strength for thinner layers on foam core

straw_dogMay 6, 2006


a bit of a broad question; inviting speculation or experience.

Without having a finished piece around, and in use, for a while, I'm wondering what to expect in the way of durability for some planters that I'm making.

Does anyone have an opinion on thickness, or limiting factors, for applying 'hypertufa' mixes to a styrofoam core where the intended use is as a planter?

I'm hoping to get away with about a 2cm thickness, beefed up at any sharp angles. These containers aren't huge; let's say 44cm by 22cm (18in x 9in). I read davidcdn mentioning a two layer approach, the first layer being something more substantial. I'm adding a bit of carpenter's glue as admix, and I just picked up some fibreglass drywall tape. There's a layer of freshly applied carpenter's glue before the mix goes onto the foam.

Time will tell, but are there any thoughts on this? I'm quite attracted to the idea of benefitting from the light weight as well as the form of the foam core.


Sean : s_d

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rickharmer(z4British Colum)

Morning,Sean.Seems we're both on the same page of development.I've made my styro forms,now I'm going to cover them.Since I've not gone that way before,I'm going to try the following:first coat would be with Mymud and detail mesh.This gives a hard,very thin coating to put my finishing coat on.I'll be doing it on progressive days as I don't want to lose the window to apply a second coating.A longer delay seems to cause adhesion problems for me,despite applying a little admix as a"glue" between coats.My second will be a coarse,coloured mix,going 1:2 or 1:3 to finish.This would be applied as a thin coat as well,but,completely covering the primary Mymud.I'd consider a third coat,if coverage is inconsistent.Like you,I'm doing this on a smaller piece(12 by 12 inches) as well as a base for a fountain display(18 inches high,12 inches wide),both of which I did in the blue styro from Dow.For now,I'm using the 1 inch thick stuff,although it does get up to 3 inches thick.
I know in telling this that some of the ingredients that I use are hard to find,and it seems they only come in contractor sizes-large!But,you asked...
Anyhow,cheers from the sunny shores of British Columbia.

    Bookmark   May 7, 2006 at 1:47PM
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Hi Sean
Why are you using the styrofoam? To add thickness or just as a mold? It won't add strength. It will shrink away from the crete over time and leave you with unsupported space. Carpenters glue on the styrofoam won't help and drywall tape will just fall apart. Forget those.
Steel mesh is a little heavy for a 2 cm thickness.
Hardware cloth (it's really wire mesh) is the stuff I would recommend. A nice 1/2 mesh or smaller. Apply one cm of mud to your styrofoam then press in the hardware cloth And apply another cm one top of that. It's important that your wire or mesh is well embeded in your crete or it simply doesn't do it's job. You can cut the hardware cloth into pieces and use it where you need the strength.
As far as layering crete be very careful about your admixes and concrete glues. They must be compatible with each other or delamination can occur. You don't need glues if you keep your work damp and add the layers within a day or two.
Admix in the crete is all I use under these conditions. I never use a bonding agent between the layers it can do more harm than good IMO.

    Bookmark   May 7, 2006 at 2:42PM
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Thanks for input guys(?).

Rick, I've reading your posts. As a matter of fact, I was thinking (read hoping) that the mesh you were referring to was the fibreglass drywall tape that I'd picked. Made by 'Sheetrock'. Looking at the data sheet now it doesn't mention alkali resistance. When I can get my ass out to Burnaby I can see about getting some of that silica fume to try out.

truegrit, there should be a snap with this post. The idea was build the planter around a styrofoam form. It would be lighter, styrofoam boxes are freely available, and easy to work with. It seems like it would fine for a wall hanging or somesuch. It was the potential for abuse that a planter full of soil and plants present that has me wondering.

btw, here's a neat link. More than you'd ever want to know about fibres in concrete. "Use of short fibres in structural concrete to enhance mechanical properties". I found it pretty interesting.



    Bookmark   May 7, 2006 at 6:14PM
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rickharmer(z4British Colum)

Hi,Sean,Just did 4.5 hours of creting my fountain base.Using my mix,I can work upside down if necessary,but the drawback is very quick set time.I use the detail mesh as described in an earlier post.The base is 12 inches a side,18 inches tall,with a foot and a top that's been cut out to accept a ball.I'm keeping it simple at this point.The blue styro that I use,according to the manufacturers notes on the Dow website,does absorb water,but minimally at best.I can't speak to the packing styro that you're using.I know of at least one artist who uses styro extensively in her works(Check Marion Lea Jamieson-saw her stuff a while ago,very impressive up close,also does complimentary paintings to her pieces).Some of the Dow stuff is up to 3 inches thick,so I daresay there would be some support there.I have a spare piece of 2.5 inch,I'll play with that at another time and let you know the results.Unlike the previous poster,I do use a light coating of admix between layers to assist in the bonding process.I mean,a little.Too much can negatively affect your mix.Hope you're having success with your work in styrofoam.
Cheers from here

    Bookmark   May 7, 2006 at 6:26PM
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Hey Rick,

I'll be interested to see how that turns out.

