Grinding a concrete slab

Getting our floors done at the moment. Part of the house is the traditional bearers and joists - they butt up against the back room in the house which is on a concrete slab. We are laying yellowtongue then Sydney Bluegum on top and wanted to continue the bluegum onto the back room slab.

We have been told that for the wider boards (130mm) we need to lay the bluegum onto yellow tongue as well over the top of the concrete slab, however this is going to bugger up our levels. The slab + yellowtongue + the floorboards is going to be about 30 or 40mm higher than the rest of the house. We really want the whole thing flush if we can as a 30mm step is going to look stupid.

So I want to get someone to grind 30mm or so off the slab. My question is, is this feasible and if so, how much would I be looking at paying to get this done? The room is about 4m x 7m.

Thanks in advance.

I think grinding 30mm of the top of a slab would be difficult and would take a long time and make a lot of dust.

If the slab is going to be covered it would be better to hire a jack hammer and scabble the 30mm off. It will be quicker.

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Originally Posted by Barry_White

I think grinding 30mm of the top of a slab would be difficult and would take a long time and make a lot of dust.

If the slab is going to be covered it would be better to hire a jack hammer and scabble the 30mm off. It will be quicker.

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Thanks Bazza. I just rang a couple of concrete places and they said pretty much the same thing. Grinding that much off is pretty much out of the question.

If I get it jackhammered, is that something I could do myself? Then can you put that self levelling stuff over the top of a roughly jackhammered slab?

Thanks

You could most certainly do the scabbling yourself and then I would think it would be better to just top it off with a sand and cement mix using something like Bondcrete to get it to adhere to the concrete slab. You could perhaps get a Ceramic Tile layer to do a topping for you.

The self leveling product is usually used in situations where you have dips and hollows in a concrete slab where you want to get it level to lay lino or cork.

vGolfer,

How deep is the reo' mesh in your slab? I would be concerned about hitting/exposing it.

Cheers,

Jill

Quote:

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Originally Posted by Jill

vGolfer,

How deep is the reo' mesh in your slab? I would be concerned about hitting/exposing it.

Cheers,

Jill

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Not sure Jill. I think the slab is pretty thick though. The mesh would be deeper than 30 or 40mm though wouldn't it?

Quote:

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Originally Posted by vGolfer

Not sure Jill. I think the slab is pretty thick though. The mesh would be deeper than 30 or 40mm though wouldn't it?

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There is usually a layer of reo at around 30 to 40 mm below the top. That is there to stop shrinkage and surface cracking in the concrete.

More importantly is the slab on the ground or is it suspended and how thick is the slab.

Quote:

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Originally Posted by Barry_White

There is usually a layer of reo at around 30 to 40 mm below the top. That is there to stop shrinkage and surface cracking in the concrete.

More importantly is the slab on the ground or is it suspended and how thick is the slab.

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I have no idea unfortunately!

I did what you are planning about 2 years ago. It wasn't really that hard, although I only took off about 30mm.

What I did with an angle grinder was to cut 30mm grooves in the concrete about 150mm apart in a chequered fashion. Then with a Kanga and the spade bit, lift off the top 30mm. Finish off with a lump hammer and comb chisel. I only had to do an area of about 4 sq meters. It took a couple of hours.

Pretty dirty job tho, cement dust everywhere.

Your main potential difficulty is the depth of the reo, as everyone says

Good luck

I've done it the same way as Bodgy. Difference was that I cut the grooves with the angle grinder about 50mm apart, in both directions. The bits between each groove just popped off as a single square chip when the jackhammer got under them. The negative was that I could scarcely see for the dust. Horrible job.

If I were in your position I would definitely do it - though I dont think a 30mm step is really that noticeable in practice.

Just a thought (probably a stupid one), could you cut the grooves with a concrete saw, and thus get it over with quickly. I know some of them have depth stops.

