dusty concrete

[AndrewPatrol]

I'm extending a shed slab and the client wants me to seal his old slab which is as dusty as. Whats the go - do i just water blast it then paint with goo ( acrylic? ) I suspect the old slab has no plastic under - could this be why the floor has turned dusty - 'cos the moisture was sucked out too quick by ground soil. BTW he's doing up an old car in this shed, so is acrylic floor stuff ok to use?

[TermiMonster]

If a slab get rained on before it's gone off it will be soft/dusty. Can't really answer your other questions (the important ones)
TM

[johnc]

There is an etching wash you put down first, wash off then apply paint. Look at Dulux paving paint for starters the instructions pretty much say it all.

John.

[Theremin]

Pressure wash. Then etch with diluted hydochloric acid (1:10 rings a bell but the container will say). Then pressure wash again.

[Terrian]

'dusty' concrete could be because the slab was worked too much in the first place, no matter, as for sealing, a good sweep and apply the sealer, 20lt drum should be around the $120 mark for a decent sealer, depends on wether you want colour or clear. Can be applied with a normal everyday house broom, read the can for instructions

[billbeee]

Good concrete has a correct water/cement ratio.

Crap dusty concrete has usually had extra water added, one way or another. It could have been added at the truck to make it easier to pour, then the water rises to the surface so that the top few mm is diluted fines and little cement (laitence). Or water was added later by inexperienced finishers (going off on them too quick). Same deal, the cement is diluted out and a flaky surface is the result.

If it is fairly bad, and the owner wants it like a new floor, you might have to scabble the whole surface to shift the loose crap, then a possibility is to use an epoxy coating on top of a thin (soak in) epoxy primer. Of course this is a lot dearer than paint.

Acid etch with hydrochloric is a good preparation,(although even this won't shift oil stains) but watch the fumes and then rinse and rinse again. Don't leave any acid residue. It is not the best acid for attacking steel, but it does do it eventually.


Water based paints won't stick to oily concrete, use same with same, so use oil based paint if the old workshop floor is oil stained.

Best of luck with it.

Cheers
Bill

[AndrewPatrol]

Thanks all for your help. pour went well ( bloody heavy though, truck driver asked me if I wanted more water, thank god I said no) Surface finish went ok but could be a bit more level.

Tips to pass on for the day that I think i learnt:
1 .. someone said plastic under and not playing with surface too much will help to not create another dusty finish. seems to have held true but probably early days yet.
2.. I used treated pine fence plinth board for formwork ( cheap and straightish 6x1 ) with HW pegs about every 300mm and some bracing. worked well.
3.. covered slab too soon with plastic and it marked surface. wait till its gone off enough. lucky this is just a shed.
4.. should have probably gone over twice with screeding board to get surface flatter. timber float left a nice rough grippy surface.
5 .. ANY concrete pour where you need to use a screed board longer than about 2 feet, I would say have a helper handy, concrete is &&^%% heavy.

Andrew

[Terrian]

Thanks all for your help. pour went well ( bloody heavy though, truck driver asked me if I wanted more water, thank god I said no) Surface finish went ok but could be a bit more level.

Crap dusty concrete has usually had extra water added, one way or another.

Please, the major reason for a new concrete surface going 'dusty' is due to the concrete being over worked.

If you are going to add water to the truck before you start the pour, let the truck mix for 5 minutes or so to make sure the extra water has been well & truly mixed in.

FFS, I have poured concrete that was 100 slump (ie, pea soup, self spreading,self leveling) and the finished product has never been 'dusty'

The main problem with a wet mix (ie, more than about 70 slump) is shrinkage

Tips to pass on for the day that I think i learnt:
1 .. someone said plastic under and not playing with surface too much will help to not create another dusty finish. seems to have held true but probably early days yet.

Plastic under the concrete helps slow the drying process, concrete dries by 1) chemical reaction, 2) water drawn out to the ground 3) air temp

it is a good idea to try and slow the drying process when the weather is warm / windy

3.. covered slab too soon with plastic and it marked surface. wait till its gone off enough. lucky this is just a shed.

If you are going to cover the new pour, use hession, allows the concret to breath still, while keeping the likes of blasted magpies off the still wet concrete

4.. should have probably gone over twice with screeding board to get surface flatter. timber float left a nice rough grippy surface.

Working new concrete too much leads to 'dusty' finnish, I have found the an initial rough screed then allow the concrete to go 'off' a bit (check every 1/2 hour or so on a small patch, if you start working it and it gets wet, leave it for a while longer)

5 .. ANY concrete pour where you need to use a screed board longer than about 2 feet, I would say have a helper handy, concrete is &&^%% heavy

last 1/2 decent slab I did was 7m wide x 9m long, 5.5m3 of 25mpa, 80 slump concrete, 2 on barrows, 1 on shovel. me on 2.4m screed (should have hired a power screed for that job, and it was a damn 'love job' for a work mate to boot.
12 months later the slab looks fantastic.

just as a side note, best sealer we ever used was this