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Concrete base


Dogstar45
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I am about to start my observatory build. I found an earlier post (apologies for forgetting whose it is, but many thanks) showing different profiles for the concrete slab. I am going to install a Pulsar 2.1 dome.

My question is has anyone here constructed a slab similar to example D on this diagram? I am having a problem figuring out how to make the former to produce this profile.

Thanks for reading!

Slab Profile.png

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I've done a bit of DIY concrete work and if I was doing what you are doing I'd use timbers to make something like this.

The key measurement would be the top so that the shed fits over the edges so work that out then work from there.

base.png.92e69049d9523002e557014876e2a26a.png

Michael

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Do you really need the rebate to be in the concrete? I can see that having the floor slightly higher than the bottom of the walls would be good for keeping water from seeping under the walls but would it not be easier to make a flat base then tile the floor only inside the dome to raise its height?

Olly

Edited by ollypenrice
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Just forming a good circle without the rebate will be hard enough. Bendy ply and the like will certainly bend adequately but tends to have inconsistent bending characteristics even within one piece. So retaining it with perfectly laid out stakes won't guarantee a perfect circle.

But if you do get that far, just form the rebate by layering some strips of bendy ply at the top of your curved former. As I say, it had a tendency to run off and buckle which is only compounded by layering.

I did build a flat circular base at my old house back in about '89. It was an inch or so out of circular and that took some doing. So care would need to be taken that any rebate in the slab was circular enough that it doesn't deform the obsy.

If the former shown in blue in your diagram was used then that would ensure a better circle but the upstand from that might be fun to construct.

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If it were me, I think I’d take @ollypenrice’s approach. I suspect bending sheets of ply into a circle wouldn’t give perfection, so you might have to undersize it a bit or take a cutter to it when fitting the obsy. 
 

Why choose profile D when profile B has the same protection against water ingress? How high do you want the rebate? As I see it, if you want a perfect circle you’d have to cut circles out of 18mm or 1 inch ply sheet and stack them to the desired height-effectively lose the orange bit in the diagram from @Synchronicity and increase the thickness of the blue bit…

You’d have a load of wood wasted, but at least you could make a bench seat and or worktop/shelf unit for inside the obsy with it😜

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Does the observatory have a base piece?  If it does you might be able to use that as a former, protecting it with plastic sheeting or similar.
The other way would be to use ply or whatever to make a circular former but make it undersized.  Have stake - piece of rebar or similar - dead centre of the base with a cord the correct radius for the finished base.  After the concrete is set enough to hold its shape remove the former and build up the base to the correct size.  Make a shaped piece to give the profile you want, fix that to the cord and use that to go round filling and trimming.
Remember, if this is for the edge of the obsy to hang over it doesn't have to be perfect as it'll be hidden and any small gaps can be filled with suitable material later.

Michael

PS.  That all made sense to me - I hope I explained it well enough!

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On 17/09/2022 at 15:10, Synchronicity said:

Does the observatory have a base piece?  If it does you might be able to use that as a former, protecting it with plastic sheeting or similar.
The other way would be to use ply or whatever to make a circular former but make it undersized.  Have stake - piece of rebar or similar - dead centre of the base with a cord the correct radius for the finished base.  After the concrete is set enough to hold its shape remove the former and build up the base to the correct size.  Make a shaped piece to give the profile you want, fix that to the cord and use that to go round filling and trimming.
Remember, if this is for the edge of the obsy to hang over it doesn't have to be perfect as it'll be hidden and any small gaps can be filled with suitable material later.

Michael

PS.  That all made sense to me - I hope I explained it well enough!

Like a Heath Robinson, string and scraper concrete router?

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Thank you all very much for  these thoughtful replies and ideas. I am now leaning towards going with a simple flat base and trusting that whatever sealants I use will suffice. Have those of you who built a flat base had any water ingress problems? Also this would require less precision, as long as the slab is wide enough, job done!

Michael - the observatory does not have a base as such - at the bottom of the wall section there's a 90 degrees bend about 75mm wide that it sits on.

