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Thread: Yet more concrete-related questions.

  1. #1
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    Yet more concrete-related questions.

    After much searching, I've decided that I may as well just use concrete for my hollow-sphere speaker project. By using casting foam to make a mold of a hemispherical wooden bowl ($20 from Ikea) with all the requisite details added, using modelling clay, I can cast four identical concrete hemispheres which can be combined into two nice neat spheres.



    The problem, however, is that I don't know much about concrete beyond having once helped put a bit in my backyard. With the right additives and enough mesh, a ~1/2" thick wall should be possible, but I'm not sure exactly how. Anywho, I have the following questions:

    1. How do I make it stronger? There are several answers to this questions, including mixing in chopped fiberglass, chemical additives, and various types of plastic and metal mesh. What method would you recommend?

    2. How do I make it easier to pour? I'm aware that there are several types of plasticizers and similar chemicals to choose from - which ones work well?

    3. How do I improve the aesthetic appeal? If anyone knows any tricks for tinting, mixing, or polishing, I'd be open to hearing them.

    4. How do I join the hemispheres together after casting? I'm a bit stumped on this one.

  2. #2
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    I don't know much about concrete myself, but I do know that the big borgs often sell an additive that will color the mix... also know that if you let it set just slightly, you can take a trowel and water to smooth the exterior... When I had a patio poured, the concrete guy just kept trowelling the surface with a little water until it was really smooth...
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  3. #3
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    Sounds like a fun project. I've not really worked on anything quite like what you're thinking of, but I'll take a stab at your questions. Others will likely have more ideas (and experience). And if I get a chance, I'll bounce your questions off my dad. He's a PE who's been engineering concrete mixes and applications for over 50 years.

    Quote Originally Posted by Joseph Shaul View Post
    1. How do I make it stronger? There are several answers to this questions, including mixing in chopped fiberglass, chemical additives, and various types of plastic and metal mesh. What method would you recommend?
    There are two kinds of concrete strength I used to deal with...compressive and flexural, and neither should be a problem for your project. For molded shapes like a hemisphere, I'm pretty sure the actual strength won't be real critical. Keeping it from cracking will be an issue, though. Chopped fibers or mesh will help in that regard. I think Sakcrete sells a "reduced cracking" redi-mix with fibers in it. Pouring around a metal or plastic mesh would have a similar effect, although keeping the mesh in the middle of the wall thickness would be a real challenge.

    Another important consideration for minimizing cracking is the amount of water you use in the mix. If you make the mix too wet, the finished product will have shrinkage cracks from drying.

    And yet another thing that can reduce cracking is how much air is in the concrete (in the form of microscopic bubbles). There are chemicals that can be added to a mix to add the air bubbles (called "air entrainment admixtures"), but I suspect they'd need to be purchased from a concrete supply house, or perhaps your local concrete batch plant. For your application, I'd think you only need a few ounces at the most.

    Quote Originally Posted by Joseph Shaul View Post
    2. How do I make it easier to pour? I'm aware that there are several types of plasticizers and similar chemicals to choose from - which ones work well?
    I've been out of the business too long to know any name brands, but a plasticizer would be the hot ticket. Adding a little bit of plasticizer to a load of relatively dry mixed concrete makes it as soupy as if you'd added lots of extra water, but without the risk of shrinkage cracks that water would include. Like air entrainment admixtures, I don't think you'll find plasticizers at a home improvement store. Here again, you'd only need a little bit and probably need to get it from a concrete supply house a concrete batch plant.

    Quote Originally Posted by Joseph Shaul View Post
    3. How do I improve the aesthetic appeal? If anyone knows any tricks for tinting, mixing, or polishing, I'd be open to hearing them.
    There are a number of tints you can add. Some are at the home improvement stores, abd you may find others at a concrete supply house. A lot of architectural concrete has an extra layer of grout rubbed into the surface after the forms are removed. It's basically a slurry of cement, sand and water rubbed onto the surface with a foam trowel to smooth out the surface and fill any minor voids in the surface. Once you have a smooth surface, there are several concrete sealers (often made for floors) that would add some shine to the finished product.


    Quote Originally Posted by Joseph Shaul View Post
    4. How do I join the hemispheres together after casting? I'm a bit stumped on this one.
    Epoxy. A concrete supply house will have specialized epoxies for concrete, but I'm pretty sure any of the clear epoxies you'd find at the hardware store would work. I worked on a project once where a school had shifted due to ground settling, and the foundation had sheared off the auger-cast concrete piers it was sitting on. We jacked the building back into place and literally glued it back onto the piers with pressure-injected epoxy.
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  4. #4
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    I hadn't considered an epoxy - it sounds to be a pretty good solution.

    One product that came highly recommended on a discussion forum for concrete countertops is called Liquid Z. I'm a bit perplexed as to what, exactly, it does, or how I should go about using it, but it's quite a lot cheaper than the competitors and is sold by the gallon.

    http://www.blueconcrete.com/countert...s/liquidz.html

    I was thinking about placing several mesh hemispheres between the two layers of the mold, taking care that they shouldn't poke through the outside. (If they poke through the inside, it's not a big deal - nobody will ever see it.) I'm told that regular fiberglass mesh works quite nicely for this.

