How thick should the concrete pad be?

[glend (Glen)]

Folks I am going to put in a purpose built concrete pad for my 16" dob. This is to be a viewing platform for now. The scope will be housed in my big shed and rolled out to the pad which is positioned for best sky angles and less obstruction. The question is, how thick do I need to make the pad? Will 75mm be ok and does it need to be reinforced at all? Any advice appreciated.

[Paul Haese]

Originally Posted by glend

Folks I am going to put in a purpose built concrete pad for my 16" dob. This is to be a viewing platform for now. The scope will be housed in my big shed and rolled out to the pad which is positioned for best sky angles and less obstruction. The question is, how thick do I need to make the pad? Will 75mm be ok and does it need to be reinforced at all? Any advice appreciated.

Make it 150 and you should not get much vibration showing up.

[rogerco (Roger)]

You don't say what size the slab is. You should consider the foundation material you are putting it on (soil, fill etc). Some soils are reactive and will swell quite a bit with wet and dry periods causing the slab to go up and down. You would be well advised to put reinforcement in to hold the slab together. I would save a bit on the concrete though and go for 100mm (but make sure it is 100 all over) with edges thickened to 150mm.

It doesn't cost much to put it in when you pour it, bit hard to get it in afterwards and you never know what you might decide to do in a few years time with it.

[glend (Glen)]

Sorry forgot the size. My16" dob sits on a custom mount and the ground board diametre is 700mm. I was thinkng that a 900mm diametre circular pad would be all I need and uses less concrete than a square pad of say 900x900. I would mix it from bags so its a DIY pad. I don't want to create something that's there forever but just stable enough to support the dob. Re vibration, from what? There are no truck or buses going past my place and there are no motor drives (pushto only at this stage although ServoCat is a possible in the future. It does have a fan but there is no vibration from that. The pad would be in the ground, on a sloping area cut to level with a compacted roadbase underlay.

[Paul Haese]

Glen I was thinking you might be walking on this so it being thicker helps reduce vibration is you or others are walking on it while viewing. If no one is going to walk on the area, then 75mm should be fine.

[gregbradley]

I agree, 75mm should be fine for something that small but being so small why not 100mm and some reo? It won't cost much extra.

Greg.

[rogerco (Roger)]

Sorry I thought you meant something a goo deal bigger. Could you not just buy a 900mm paving slab from a landscape supplier, save all the work of forming it up and smoothing it off.

[glend (Glen)]

Good idea Roger I will try to locate one. Thanks.

[I.C.D (Ian)]

G'Day Glen,
I agree with the rest nothing less then 100mm with reo the thicker the better ,I have a mate who has 100mm slab and if you stamp your feet it will vibrate the scope.
If you want to use less Concrete ,do it in two pour's.Frist pour were the dob will go and then other were you walkaround , place expansion joint between the two pour's this will reduce the vibration .
Ian.C