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My husband and I have a design for outdoor seating we would like to create that is organic (curvy) in shape and surfaced in mosaic designs. Think of a dragon undulating in and out of the water, the part that appears above the water's surface would be something like what we want to create.

I have experience using chicken wire as an armature under plaster and I know this degrades with use - the bendy nature of the armature causes the plaster to break. I don't have experience with concrete but I suspect it would be likely to degrade in the same way. Is there a way to improve the plasticity of concrete? Is there a particularly good way to create an organic form, or should the seating be solid concrete?

Is fiberglass a better choice?

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I think you're on the right track, you just have to upgrade the chicken wire to something a bit beefier. Take a look at the underside of this fake rock, and notice the rebar frame with heavy wire mesh form. The frame itself should be rigid enough to not bend/flex, which will then hold the concrete firmly in place. –  Tester101 Jan 30 '13 at 12:24
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up vote 2 down vote accepted

Concrete will definitely be strong enough if done right.

Make sure you use the right mix. I would use sakrete 5000 plus, NOT quikcrete 5000, and add a hand full of Portland cement to better hold the mosaic. Then add superplasticizers. More water is bad, but you want it to flow into an odd shaped mold and around mosaic tiles, superplasticizers will solve this. You can get this at http://www.expressions-ltd.com/default.asp You can get some integral coloring for the concrete there too.

The forms will be tricky, but i think i would build a wood box and drape some cloth in it the way you want. Then spread some glue on it to stiffen it up. Then use bondo and sand it down to smooth. Then you can use some spray adhesive or silicone to put your mosaic tiles in upside down.

You are going to need to vibrate it after you pour it. Maybe just hammer on the wood box.

I am saying this would be solid concrete, but this will be very heavy when you demold it. You could put something lighter in the mix after you pour it, maybe some empty bottles?

Anyway, yes, this can be done with concrete. Concrete would be very strong, and no you don't need mesh in this setting. The tensile strength of concrete is terrible but the compressive strength is amazing.

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Thanks for all that! Super useful info. The superplasticizer part is especially helpful. I was expecting to build the form and then apply the mosaic tiles separately. I hadn't considered that I could do it all in one step. Other than the difficulty of thinking upside down and inside out :) is there any benefit to doing it all in one step over adding them after the form is built? –  kewpiedoll99 Feb 4 '13 at 16:15
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Yes. It would be much stronger to embed the tiles in the whole vice attach them with mortar. Visualizing it upside down and inside out will be difficult for sure. Sounds like fun though. –  Tatton Chantry Feb 5 '13 at 0:09
    
would it affect the strength of the piece if you put a lot of styrofoam pieces in the mix (after pouring) to take up space and lighten the whole? –  kewpiedoll99 Feb 14 '13 at 21:25
    
That should be ok. There are a lot of concrete artisans who do foam cores. I have never done it, so I can't say exactly how to make it work the strongest. You might look into putting a little glass givers in your mix though. –  Tatton Chantry Mar 8 '13 at 2:03
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