thin foundation slab

[Goon_Squad]

So the plan is to pour the foundation slab over the top of an existing brick paved area. The bricks have been there for 20+ years and are concreted in place, so I expect they are a pretty solid base to utilise. Mind you it has moved over time, developed cracks here and there, etc.

I still expect that a foundation slab is needed to level it off and to distribute the weight from the oven over the top. But I suspect that the full 5 1/2 inch slab as per the plans is overkill when taking into account the pre-existing brick paving.

Any suggestions on (firstly) am I correct in reducing the thickness and (secondly) what is the thinnest I should be considering? I'm thinking 2-3 inches is heaps but that is not based on any engineering!

[Toomulla]

Re: thin foundation slab

You take a chance pouring on top of the paving you may trap water that previously evaporated resulting in a wet sub grade and movement. I would recommend removing the bricks and digging down at least 300 mm. But if you are going to proceed on top of the bricks you should be at least 100mm preferably 150mm thick

[Goon_Squad]

Re: thin foundation slab

You take a chance pouring on top of the paving you may trap water that previously evaporated resulting in a wet sub grade and movement. I would recommend removing the bricks and digging down at least 300 mm. But if you are going to proceed on top of the bricks you should be at least 100mm preferably 150mm thick

Good point regarding water carry, I suppose a sheet of plastic under would be sufficient?

I'm not sure why you think removing the bricks would be better, i would have thought the ground would have well and truly settled by now. Keep in mind that these have been laid embedded in the ground so are already 80mm into the ground. Prob worth mentioning that we have pretty heavy clay soils around here which is why I'm not too concerned.

[kstronach]

Re: thin foundation slab

imo i wouldn't be too concerned at all, my ovens built on existing 2ft square flags on a compacted hardcore base, no slab or re bar, its been there 2 years with no movement at all, i'm not advising you do the same just saying how mine was built. 100mm will be fine for you, all i'll say is if you pour a slab on top of existing ground will this be seen when the builds complete or will you be landscaping up to it? might not be aesthetically pleasing if you can see a slab of concrete when built

[wotavidone]

Re: thin foundation slab

Melbourne only gets half the rain that Brisbane does ( I looked it up ) but that's still 600 mm per year, so you should give some consideration to water management. If the pavers are laid with a fall and you can build on the high side so the water runs away from the oven, you should be OK.

This would apply whether or not you pull the pavers up. I wouldn't be building in a low spot that the water will run towards.
100mm with reo would be my personal absolute minimum for a base for a full-on concrete block stand and brick oven.
The thing is, with the common Besser block stand layout, the load is distributed to the outer perimeter of the slab. So, IMHO, the slab should be bigger than the foot print of the stand. This would be easier to achieve without a trip hazard if the pavers were pulled up and the slab laid so the top is level with the surrounding pavers. But still, it'll probably be fine even if you lay it over the top. It's not a lot of pounds per square inch when you think about it.

[Goon_Squad]

Re: thin foundation slab

Melbourne only gets half the rain that Brisbane does ( I looked it up ) but that's still 600 mm per year, so you should give some consideration to water management. If the pavers are laid with a fall and you can build on the high side so the water runs away from the oven, you should be OK.

This would apply whether or not you pull the pavers up. I wouldn't be building in a low spot that the water will run towards.
100mm with reo would be my personal absolute minimum for a base for a full-on concrete block stand and brick oven.
The thing is, with the common Besser block stand layout, the load is distributed to the outer perimeter of the slab. So, IMHO, the slab should be bigger than the foot print of the stand. This would be easier to achieve without a trip hazard if the pavers were pulled up and the slab laid so the top is level with the surrounding pavers. But still, it'll probably be fine even if you lay it over the top. It's not a lot of pounds per square inch when you think about it.

The pavers tend to fall away toward the lawn, so reasonably well drained and definitely in a high point. But I do like the idea of pulling up the bricks and having the slab level, as kstronarch mentioned it could be a trip hazard and not a good look being visible. Advantage is the the remaining bricks would make pretty good formwork. Disadvantage is that I would need to dig down a bit further to get 100mm, not that big a deal in the scheme of things.

[Gulf]

Re: thin foundation slab

..........Disadvantage is that I would need to dig down a bit further to get 100mm, not that big a deal in the scheme of things.

I heard that! GS .

I remember digging my foundation, setting the steel, and pouring the concrete.......It was a chore. But, it does not stick out in my memory, near as much as fitting the last of the dome brick............in close quarters .

I like 140mm to 150mm better. If my conversion is right, 140mm is pretty close to the dimensions of a 2"X6" (american). I don't know anything about your local soil conditions. But, I do know that if you pour a thick enough monolithic foundation, with the proper concrete mix and reinforcement: Your entire oven will rise, fall, and settle as one complete unit.

