Dusty Concrete Floor

[Ex-repairman]

Hi all. We had a conservatory built around seven weeks ago and have arranged for a tiler to come in another couple of weeks to do the floor. I have two questions you knowledgeable lot may be able to help with:

1. Is 9 or 10 weeks sufficient drying out time for the floor? At it's deepest I think the slab was around 5" thick with DPC under.

2. The surface is very dusty. We brush it then a couple of days later it needs doing again. A scratch with a key leaves a mark. Is it possible that the mix was weakened by rain sitting on it when it was laid, it was torential!! Obviously this needs to be sealed before the tiles are laid, can anyone advise of a product that will remedy this or are we doomed to never be able to succesfully tile the floor?

Any advice you can offer will be much appreciated.

 

[noseall]

i would have thought a bit of aggressive brushing to remove the loose stuff with a diluted coat of pva or floor tile primer to stabilize, will do the trick.

 

[Ex-repairman]

i would have thought a bit of aggressive brushing to remove the loose stuff with a diluted coat of pva or floor tile primer to stabilize, will do the trick.

Thanks Noseall, do you think it is likely that it was the rain which just weakened the surface mix then?

 

[noseall]

absolutely 100%.

if i want to get a nice tight surface on trowelled concrete then i will need to mix it fatty but relatively dry. at no time do i want a lot of water near the surface as this spoils the finish.

powerfloated floors are left to dry for hours before the mechanical float can get to work.

i'm afraid the rain killed the surface of your floor.

 

[Ex-repairman]

mmmmmm.... had a horrible feeling that was the case!! Just been reading an article on 'laitance' on the net which suggests shot blasting to remove it, although I think this was for laying a vinyl floor.

Have got the tiler coming around on Saturday to have a look. Pending what he says will probably take your advice and get scrubbing!!

Many thanks.

 

[^woody^]

I doubt that rain has weakened the floor

Concrete surface is generally powdery when unfinished, and unless you trowel it smooth to give it a bit of a wearing surface (by bringing more cement to the surface), or vibrate it to get all the air out, then you will always get a [relatively] soft surface that will be 'powdery'

The floor is not weakened, all that has happened is that the resistance to surface wearing has been reduced

All that needs to be done is the loose grains to be brushed off and then you can lay the floor adhesive, and this will bond the tiles no problem.

I would ignore that net article

If you are no longer getting condensation on the glass, then you can assume the floor is dry enough

 

[noseall]

what the surface water does is bu***r up the 'skin' of concrete that is trowelled to the surface. i agree it does not weaken or compromise the slab as a whole but can ruin the smooth effect of a trowelled slab.

 

[Ex-repairman]

Woody

Many thanks for your responce. Hopefully the tiler will agree with your thoughts, but on the phone he seemed a bit concerned. I'll post his comments once he's been around.

As far as whether the floor has dried out sufficiently you've concerned me slightly. We were getting all of the windows completely covered in condensation as you'd expect. At the moment we get an inch of light condensation on the bottom of each window first thing in the morning. In your experience would you think this is too soon?

I did try to do (what they say) is the ultimate test and stick a metre of plastic to the floor to leave overnight. If only I could have found something to stick it to the dusty floor with.............!!

Cheers,
Nick

 

[^woody^]

Your bit of condensation around the edge of the glazed unit is natural and more to do with the cold glass and unheated conservatory.

There is normally a lot more during the drying process as you have noticed, and the time since the floor was laid is more or less long enough.

You still want to be leaving the windows open or in the night vent positon (locked but slightly open) to keep the air moving.

 

[Ex-repairman]

Tiler finally came around last week and seems to think that the floor shouldn't be a problem, although wants me to brush it and give it a coat of PVA (one part PVA to five water I guess). Is there any benefit to be had by giving it two coats?

I tried to get some reassurance from him but when I asked if they would go down and STAY down he said " I think so......well long enougth for the cheque to clear ha ha".

Cheers for all the advice.

Nick

 

[masona]

How about scrape all the loose stuff then use floor levelling compound over it?

 

[Ex-repairman]

Cheers Masona, may have been a possibility but running out of time to be honest. May scrape as much loose stuff off and then PVA.

May even just be worrying about nothing!!!

Nick