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Old 24th February 2012, 04:15 PM   #21
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Here is my limited experience with concrete as a architectural material... and in short. Making a slab counter top with built in sink (very, very, shallow depression) with a bunch of friends.

Materials were bought on a run to Home Depot to get a lighter shade of gray concrete for fine work, lots of gloves, thich plastic drop sheets, very smooth 3/4 inch plywood, a tonne of screws (we used drywall screws -- just a lot of them along with wood screws), coloured aggregate (it looks like beach glass, he had small amounts of clear, white, and blue, with a double amount of the green), cardboard tubes (inserts so we would have minimal drilling to do), construction glue (LePage PL9000), and one of those buckets you throw ice into for parties.

Toolwise: a big plug in single-speed drill (it was handy and had a D-handle) for the mixing thing (we used a wire spiral versus a paddle with holes), trowels, gloves, hammers, drill drivers, jigsaws, and circular saws.

Most important tool? There is this massive vibrator thing (no kidding) from Makita the friend who wanted this thing made, he had ordered it on-line. It looks like a handheld drill. SOOOOO IMPORTANT! This is the thing that got the bubbles out. You need to wear thick wooly leather gloves so your hands don't feel the vibration as much because they will get all lose and tired.

The other thing you need, is time. We thought we could cut, put together, and pour in two days -- setting and polishing is a week or two away. Nope. The mould took two days for the cutting and the putting together. Here are 4 to 6 guys drinking beer, old Pentium 3 laptop hooked up to the internet (music and instructions), and saws. The house was a mess with plumbing and sawdust (this was for a kitchen).

You get the mould done, line it, and pour, there are instructions on the web for the right mix, but you get the concrete in and put on the ear muffs. Then go to town with the Makita vibrator thing. Truly amazing how many bubbles you get in concrete and how many come out. Keep adding and mixing before it sets and done.

Over the span of 5 days you have to pour small amounts of water so the concrete will stay damp, cover it with a plastic sheet, so it will cure harder. He left it for another week after to dry out a bit.

Get back there and start removing screws, removing wood, and removing plastic. Lots of lifting (HEAVY!). And then polishing, waterproofing, and more shining, which I missed out on, but the slab was shining like no one's business afterwards. The aggregate he put in was nice for an accent and the whole thing looked pretty professional (the corners were not well thought out) and it was about 4.5 inches in thickness.

Maybe it came out to $500 with the Makita thingy, plastic sheet (that stuff turned out to be expensive when we added it up), wood, and other materials.

There is a Japanese guy who does this stuff for a living and he wrote a book on forming concrete for project like this and there is a bunch of articles on websites out there about it too. All I can say is it is a lot of time and work.

Good luck and tell us how your project comes out.
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Old 24th February 2012, 04:27 PM   #22
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I tried cheating slightly. I purchased a 8" horn from a pro-audio sight for around 8.

Lubed it up and poured in plaster of paris, securing it with clay to a granite plate.

The idea was to make a box around it and cast the negative space. This i never quite got around too.

Came out ok, pretty smooth, top wasn't flat though, and in trying to get out the bubbles, my clay seal failed slightly.

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Old 24th February 2012, 04:30 PM   #23
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Also tried casting concrete tapering tube shapes using champagne flutes.

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Old 24th February 2012, 08:04 PM   #24
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Studio Au, that is a wicked idea! That's exactly what I need to do for my tweets
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Old 25th February 2012, 08:25 PM   #25
SunRa is offline SunRa  Romania
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Hi there, regarding horn moulding out of concrete, take a look here, this guy is doing exactly that: MiWi-Horn

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Also, the author opened a thread on this forum, search for MiWi. He detailed a lot the process in that thread and there's valuable info there as he experienced with various mixes of concrete.
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Old 25th February 2012, 09:14 PM   #26
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Would attaching a "bass shaker" or tactile transducer to the mold assist an removing bubbles? You could use any stereo you have laying around Might be cheaper than trying to find a specialty too as well... I am getting more and more curious about cement as I learn more! I would love to make an econowave with a cement baffle. I could really smooth out the baffle edges if the horn shape is integrated into solid baffle mold!
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Old 25th February 2012, 11:11 PM   #27
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Quote:
Originally Posted by SunRa View Post
Hi there, regarding horn moulding out of concrete, take a look here, this guy is doing exactly that: MiWi-Horn

Also, the author opened a thread on this forum, search for MiWi. He detailed a lot the process in that thread and there's valuable info there as he experienced with various mixes of concrete.
Some beautiful work done there... I wonder what the concrete would do to the sound compared to wood?
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Old 26th February 2012, 07:44 AM   #28
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I have used a standard palm sander as a panel vibrator, of course without sand paper on it.
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Old 26th February 2012, 10:54 PM   #29
SunRa is offline SunRa  Romania
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Hi again,

I found the thread I mentioned above here it is: Introducing me and my speakers

I recall that one of the major problems with the concrete mix curing was the exothermic effect. The guy actually submerged the molds in a water pool to prevent thermal craking. It seems that is a serious issue, along with compacting.
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Old 26th February 2012, 11:20 PM   #30
Nanook is offline Nanook  Canada
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Default Perhaps the "wonder board"?

Looking at the use of "wonder board" or similar, this could make an excellent enclosure. But the enclosure would have to be quite basic, a very simple horn or BR type. Any real complications and the benefit would be wasted. Just make a fame work to glue/screw the board pieces to, and be prepared to spend some time finishing the exterior. or adhere pieces cut for complex shapes and expect some time to do some filling and smoothing.

Rauna made some loudspeakers using concrete in the late 1980's and 1990's. Not sure if they continued along with them or not.
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