'Dust-free grit blasting' for concrete - how does this work?

[mangocrazy]

About 6 weeks ago I had a load of fibre Readymix tipped into my cellar (on top of a DPC, I hasten to add) to level the floor and dry it out. Unfortunately I got approximately twice as much concrete delivered as I ordered and as a result all my level markers were covered and the floor level is higher than anticipated. Apparently I was the last order of the day, so the driver just emptied his tank into my cellar. It also meant that the concrete dried faster than anticipated. And once concrete has been pumped into a cellar it's damn near impossible to get it out again easily.

So it was a race against time to get the floor approximately level before it all set solid. As a result the floor is sort of level, but a long way from smooth and I'm now planning to get the floor screeded to get it more how I want it.

My problem is that everything I read about screeding tells me that the surface laitance (weak milky concrete top layer) needs removing prior to screeding in order to get a proper finish. There are a number of ways of doing this including acid etching, dust-free grit blasting, grinding and scabbling.

The general opinion is that dust-free grit blasting is the best method. Does anyone here have any knowledge on this method. I know Roger (rtbcomp) is a dab hand at blasting metal objects and the like, but mobile blasting of a concrete floor is another game altogether, I suspect.

Any responses gratefully received.

 

[Shox Dr]

Give PWM Sales* a call. They are specialist in Concrete finishing. They will not do the job, but might be able to suggest a company that would.


*(No connection to them)

 

[Pete.]

I've broken out thousands of square meters of screed and I've never seen concrete treated in the way you describe. Just hoover the dust up, unibond the floor and have your screed laid.

Bit of a poor show from readymix. The driver should have realised he wasn't dealing with a professional and taken a bit more care. Should have put some inhibitor in it for you too especially if it was the last load.

 

[sean-1]

Concrete blasting using the closed circuit blasters are great on large areas (warehouses ect) but not realy practicle for a basement floor, to start off with they are big bits of kit. They work by shooting steel shot onto the floor which then rebounds back up into the hopper and fed back into the impeller in a continuous cycle.
For a basement floor I would consider hiring a planer with scarifying flail cutters fitted. They can be hooked up to an extractor and will remove the latents from the top of the concrete in no time. Easy to do yourself and will save you a small fortune over blasting costs as there are not many companies that have the relevent machinery so they seem to charge whatever they like.
Sean

 

[Pete.]

We have one and yes they are a large piece of kit and run on 3-phase.

Another way of gaining adhesion is to mix up a heavy slurry of cement, water, pva and a little sharp sand. Scrub that into the surface with a stiff broom and let it set. You'll have a surface akin to large-scale coarse sandpaper which will key the screed very well.

 

[mangocrazy]

Thanks for all the very helpful replies - much appreciated. Thinking on what has been said, if I want to mechanically prepare the surface then would something like either of these be about right? Floor area is about 10 square metres.

http://www.brandontoolhire.co.uk/directory/prodView.asp?idproduct=557

or

http://www.brandontoolhire.co.uk/directory/prodView.asp?idproduct=449

(I only mention Brandon Hire as they have a depot less than a mile from me).

I'm leaning toward mechanical preparation, as in my haste to finish the floor I left a fair few lumps and bumps that could do with grinding down. I want to use as little depth of screed as is reasonably possible, due to having lost more headroom than I really wanted, and at present there is a fair bit of variance in the floor level.

And I'll be using a vacuum extractor to keep the dust down.

Once that's done 'll be doing as Pete suggests as regards a cement, pva and sharp sand slurry.

 

[sean-1]

I would use this-

http://www.brandontoolhire.co.uk/directory/prodview.asp?idproduct=459

Sean

 

[Justme]

What about having the existing concrete floor polished?

 

[mangocrazy]

What about having the existing concrete floor polished?

It's far too lumpy to make polishing worthwhile. If it was flat enough, I'd just leave it as it was and seal it. Unfortunately, it needs flatting and screeding.

 

[brightspark]

use ardit floor leveler. it aint cheap but it levels floors min depth 6mm up to 30mm

 

[Justme]

It just seems silly to "fix" a floor that's too high by adding more height to it.

Angle grind out the lumps & use the thinnest self level that you can or fit soft flooring.

 

[Xsite]

Get it open blasted, yes it will be dusty but cover and seal everything up and it's very fast.

We used to open blast 500sqm of reasonably green concrete a day and 200sqm a day of well cured rock hard concrete, so taking that into account it won't take a decent blasting contractor long.

 

[mangocrazy]

It just seems silly to "fix" a floor that's too high by adding more height to it.

Angle grind out the lumps & use the thinnest self level that you can or fit soft flooring.

I agree. I really don't want to add any more height than I need to. I'm aiming for a screed depth of 5-10mm maximum, after having taken off the high spots.

Get it open blasted, yes it will be dusty but cover and seal everything up and it's very fast.

We used to open blast 500sqm of reasonably green concrete a day and 200sqm a day of well cured rock hard concrete, so taking that into account it won't take a decent blasting contractor long.

Bear in mind that it's a domestic cellar/basement I'm talking about with an area of only 10 sqm. Would the open blasting kit even fit down there?

use ardit floor leveler. it aint cheap but it levels floors min depth 6mm up to 30mm

That looks like it should do the job. I'm also looking at Tremco CS375 - it's about £25 per 25kg bag. Not sure how that compares with Ardit.

http://www.tremco-illbruck.co.uk/celumdb/documents/CS375_DS_GB_189.pdf

 

[brightspark]

I agree. I really don't want to add any more height than I need to. I'm aiming for a screed depth of 5-10mm maximum, after having taken off the high spots.

Bear in mind that it's a domestic cellar/basement I'm talking about with an area of only 10 sqm. Would the open blasting kit even fit down there?


That looks like it should do the job. I'm also looking at Tremco CS375 - it's about £25 per 25kg bag. Not sure how that compares with Ardit.

http://www.tremco-illbruck.co.uk/celumdb/documents/CS375_DS_GB_189.pdf

theres a lot of other makes that do the same job .probably ardit u pay for the name just like u do unibond over priced

 

[mike os]

stiff brush up, unibond as instructions, cement slurry before screeeding & screed as normal, no problem. min 40mm screed,

 

[Pete.]

Yes 5-10 mm is just not practical. That's latex screed territory and for that you need a pretty good floor to begin with. Just going by what you've put here you need something a lot more substantial than that.

Also be aware that no grinding machine will work right up to the wall

 

[mangocrazy]

In that case I may need to reconsider. I really don't want to be adding 40mm height to the floor. If I were to grind/scabble the floor to get it acceptably flat and lump-free, would it then be OK just to seal it rather than screed on top? It's only a cellar after all and doesn't need to look pretty.

 

[mike os]

grind & scabble or scale, then unibond and use a self leveling compound to smooth things out, tend to leave surface a little rough on their own

expensive to hire, expensive consumables & nasty job in a confined space tho

 

[Pete.]

You won't be happy with an un-finished floor.

Faced with that, I would break out the jackhammer. Crappy job but if it's not too big a floor area you should be able to strip 4" off it quite handily. Shame you didn't do it right after pouring it would have flown up - six weeks of curing is a whole new ball-game.

Buy a cheap swivelling lazer level (and check it after you set it up) and staple a dressmaker's tape to a long batten, use this to survey the floor and keep a track of your work as you reduce it. Gonna be a long nasty process though. Get a patch down to heght and work out from there.

 

[mangocrazy]

Thanks for all the replies. I think I'll go away and have a good think about it. Sounds like there's no easy fix...