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Concrete slab between trees

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Chris152

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I want to put a 1m x 2m x 12.5cm concrete slab in my front garden, to use as the plinth for a sculpture. The area I've marked out is between two trees and about 1m from the base of each (it's the only place that'll really work). I planned to dig down deep enough that the top of the concrete was just above the ground level, but having started discovered a 6" root (among others) that I don't want to cut through.

So - I'm now thinking to go down just a turf (1") thickness and have the concrete stand proud. Do you think the roots directly below the slab will damage the slab and if so, what could I do to limit damage? Steel reinforcement in the concrete, something like that? I plan to bolt into the concrete to keep the work stable, and may change the sculpture from time to time so would want to drill more holes. Doesn't matter if it breaks apart in a few years, but anything less than that'd be annoying.

Thanks, C
 

dzj

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The roots below will probably affect the slab.
A steel mesh would prevent the pieces going on walkabout if/when the concrete cracks.
Try anticipating where the cracks might occur and work around that.

HTH
 

Chris152

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Thanks dzj, I'll do as you suggest (steel mesh and anticipate possible problems!). If I fill with gravel up to ground level and then have the concrete start at ground level, will that work (or does the concrete really need to start below ground level)? Everything below that could then be compacted gravel allowing for some movement of the root?
 

Hlsmith

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Chris152":155shqts said:
Thanks dzj, I'll do as you suggest (steel mesh and anticipate possible problems!). If I fill with gravel up to ground level and then have the concrete start at ground level, will that work (or does the concrete really need to start below ground level)? Everything below that could then be compacted gravel allowing for some movement of the root?
Not with gravel you need type 1 aggregate compacted with a Wacker plate
It's the dust in the type 1 that binds it washed gravel moves
Type 1 should also be cheaper than gravel
You can also get fibre reinforced concrete to help with the strength
 

Chris152

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Thanks - type 1 aggregate it is (I see they sell it in smaller quantities at Wickes nearby).
Apart from raising the earth as nev suggests :), will it be ok for the concrete to start at ground level?
 

Hlsmith

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Do you mean the 1 ton bags or the 20kg bags as I would think you could easily loose a tonne bag under it over 2m2 area once it is well compacted
Put it in in layers compacting as you go
 

Chris152

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Hlsmith":233sbpl8 said:
Do you mean the 1 ton bags or the 20kg bags as I would think you could easily loose a tonne bag under it over 2m2 area once it is well compacted
I'm learning here - I was assuming 8 or 10 20kg bags stamped in by foot initially. I can hire a whacker for £27 locally so not too bad, though. Does the amount I need/ how densely it's compacted depend on the weight I'm going to put on the slab? It's only ever likely to be a few cubic feet of oak and some light-weight metal, and not extending high so wind shouldn't put too much pressure on the work. Sorry to be so clueless, I did do some searching on the net but maybe in the wrong places...
 

Hlsmith

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Due to the trees the moisture level will vary more than usual de stabalizing the ground the compacted base layer helps to alleviate this issue
Without this layer I wouldn't be surprised if a thin slab cracked of its own accord
Putting 2 smaller slabs next to each other may be another option only needs a piece of ply to separate them
 

Woody2Shoes

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Just a quick thought for the trees themselves - most of the root system is in the top 600mm of soil and roots need both water and oxygen from the air. I certainly wouldn't dig deeper than about six inches. I would also protect the area from leakage/spillage of cement/grout from the construction process - it's quite strongly alkaline and can burn the roots and easily kill a tree - so a bit of DPM under the concrete (turned up at the edges) should prevent the grout leaking out and also improve the curing of the concrete. Cheers, W2S

PS some trees e.g. cherry have roots visible at the surface
 

Chris152

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Thanks both.

W2S - the more I've been reading and thinking about this, the worse the idea of a concrete slab there seems. The last thing I want to do is damage either tree and what you've written convinces me I'd be right to go another route.

My meanderings have led me to plastic grass paving grids filled with gravel, on a (not too heavily) compacted level of gravel and with a timber frame around the edges (sunk into the ground) so it can stand an inch or so proud of ground level and has a nicely defined edge. Hopefully any movement of the roots/ ground won't make much of a difference as each section of the grid can move and that movement shouldn't have significant impact on stability (time will tell) and I'll take this - and the fact I can't bolt the sculpture down - into account when I make the work.

Does that sound like a reasonable plan?
 

DrPhill

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Chris152":2gfnmlxc said:
Does that sound like a reasonable plan?
I know nothing of the practicalities of concrete etc, but a wooden frame with gravel sounds much more natural, and pleasing to the eye. It also sounds kinder to the existing inhabitant sof the space. Indeed, the gravel could be one (or more) of the many and varied natural coloured chippings that you can get in most garden centers.

You may also save the environment a little by not needing the energy input to the cement, and when the display space is no longer wanted it could be removed much more easily.

Just my tuppence....
 

Chris152

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Your tuppence gratefully received, DrPhill - my meanderings led me to read about the environmental impact of concrete and I'm sure where there's an alternative, it's a good idea to take it.
 

The Bear

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Chris
2x1m concrete plinth makes me think this sculpture is pretty big. What is it, how big is it, how heavy is it. Post a picture, there may be solutions that don't require a big base but its hard to understand without seeing what you want to put on it
Mark
 

Jacob

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What about just laying three ready made 2'x3' paving slabs on sand and gravel, no concrete or mortar at all? They might move as the tree roots grow but would be easy to straighten up.
In other words make it floating and adjustable.
It looks like your other solutions would need maintenance anyway
P.S. Slabs could be concrete or natural stone
 

Chris152

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Thanks both.

Mark - I thought the idea up a couple of years ago but never got round to it - a plinth that I could build a different sculpture on every couple of months, as a way of trying out ideas (and entertaining the neighbours). Making little versions works to some extent but you don't get a sense of scale. So the idea's that there's no one sculpture and the plinth needs to accommodate different pieces, but I think they'll be fairly low and wide - hence 2m x 1m.
This is a maquette for the first one, I have oak for the two left blocks, not sure what to use on the right and need to order an aluminium tube for the cross piece.
_MG_8504.jpg

But the point is it'll change when it comes to making it on the plinth, depending how I think it looks.

Jacob - I did think about concrete paving slabs but think if they move out of alignment - which there's every chance of with me installing them - they'll look bad, whereas with the plastic grid/ gravel option I can shift gravel around within the wooden frame and make that level, even if the plastic grids have shifted a bit.
 

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Chris152

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Jacob":iisff5am said:
Trad Japanese timber buildings are plonked onto stones:
Brilliant! Not the look I was aiming for but that's great.

W2S - I did wonder about a frame built onto fence spikes but given how simple the sculptures will be, thought the plinth might steal the show.

Anyway, I've ordered 2 sq metres of the plastic gravel things so will start digging out ready for those. I'll post an update once it's done.
Thanks all - complete change of direction from the start but very happy with the plan as it stands.
 

MikeG.

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Whacker plate for 1m x 2m!!!!!! Don't waste your money. You can tamp almost anything with a sledge hammer held vertically and pounded down on the end of the handle/ hammer head. Ten minutes work for 2 square metres. A whacker plate in these circumstances would be just silly.

Having said that, I wouldn't be casting a slab there at all. First you'll damage the trees, then next they'll damage your slab.
 
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