[Toomulla]

Re: thin foundation slab

Good point regarding water carry, I suppose a sheet of plastic under would be sufficient?

I'm not sure why you think removing the bricks would be better, i would have thought the ground would have well and truly settled by now. Keep in mind that these have been laid embedded in the ground so are already 80mm into the ground. Prob worth mentioning that we have pretty heavy clay soils around here which is why I'm not too concerned.

There a couple of issues 1 the pavers allow ground water to evaporate and are also able to flex as the clay gets wet and swells then drys and shrinks. 2 the slab will accumulate water under it and the clay will swell. The weight of the oven will not resist that swelling and may result in oven going out of level. In the end however it is up to what risk you are prepared to accept

[Laurentius]

Re: thin foundation slab

Hi Goonie,

If those bricks have been there for 20+ years, forgetaboutit! If you are worried, place a moisture barrier on top and drop your slab. Your yard is a stable as its ever will be, unless you live in an area of eartquakes and landslides (I do). Worry about your house falling down not your oven. Your neighbors will complaint about your smoke and not your cracks, with the exception of the plumber`s kind. Get building, so you can cook something.

[Goon_Squad]

Re: thin foundation slab

There a couple of issues 1 the pavers allow ground water to evaporate and are also able to flex as the clay gets wet and swells then drys and shrinks. 2 the slab will accumulate water under it and the clay will swell. The weight of the oven will not resist that swelling and may result in oven going out of level. In the end however it is up to what risk you are prepared to accept

Having had a better look at it today, I think that the amount of water getting through to the ground underneath is limited; remember, these are solid bricks used for paving that have been concreted in. There are a few cracks here and there in the mortar between but overall it is actually pretty solid.

My main issue to deal with is the fall; it is around 40mm overall. The other is that the bricks have been laid in a decorative pattern, with curves and so on so lifting out is not going to be as easy as I thought. So the path of least resistance for me now is just to lay some formwork over the existing bricks and save myself the hassle of cracking the mortar/concrete and dealing with the curved bits.

Seems like the consensus is to go with minimum 100mm, that will then grow to 140mm at the deep end. I'll keep the edges to a shorter 40-50mm so that it doesn't stand out so much from the brickwork.

[TropicalCoasting]

Re: thin foundation slab

Clay moves a fair bit
Ive seen cracks in the inner eastern suburbs you could put your fingers into during a drought close up after some good rains.(double brick house)
This was the orange stuff you get in the east,the north and west has the black clay with lots of rock in it.
My limited experience says that seems more stable,friends houses in those areas are crack free.
I would take the effort to do it properly or its a pain if it fails.

[cobblerdave]

Re: thin foundation slab

G'day
Those brick commons have been in the ground for 20 years and would be pretty soft. I'd experiment with a circular saw with a diamond blade or masonry blade if it will not cut then throw the slab over the experiment .
I recon it will cut though and I'd cut out the shape of the slab through those curved brick patterns. Pull back 200 mm of bricks leaving a central island of brick in the middle. This will then give you a 75 mm thickening beam right around slab to the original ground.
Boxing, thin ply buttered up to the cut brick, then heavier wood frame on the outside it. A long rackets strap around the lot for good measure.
The 40 mm fall is nothing my slab is on sloping ground thinnest corner is 120 mm and the deepest is 300 mm.
Regards dave

[NCMan]

Re: thin foundation slab

Hi Goon_Squad. I know you are getting a mix of opinions here. And that is a good thing. I have poured lots and lots of concrete, so I wanted to jump in here w/my own 2?. Unless your build area is in a low area or is susceptible to flooding, I feel there is no need at all to remove the bricks. I would place a layer of 6 mil plastic over the bricks, install your formwork and proper reinforcement and pour away. As for your exposed slab edge that you may have, you can easily cover that up in a number of ways.

[Campmaki]

Re: thin foundation slab

Hey, I would do the extra work and remove the brick and pour the slab per local, regional building codes. In other words build it the way it should be built per your climate, which I have limited knowledge. My thinking is you do not want anything between your slab and the ground it rests on. Did you put the brick in? What is directly under the brick? If it is a wet region, I would tend to lean towards a gravel pad under the concrete slab, just my opinion.

[Laurentius]

Re: thin foundation slab

Hey, I would do the extra work and remove the brick and pour the slab per local, regional building codes. In other words build it the way it should be built per your climate, which I have limited knowledge. My thinking is you do not want anything between your slab and the ground it rests on. Did you put the brick in? What is directly under the brick? If it is a wet region, I would tend to lean towards a gravel pad under the concrete slab, just my opinion.

Normally whats under driveway paver`s is dirt! In 20 years those bricks have conformed to nature more than nature had conformed to the bricks. If there were a drainage problem you would know it by now. I would put the money from that to purchase ceramic fiber boards and blankets to insulate that puppy good.