View Full Version here: : What to do to prepare for concrete?
09-04-2011, 01:05 PM
I'm David and from Michigan, US. And I've got a Meade LX200GPS on an eq wedge, but it's flemsily mounted on a Scopebuggy and I usually roll it out onto the grass front yard from the garage (where I keep it).
Well my parents, at least especially my mom (we live in the country), will let me have and build a kit observatory.
But for now I'm considering it and needing to save up money, because now we live from check to check from the government. I need a job.
Also the place where I'd like to put the building is going be on the land (we own) but part of a farm field that's been used to being tilled. And I wondered if there's any special considerations that you guys would have about pouring concrete there. I don't know if it will be a concrete pad or a pier or a roll-off roof or a dome, but I know that I'm excited, but know that I want to take it slow to do it right.
About the concrete: I guess, like the concrete men say is that it (the soil) needs to be level. But it also needs to be compacted so it won't give after the concrete is dried and make it crack. How wide (deep) should the 2X whatever's need to be that will confine the concrete?
My parent's say that we might be moving from this place in maybe 2-4, a couple of years.
So mom's wanting the structure (the part other than the concrete) to be movable and transportable by a truck.
If we get a dome, I'm considering Technical Innovations 10 foot dome.
It is a little expensive, but I believe (of course) that is better built than the "pod" from SkyShed.
PS: Do I need to use Reebar in the concrete?
Thank you for your advice.
09-04-2011, 01:17 PM
Rebar (RIO) is nice if you have it, but not totally necessary in a pad that is not likely to support heavy machinery or vehicles. It is good to have in your weight-bearing foundations and also in your pier if it is a concrete one. 90 to 100mm, (3 to 4 inches) should be plenty to make a nice floor for your observatory. Make sure it is thicker on the edges where it supports your building walls.
Edit: you might want to consider leaving the rebar out of it. If you have to move, you may be asked to break up and remove the concrete. It's a lot harder to do so with rio in it...
Once you have levelled off the soil, we commonly use a gravel base like blue-metal, crushed granite or crushed concrete to a depth of about 50mm, screed flat and level prior to pouring the concrete.
Your form work to give your edges a nice finish can be anything from 4 x 2 timber, to strips of steel or plastic, 100mm wide. As long as they are well supported so they don't move when the concrete is poured in, you can use whatever you have got.
One thing you might want to consider, is leaving a gap around the pier footing, to allow for expansion/ground shift and vibration, to isolate the pier and telescope from the rest of the floor and building.
Dig a square hole in the middle of the floor before pouring, neatly line the sides with polystyrene sheet, then pour your concrete floor and in the hole. Once it is all set, all you need to do is pour about a cupful of petrol slowly onto the polystyrene and it will disintegrate. Voila! One isolated pier foundation.
These are some ideas, I hope they help. Every observatory is different, which is what makes them fun!
09-04-2011, 03:54 PM
mmmm Napalm :P O so much was made when i was a kid, those poor ants it was vietnam for them :P
09-04-2011, 11:27 PM
You are living from check to check from the government, yet you are considering a $6,950.00 dome!
You could build a very spacious roll-of-roof obs for under half that cost. :thumbsup:
or even buy a ready built Roll-off-roof obs fully installed from places like this starting at $2,960
13-04-2011, 05:55 AM
David, Once you know the diametre of your dome, prepare a pad that is a little wider than the diametre. Excavate all of the soil out of your pad area - soil is organic and will move even after compaction, so find a solid base ie clay, and compact the base, and as Baz said allow for an edge beam, if you are wanting to put in power, now is the time to run your conduits if you want them to come into your dome area from under ground. Put in a layer of road base that is at least 100mm thick - if you get it dry you will have to add water and make sure that it gets all the way through the road base - you will know that you have the right amount of water in it when you can sqeeze an amount in your hand and it makes like a sausage that you can break into two. If it crumbles it's too dry. Compact the road base, then apply further water and compact again - this will create a slurry on top and close up the gaps in your road base, and let it sit a couple of days to bake out (if you find that you need a thicker layer of road base, compact it in two layers - assuming that you will use a vibrating plate). Set up your form work to allow for 100mm of concrete and install the Rio (rebar), here in Australia we would use F62 - a sheet of mesh, this will stop shrinkage cracking. Make sure that you leave about a 50mm gap between the rio and the formwork, so that no rio is exposed after you've poured the concrete. Pour the concrete and level and round the edges, once the surface has a dry layer over it, run a broom over it from one side to the other - this will make it non-slip. If you are in a hot area, you could sprinkle water over the top of the concrete regularly (morning and afternoon)whilst the concrete is drying - this will stop surface cracking. Let the concrete cure for at least 4 days, best to let it sit for a week (concrete will have 95% of its strength after 4 days). Then construct your dome. I hope this helps a little. If you don't have a solid base, your concrete will crack eventually.
13-04-2011, 08:53 AM
The other thing that may affect your process is something most of us dont have to consider here in Oz and that is the depth the soil freezes.
Your concrete, if its is not to end up being subject to soil expansion due to freezing, needs to go below the frozen depth level.
While this may or may not affect the dome part of the slab, it will be critical to the pier footing.
Read this thread and look at the CloudyNights link I listed
13-04-2011, 10:04 AM
Nice on Dutch. You've obviously done this before.
Yes to F62 mesh, turned down into a 250 x250 deep trench at the edges Visqueen or similar dampproof poly sheeting over the roadbase will stop rising damp. Put the mesh on bar chairs to hold it off the dampcourse.
This would make a platform that won't vibrate or slide around on top of the substrate. The pier plug should be isolated from the floor, but not necessarily from the substrate. Isolation can be something like a strip of foam and left there after it is all set - have a look at CJs (construction joints) in your area's footpaths.
13-04-2011, 06:43 PM
Just a thought. You could put a piece a PVC pipe in the pier section to allow power, wires and cables, etc. Just use a straight section and add an elbow at each end then straight sections through the formatube. Use plenty of duct tape to hold everything in place.
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