Arron

Yep, Aaron's correct, a concrete saw would be far better and more accurate too in depth. Less dusty but you get a cement slurry evrywhere instead.

Incidentally, you'll know if your reo is set high - Guy Fawkes night!!!!

I wouldn't be taking 30mm off if I was you.You will destroy the structural integrity of the slab.It is highly probable that you will expose the top mesh and have to cut it out.Not good.And you will be reducing the slab and beam thickness by 30mm.Again,not good.the slab is a certain thickness for a reason.

Tools

VGolfer,

If your slab is only 100mm thick, as some are, you may need to rethink, also. Our slab has pexel piping for inslab heating laid on the reo', so we won't be going near our slab with a drill, let alone a jackhammer!!! I've drawn up a colour coded plan with every measurement for built-ins recorded carefully. NO ONE is allowed to touch the slab without consulting THE PLAN!!!!

Is this section a full extension (i.e. without a 'doorway') of a living area? If not, you could perhaps taper the slab in the entry area, to form the timber flooring into a shallow ramp?

What about terracotta tiles - (although levels still could be out, there), with timber look tiles framing them (in blocks of four, on the diagonal - think of the fun you could have laying them!!!) to help tie it all in? Just a suggestion that may not be an option, though.

If all else fails, just jack the house up!!!

Cheers,

Jill

vGolfer,

Two other options come to mind:
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1 - Raise the bearer and joist section of the house by 30mm to compensate, using 30mm battens over each joist.
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2 - Why use yellow tongue over the slab? I know people who fix hardwood floors straight over concrete. Well I know one person who did this, absolute beginner with the help of a carpenter brother in law. beautiful floor resulted.
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Personally, I would go with option 1, would anyone miss 30mm doorway height?
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And I agree with some of the other posts, watch out for rio and don’t degrade the slab integrity. And sounds like a bugger of a job for the size area you are talking about.
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Having said that, the 30mm step would bother me no end. So I see why you want to make it all level.

vGolfer,
concrete is not cheap and I doubt that your floor has any more than the bare minimum of the requirement for thickness. Even if you do have 30 or 40mm to burn it will very likely reduce the topcover for the steel. The eventual outcome might be structural failure of the slab and that section of the walls and roof :eek: . It's not that likely but I bet you won't find an engineer that will stake his professional indemnity insurance on it :rolleyes:

Mick the cautious

We had our floors ground back by a concrete cutter. He has a machine that shaves off the top..Not sure how much it would cost as we used our neighbour who we swapped favours with and it was only the lounge that we had done. Very very very messy but worth calling someone and seeing if they can do it for you.

vGolfer - I found this site for you - not sure if it will be any help or not, though - bretts.com.au/sound-proof-fence/TQL18TimberFloorsOverConcreteSlabs.pdf.

Cheers,

Jill

I wouldn't give any thought to cutting into the slab unless you've got a nice bank manager to pay for the litigation in the years to come.
The slab will have been designed with [in 99% of cases] 100mm thickness, a set thickness [depth] of edge and internal beams and a requirement under AS2870 - residential slabs & footings - for a minimum of top cover of concrete to the reo.

Hey thanks everyone. I didn't get an email notifying me there had been responses!

I chipped away some of the tiles on the slab...seems like there is a bed of mortar that is at least 10mm so getting rid of that will help.

I think we may try and lift the floor height in the whole house say 20mm...hopefully that would be enough to have it flush with the slab. Once all the tiles are up and the bare slab exposed, I'll get a laser level and see how much I have got to play with.

The whole idea of doing anything to the slab i sseeming more and more unwise!

It sounds like you have a tile bed on top of your slab. Run a 9 inch grinder vertically in a grid pattern to the required depth(so it looks like a block of chocolate) then chisel out with small jackhammer, reinstate with a thin screed of sand and cement . There will be tons of dust !!!
. DONT INTERFERE WITH YOUR SLAB!!!!
Jamster