Jim - I was not aware of flexible plywood, thanks. That has some real possibilities.

 

 

Edited by Dogstar45
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18 hours ago, Anthonyexmouth said:

Surely the easiest thing would be to have a flat slab, fix the dome then lay a screed inside over a membrane. Little bit of shuttering across the entrance if there isn't a raised threshold 

That was my thoughts, lay the base, add a membrane, fit the dome, then do whatever after using spacers between dome and inner base former to create the shape you want. Flexible ply or even a bit of hardboard would probably work. I wonder if a piece of white plastic trim would be flexible enough.

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1 hour ago, M40 said:

That was my thoughts, lay the base, add a membrane, fit the dome, then do whatever after using spacers between dome and inner base former to create the shape you want. Flexible ply or even a bit of hardboard would probably work. I wonder if a piece of white plastic trim would be flexible enough.

No, fit the dome, lay the membrane inside and lay a screed that then gives you a guaranteed dry internal base to lay a carpet if you wanted to add a little fall protection if you drop anything. Only place you need any shuttering is the entrance and only then if there isn't a raised threshold. 

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Hi Dogstar,

good to see you found my drawing, which I posted a while ago here.

The circular rims can be made from thin plywood, if needed multiple layers, of from a plastic like HDPE, which can be sourced in various thickness. Even at 20mm thick HDPE bends quite easily.

Nicolàs

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Despite taking extra care to get the floor flat I had two areas where rain would creep under the edge of the dome.

Fixed by using an angle grinder with a stone disc to cut groves to take away the water, Look closely and there is a grove right next to the green RTV.

Making the groove next to the dome wall was straight forward but had to use one of those oscillating stone cutters to cut the radial groove from the inner to the outer ring

Also I screwed batons to the inside and covered with waterproof chipboard flooring. Gives lots of gaps to run wiring.

IMG_20220921_111717.jpg

IMG_20210417_145854(1).jpg

Edited by Tomatobro
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5 hours ago, Tomatobro said:

Despite taking extra care to get the floor flat I had two areas where rain would creep under the edge of the dome.

 

I'm actually doing the final fixing of my new dome on Saturday so were you getting water in even after applying silicon around the base?

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1 hour ago, scotty38 said:

I'm actually doing the final fixing of my new dome on Saturday so were you getting water in even after applying silicon around the base?

Silicon should only be used as a last resort as there is a fair chance that it will start leaking again after some time. Better to avoid water can ingress at all.

Nicolàs

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7 hours ago, Tomatobro said:

...

Also I screwed batons to the inside and covered with waterproof chipboard flooring. Gives lots of gaps to run wiring.

 

IMG_20210417_145854(1).jpg

I see that you have fixated the batons both to the outer slab and to the central concrete square on which the pillar stands. This more or less nullifies the effect of having those two parts separately pored. Why not creating a framework that does not touch the central concrete?

Nicolàs

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1 minute ago, inFINNity Deck said:

I see that you have fixated the batons both to the outer slab and to the central concrete square on which the pillar stands. This more or less nullifies the effect of having those two parts separately pored. Why not creating a framework that does not touch the central concrete?

Nicolàs

You beat me by seconds on that comment. 

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4 hours ago, inFINNity Deck said:

Silicon should only be used as a last resort as there is a fair chance that it will start leaking again after some time. Better to avoid water can ingress at all.

Nicolàs

I have the dome going on to a flat concrete base so how do you suggest I stop water seeping underneath without a bead of silicon? I mean even Pulsar themselves put this method in their instructions. I realise if I'd built a fancy base it would be a different story but I didn't.....

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11 minutes ago, scotty38 said:

I have the dome going on to a flat concrete base so how do you suggest I stop water seeping underneath without a bead of silicon? I mean even Pulsar themselves put this method in their instructions. I realise if I'd built a fancy base it would be a different story but I didn't.....

I agree if the base is already done, then there is little options left, so then it indeed is your last resort....

Nicolàs

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