoat produce coloured and clear resin finishes that appear to be compatible, they also do a 1-part floor paint. Does anyone have experience of finishing an SLC with this kind of product?

I’m a bit wary of mere “paint” as by previous experiment of painting the rough screed directly didn’t work very well, though the paint adhered beautifully (annoyingly, as now I have to get it off before priming for the SLC…)

Thanks again.

D

 

[Deleted member 174758]

Whatever you use you'll need to do a test to see if your chosen resin coat is compatible with the SLC. What do Resincoat say, and do they offer a barrier coating to overcome compatibility issues?

BTW resins often generate nasty chemicals whilst they cure, so industrial fume extraction to outside is an absolute must, togethervwith adequate masks

I am hesitant to say this, but maybe you could consider something such as an epoxy flooring coating, as sold for industrial (factory, workshop, etc) floors. They will often go over screeded floors, but it will still be necessary to prep the floor to a very high standard in the first case and the results might look a tad industrial for you. Also 2 coats are normally required.

 

[Barbicanite]

Thanks again! I was looking at industrial epoxy floor coatings from companies including Watco, where I came across their floor repair self levelling compound, Flowtop. This seems like a much better choice than the Mapei 3240 for my purposes. Most importantly, it is suitable as a finished floor surface without further coating, though it can also be finished with an epoxy coating or another suitable floor paint. However, I’m attracted to the raw concrete floor look. It comes in several colour options but I like the natural one.

The instructions indicate it’s quite straightforward to use—the concrete to be repaired should be degreased and as much old paint scraped off as possible but it doesn’t need to be completely stripped or sanded back down to the virgin concrete screed. I’ll give them a call tomorrow.
D

 

[Barbicanite]

Talked to Watco. They advised that indeed it's not essential to remove all the old paint, but it should be sanded or roughened to give the Flowtop a good key. Since the old paint will have already sealed the underlying screed, it's not necessary to apply the Primer 1, just the Primer 2 (P1 will be needed for the screed under the carpet, which has never been painted).

They warned me that if the old underlying screed is too soft or crumbly, the Flowtop will crack. This is difficult for me to assess. The cement screed was laid in 1973 but it's been perfectly dry since then. There are a few cracks, which of course I'll repair, and I'll make good the damage at the edges where the carpet gripper rods were (or will be) pried up.

I think I'll try my two test areas, the kitchen and hallway, and see how they look and hold up to feet, splashes, and the winter underfloor heating. If all good in a year, I'll do the rest of the flat. Obviously the larger areas are more likely to crack, but if that happens, I'll just have to lay carpet again.

Incidentally, I compared the published strength specs of Mapei 3240 (flex 5.5N/mm^2, compression 28N/mm^2) with Watco Flextop (Flex 12.8 N/mm^2 Compression 29.8 N/mm^2) and they're not much different. I know there's more to being a suitable surface than that, such as impact resistance and non-dusting, but I expected the specs to be way different.

Thanks for the advice so far.