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I am building a 'potting table' for the missus and have opted for a concrete top. The table frame is complete and I have built the form for the concrete top and before actually purchasing materials, thought I would seek advice from this worldly community. 
First, the dimensions:
L=63''
W=29''
Currently, the form is 1.5'' thick, conveniently the thickness of a standard (milled) 2x4. This thickness is also at the bottom end of several DIY resources I found so:
Q: I'm wondering if 1.5'' combined with a metal reinforcing grid would be enough. (queue jokes) The frame has a cross-member at the middle, with two equal areas roughly 24'' x 29'' that will be unsupported and that would need to support at most 200 lbs. (US) I'm not opposed to adding a second cross-member or another .5'' of thickness, I just don't want to make it any heavier than it needs to be.
If any of you have done this before, understand the structural properties/limitations/best practices of concrete, please share! Thanks and have a great weekend!
"Go forth into the source" - Neal Morse
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I can't speak to the properties of concrete or best practices on using it, however reading your plans...
I'd suggest pouring the top as two separate pieces if it's feasible. That'll make it easier to get them in there once set, and far easier to move later if you have to.
My two cents.
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I (and my back) agree with you, but she who must be obeyed loved the thought of concrete, as it would be seamless...  I'm guestimating the finished weight at around 225 lbs. as currently designed...definitely a two person job to move it! 
"Go forth into the source" - Neal Morse
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Instead of an in situ pour.
Why not pour it as face-down, then you can fill some of the volume that won't show with a sheet of 3/4 plywood which is lighter than the equivalent volume of concrete and gives strength and, if you leave it exposed on the "bottom" surface (top of the inverted pour), it can be used to attach the slab to the frame?
Just a thought...
A positive attitude may not solve every problem, but it will annoy enough people to be worth the effort.
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The problem there is finding something smooth enough to pour onto -- every wrinkle, nook, and cranny will be there for ever, once it's cured.
There's no reason not to put the ply in the shutter, though. It's quite a common practice to use ply or expanded polystyrene to reduce the load. They also absorb vibrations, which can actually make the slab more durable.
I wanna be a eunuchs developer! Pass me a bread knife!
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Hi Mark, I'm actually using a sheet of thin furniture backing board on top of a sheet of .5'' as the bottom of the mold, which will be the table top. I'm also putting in concave trim around the edges hopefully for a nice rounded edge. If I have to go thicker than 1.5'' I may need to worry about the weight.
Also, regarding your other post below about getting the air bubbles out, I don't have a roto-hammer, but I do have an old 2x12 stage monitor and a 300W amp! The forming/pouring will be done on my 4x8 workshop table so I can put the speaker underneath and vibrate the hell out of it...I just need the right music...something like the black album by Metallica should work! (evil laugh) This is sure to annoy the neighbors!
"Go forth into the source" - Neal Morse
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kmoorevs wrote: This is sure to annoy the neighbors!
I know some other ways to annoy the neighbours, if you need help with that.
the vibration isn't only to get rid of air bubbles, though; it's to form a layer of grout on the outside of the slab, so make sure you do enough neighbour-annoying! It wouldn't do to not annoy them enough!
I wanna be a eunuchs developer! Pass me a bread knife!
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Over here you can usually rent a cement vibrator at a hardware store.
Tomorrow I'm seeing my brother that's a mechanical engineer, I'll check with him if he has any pointers.
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I've seen it (on TV) done with sheet acrylic. Exceptionally smooth!!
I'm wondering about the vibration issue.
If the stones (aggregate) in the concrete are more dense than the cement, won't the vibration cause them to sink to what will become the surface?
Again, on TV, it appears they've used high strength cement with a fiber (fiberglass?) mixin for strength, instead of a concrete mix.
A positive attitude may not solve every problem, but it will annoy enough people to be worth the effort.
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The vibration causes the liquid to go to the outside, because it vibrates more than the stones and sand. Because the sand reacts more to the activity of the water, it goes along, giving a layer of what we call "grout".
