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Old 03-13-2013, 08:43 AM   #1
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Large Rocks into a 'Concrete Cube' ??


I am going to be building a permanent pier for my telescope and (obviously) the pad that it sets on is the first construction step.

Telescope piers present pretty low loads to the supporting pad, but (if you are going to do astrophotography) ABSOLUTE STABILITY is critical. No movement and no vibration.

My design is basically a 2.5 foot cube (ending maybe an inch above grade). This is probably 1.5 feet away from any reasonable assessment of our frost line here in NC. And that is kind of the upper end of what I think that I can deal with using Sakcrete bags (BTW, I have ZERO concrete experience). I'll be using some undecided mix of rebar and/or mesh (pier itself bolted with J-bolts). However, I have a bunch (say 3-4 80 pound bags worth) of roughly brick sized rocks left over from an old water diversion project. It occurs to me that these could be most useful here - just place them 'randomly about' inside the cube being careful that there are no 'voids' around them. This would (I think)

1) Make my wife happy (rocks would be gone)
2) Have to buy/mix a few less bags
3) Might make the whole thing stronger ??

Comments on this? Thanks.

dave

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Last edited by DaveLeeNC; 03-13-2013 at 09:23 AM. Reason: clarity
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Old 03-13-2013, 11:17 AM   #2
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Large Rocks into a 'Concrete Cube' ??


http://www.calculator.net/concrete-calculator.html

Just how big is this?

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Old 03-13-2013, 11:59 AM   #3
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Large Rocks into a 'Concrete Cube' ??


If you mixed them into the concrete no problem. Filling the forms with the rocks and trying to place the concrete around, problem.
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Old 03-13-2013, 04:38 PM   #4
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Large Rocks into a 'Concrete Cube' ??


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It is a cube (roughly) 2.5 feet on a side. Comes out a tad over a half yard of concrete. As I said the upper limit (for me) of what I am willing to bag/mix.

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Old 03-13-2013, 04:38 PM   #5
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Large Rocks into a 'Concrete Cube' ??


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If you mixed them into the concrete no problem. Filling the forms with the rocks and trying to place the concrete around, problem.
Thanks - that is pretty much what I thought.

dave
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Old 03-13-2013, 05:10 PM   #6
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Large Rocks into a 'Concrete Cube' ??


i would also,,, you wouldn't believe what i do 'round this house to keep my bride, nagzilla, happy
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Old 03-15-2013, 09:07 AM   #7
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Large Rocks into a 'Concrete Cube' ??


I'd be tempted to form up a box on the inside and pour the concrete walls leaving a hollow in the middle. Put a cap on it to pour the top 3-4" thick and form the top edge to just below ground level for appearance. Save on concrete and you don't need it to be solid to carry any weight. Set some galvanized or stainless carriage bolts into the top to fasten your equipment down solid.
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Old 03-15-2013, 09:43 AM   #8
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Large Rocks into a 'Concrete Cube' ??


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I'd be tempted to form up a box on the inside and pour the concrete walls leaving a hollow in the middle. Put a cap on it to pour the top 3-4" thick and form the top edge to just below ground level for appearance. Save on concrete and you don't need it to be solid to carry any weight. Set some galvanized or stainless carriage bolts into the top to fasten your equipment down solid.
Ayuh,... That sounds like Alota Extra work, compared to droppin' big rocks into wet concrete...
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Old 03-15-2013, 10:49 AM   #9
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Large Rocks into a 'Concrete Cube' ??


Yeah, there's no need to reinvent the wheel on this one. Besides, you'd think a solid block would be less prone to vibrations.

I would add that throwing organic material, like sticks and dirt clods and stuff, would also be bad because they'll cause big voids when they break down.
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Old 03-15-2013, 11:13 AM   #10
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Large Rocks into a 'Concrete Cube' ??


For the purposes of vibration damping, mass and isolation are the keys. The larger the mass, the more vibration it takes to 'move' the mass. Isolation prevents the vibration from affecting the target.

If you look at some of the large high accuracy machines being put in...they will cut out a big hole and pour a new block of concrete in that hole with no mechanical connection between it and the surrounding concrete. Basically it's a float monolithic slab.

In the OP's case, strength is not an issue......hence, I see no reason to worry about voids...he just wants a have mass that is isolated from the ground.

Me...personally.....I see nothing wrong with lining the bottom of the hole with all that left over rock. It's really no different than a packed base of gravel that foundation guys use when pouring a slab.

More important....I would put 1/2" foam around the edge to vibration isolate the physical ground from sides of the slab. Foam on the bottom would also help....it's done all the time for regular slabs....

Having spent some time behind a telescope, I can understand why you want a stable base...especially if your doing some timed exposures....any vibration will kill the shot. Back in my day it was real film. Now I bet your all digital.
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Old 03-15-2013, 04:55 PM   #11
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Large Rocks into a 'Concrete Cube' ??


ddawg is quite correct about the issue being mass vs. strength. And absolute stability is crucial as well (you don't want 1/1000 inch of movement due to cracking for example - polar alignment would then have to be redone).

I have serious doubts about the effectiveness of ground/pier pad isolation. My inclination is to simply built it and see how it works (without isolation). If isolation is a problem I would go at that by isolating what I am standing on from the ground, which seems a more solvable problem to me.

Thanks.

dave

ps. And yes it is now a digital astrophotography world right now. But getting 'film sized' CCD chips (high quality/low noise/etc) is an expensive proposition. If you can deal with the smaller FOV then it isn't so bad. And reasonable astrophotography can be done with DSLR digital cameras.
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Old 03-15-2013, 05:11 PM   #12
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Large Rocks into a 'Concrete Cube' ??


Would love to see some pics of your setup....I know guys don't grind their own mirrors any more...but would still love to build a good 8" reflecting scope....

I still remember the excitement I had when I saw the rings of Saturn through my 700mm refracting telescope...and when Apollo was circling the moon...if you caught it just right, you could see the command module as it came around the moon.
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Old 03-15-2013, 05:43 PM   #13
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Large Rocks into a 'Concrete Cube' ??


I had actually abandoned astronomy back in the late 90's (long story). My Celestron C11 (11" SCT on a Losmandy G11 German Equitorial mount) had been sitting unused since then. The mount is in the process of getting a checkup/tuneup and the pier is being built at a local shop (based on some scrap 8" steel pipe).

Probably a few weeks before this comes together.

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Old 03-15-2013, 05:46 PM   #14
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Large Rocks into a 'Concrete Cube' ??


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Originally Posted by DaveLeeNC View Post
I had actually abandoned astronomy back in the late 90's (long story). My Celestron C11 (11" SCT on a Losmandy G11 German Equitorial mount) had been sitting unused since then. The mount is in the process of getting a checkup/tuneup and the pier is being built at a local shop (based on some scrap 8" steel pipe).

Probably a few weeks before this comes together.

dave
Let me guess.....involves women.......it always does.....
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Old 03-15-2013, 05:54 PM   #15
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Let me guess.....involves women.......it always does.....
Interesting speculation. But it was strictly a job thing (change in assignment and I suddenly had no time or energy for the hobby). Been retired for a good while now and the interest is just now returning.

dave

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