    One question I have is as to how I might add mounting points for the screws that hold down the woofer. One solution someone brought up was to place wood pegs around the perimeter of the hole before pouring the concrete, and screw the woofers into the pegs. I'm a bit nervous about moisture expansion, though - maybe I should use polyethylene?

    Another question is if anyone can tell me where I should put the sprues, and the best way to remove them after casting.

    For coloring, I've come up with a rather interesting idea: Apply some conductive paint to the whole thing and copperplate it. The cost here might be a bit high, but the net result should be really awesome.

    I should probably mention that I'm extremely fond of concrete, mostly because it seems to have so much promise as a material - with the right additives, you can do almost anything. It's strong, it's cheap, and by replacing the sand with ground-up soda bottles, you can simultaneously reduce weight and decrease the size of the landfill - and with the right construction techniques, you can knock together a complete house in a few days. There's also the issue of durability - the Romans were quite fond of concrete as well, and rather a lot of what they built is still standing.
    Last edited by Joseph Shaul; 03-18-2010 at 02:33 AM.

  5. #5
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    Sounds like you've done more reading about things like concrete countertops than I have. Lots of interesting stuff on the Blueconcrete site.

    The Liquid Z looks like some type of polymer to seal the air cells (or prevent them entirely) in the mix. Super Flowz is their plasticizer, and Super Sealz is a pozzolan, which is typically used to replace some of the Portland cement in the mix. I've seen pozzolans like fly ash added to a mix to save money (fly ash is less expensive than Portland cement), plus it slows down the curing, reduces the curing heat, and adds flexural strength. Not sure what the exact benefits the Super Sealz product adds, but the Blueconcrete folks could probably tell you more about it.

    For the screw mounting holes, I think I'd try to find a way to embed t-nuts or something similar. I suspect you could plug them with paraffin to keep the liquid concrete out of the threads, them heat it enough to melt most of the wax before inserting the screws.

    How thick of walls are you planning for the sphere to have? If they're too thin, I think you might have some trouble getting the mix into all the nooks and crannies. A lot will also depend on the size and shape of your aggregate. Here again, I'm guessing the Blueconcrete folks will have more pertinent info than I do.

    I don't have any experience with pouring concrete through sprues, so I don't really have any advice along those lines. If you're planning to build the sphere in two halves, I think you might find it easier to leave the equator of the each hemisphere open so you can pour the mix all the way around the circumference. (Basically a bowl suspended within a larger bowl, with concrete poured between the two.) You could pour level to the edge of the hemispherical form, then after removing the forms, grind the edge flat for a good mating surface to use when gluing the two halves together.

    It might be worth doing a practice run or two with inexpensive Sakcrete (mixed extra soupy) just to test various things out. Then once you figure out a workable process, cast the real thing with the fancy admixtures.
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  6. #6
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    For the size you are working with, if you use glass fibers would you really need to worry about having steel mesh? Seems like that might be overkill.

    As far as screw holes, I'd say just cast the thing and use a masonary bit to drill the holes in the proper location for the speackers and epoxy some threaded inserts into the holes to hold the speaker.
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    Quote Originally Posted by Brent Dowell View Post
    For the size you are working with, if you use glass fibers would you really need to worry about having steel mesh? Seems like that might be overkill.
    Is there really such a thing as overkill?

    I'm rather more partial to the idea of using plastic plugs for screwing the speaker into, if only because the screws that fit in the screw holes on the woofer sometimes have some weird threads. Also, I'm rather scared of shattering the concrete with a masonry drillbit, if only because some of the additives are quite expensive.

  8. #8
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    Quote Originally Posted by Joseph Shaul View Post
    Is there really such a thing as overkill?...
    The more non-concrete things you have in the form and in the mix, the harder it will be to consolidate it and get rid of the voids.
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  9. #9
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    Quote Originally Posted by Vaughn McMillan View Post
    The more non-concrete things you have in the form and in the mix, the harder it will be to consolidate it and get rid of the voids.
    Interesting - I didn't know that. Then again, that's why I'm here.

    What if I made a layup of fiberglass and concrete on the inside section (the dome-shaped part) of the mold, then place the outside of the mold (the bowl shaped part) on top of it and filling the remaining space with concrete? This would give me the structural integrity of ferrocement without the lumpiness and lack of precision.

    The best part is that I can tell people it's a "pozzolan-reinforced silicate matrix", which sounds a lot better than "I made it out of stuff from the paving aisle of Home Depot."
    Last edited by Joseph Shaul; 03-19-2010 at 05:53 PM.

  10. #10
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    I'm still not sure the fiberglass mesh is necessary, especially if there are fibers in the mix too.

    What's the outside diameter of of the sphere(s) going to be? How about the wall thickness? Let me know your estimates for those numbers, and I'll get ahold of my dad this weekend and see what ideas he might have.
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