Me, I'd rather do it with the top upward, for three reasons:
0: You've got more control of the finish (especially if you intend to harris or champher the edges, as stated).
1: You can actually see any imperfections, and give them extra attention.
2: if any cracks form while it's drying, you can rub them out.
There are all kinds of fibres you can mix in, most of which are available at larger hardware stores, but it doesn't hurt to just throw in whatever old bits of iron you've got (long nails, ferrous cables, even wire wool works, if you pull it apart and twist it to make it stringy).
The only difficulty in using fibres is that some of them can end up on or very close to the surface, which can result in ridges and micro-cracks, which "help" the surface to degrade quicker than normal.
I wanna be a eunuchs developer! Pass me a bread knife!
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Just make sure that you've got a working rotary hammer (big SDS drill) handy for the pour. Set it on hammer & no spin, and vibrate the cr@p out of the stuff for a lot longer than you think is enough (finish off by vibrating the shutter from underneath). When you see air bubbles on the surface, burst them, and keep vibrating in the same place for a few seconds.
You want a good quarter-inch grout on the surface of something like that, so that you don't get any eventual surface crumbling (where the surface comes away like sand, and you see the pebbles underneath).
I wanna be a eunuchs developer! Pass me a bread knife!
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Concrete good for compression forces but non the opposite and tends to snap. Hence why reinforcing is used. Concrete is cheap so why not pour a rough test slab about 30" square with some reinforcing and test it to destruction!
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RossMW wrote: test it to destruction
Sounds like fun!..and not a bad idea at all considering a recent DIY misadventure with a water heater... but I am using the rapid prototyping method for this build...plan carefully and hope for the best! Thanks for the advice!
"Go forth into the source" - Neal Morse
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Just as an aside, check out this company:
http://www.daviscolors.com/#concrete[^]
I know it's Saturday and this post sounds to me like you might be winging it because it is ...
Good luck!
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"You look so happy, today ... I ... I've never seen you look so happy !"
"Yes," I said, "I finally found one of you that understands me."
"Delightful ! ... you know ... you should get 'Employee of the Month' for this," She said, and she went and told all the others who soon gathered round me to wonder at how I'd changed.
Finally, one of them asked: "Well ... gosh ... who is it ? Which one of us understands you ?"
"You'll find the body in the small storage room near the fire-exit," I said, wiping away the blood that was running down my chin with a florally scented wet-wipe.
I thought it best not to tell them which parts of the body I ate because it was just before lunch-time, and I knew quite a few of them were vegetarians.
«To kill an error's as good a service, sometimes better than, establishing new truth or fact.» Charles Darwin in "Prospero's Precepts"
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I have no idea what this is but I love it.
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Bootking wrote: I have no idea what this is but I love it.
I am so happy that one of you understands me
«To kill an error's as good a service, sometimes better than, establishing new truth or fact.» Charles Darwin in "Prospero's Precepts"
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I don't know the answer either. Somehow, all of this sounds as if you are also a fan of Hannibal Lecter[^]. I feel sad for the one who understood you.
The sh*t I complain about
It's like there ain't a cloud in the sky and it's raining out - Eminem
~! Firewall !~
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Now I want to read the rest of the book. Do you have a publishing date yet?
- I would love to change the world, but they won’t give me the source code.
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He's still looking for people who understand him inspiration to write about in the final chapter.
If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson
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Luckily I don't understand you... 
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BillWoodruff wrote: I thought it best not to tell them which parts of the body I ate because it was just before lunch-time, and I knew quite a few of them were vegetarians.
Which parts of the body would you regard as food suitable for vegetarians? 
If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson
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Vegetarians eat greengrocers 
I'm a veggie myself, so I would know 
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Bill, you don't know me, but when you do know me, please don't eat me. If you have to eat me, then please sprinkle me with salt and pepper, to taste, first. Easy on the butter.
Much obliged.
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