I checked out the Marion Lea Jamieson site. I liked the paintings especially. A couple of those 'still life's would be awesome on fabric I'm thinking.

As for me, I'm kind of getting ahead of myself really with all these questions. Gotta wind in and just keep puttering along.




    Bookmark   May 7, 2006 at 8:28PM
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rickharmer(z4British Colum)

Sean-the mesh that I refer to is "alkali resistant".In many areas of the world,this mesh forms the backbone of house stuccoing.In the U.S.,it's used in EIFS(numerous websites);the place I go to(in Burnaby(!))sells to that trade area.With luck and a lotta searching,you should be able to find it.Here in Western Canada,stucco is not used as much.We have wood or Hardieplank,or(shudder!!!)aluminium siding.Sorry,people,lasts forever,washes easily,yes.Looks like the West Coast,nada.
Sean,should you want to really try the detail mesh,the tape you have might work.It's just a little time consuming to use such a small section.I cut my mesh into 5 by 5 inch sections(roll is 9 inches wide by a whole lot long!);easier to work with.Check out Laura's(ltd123) comments from last year around October.She's done(and talked about it)the ground work on the use of the mesh and Quikwall(check for availability in your area)).We're not making bridges,we're doing small art pieces.And,this stuff works.It's fun to be into lightweight concrete hernias here!!!
Cheers and good luck in your searches.

    Bookmark   May 7, 2006 at 8:30PM
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rickharmer(z4British Colum)

I forgot to mention about this foamboard I'm using.When I made my form,I rested the top piece(the ball) on the pedestal.The ball weighs about 14 lbs,and there was no flex or deformation of any sort.So,while a 1 inch thick panel has little strength on its own,it does OK when put together in a boxlike structure.So,if this thin stuff can handle that,I wonder what 2 or 3 inch stuff can handle?Not to be used in a bridge or other unsupported piece where humans can sit/step/jump on it?
Cheers and back into the studio for second coating!

    Bookmark   May 8, 2006 at 12:50PM
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Hi Rick,

yes, I think that the form would provide some stiffening, especially in tandem with a layer of YourMud. Unless there's shrinkage, like trugrit suggests. Probably less of a factor with the denser stuff that you're using.

I came across a thread recently while hunting. A lady, I think, who was torture-testing pieces of coated foam. That had slipped my mind. At any rate, it seemed pretty strong.

Here it is:

... er, the one that you mentioned. Ok, I'm going in circles. I'll salt it with some search terms and paste it below for reference. Have a good one; nice day for playing in the mud,

They must be disallowing external links on the image, but here's the url


Here is a link that might be useful: lightweight styrofoam reinforced concrete

    Bookmark   May 8, 2006 at 3:04PM
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Sean, I used stucco in a tufa mix for a planter, and you can carve a good bit away to make the planter lighter. I only used the stucco because someone gave it to me, but it has lotsa little fibers in it, I'm assuming it makes the item stronger. I'm not too good at measurements and being precise, but I can check out the measurements, if this is the sort of planter you have in mind ?

Here is a link that might be useful: planter

    Bookmark   May 8, 2006 at 9:51PM
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Hi bonnie,

yes indeedy

I think the article that Rick pointed to has answered my questions, but what's your impression with that stucco material? Seems ok?

Nice planter, btw; I like it



    Bookmark   May 9, 2006 at 12:36AM
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rickharmer(z4British Colum)

Sean-ltd123(Laura)did a lot of research on this subject,and I imagine its her thread you're referring to.She found that Quikwall(a popular stucco mix,not easily available here in Western Canada)and detail mesh(part of EIFS)worked in interesting ways.Without having Quikwall(tried a local product called Greatwall,not enamoured with it..)I came up with Mymud.Another area of interest for this kind of concrete is with websites dealing with,wait for it,concrete canoes!Engineering schools throughout the US and Canada regularly compete in races with canoes that the school builds,from shapes and formulae that they develop themselves.They get into exotics like microspheres and Kevlar detail mesh,way beyond my Stone Age recipe.Again,interesting stuff.
Cheers from here

    Bookmark   May 9, 2006 at 12:45AM
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posting a couple of pictures; proud of my new creations, and reassurance that the pestering that I'm doing for info may actually be applied somewhere down the road. These are two pieces to serve as corners on a slightly raised flower bed. Holes in angle of corner remain as small planter.

Corner pieces cut out from styrofoam cast-off; glued together; fibre screen to close bottom of planter cavity; carpenter's glue as per original internet howto.

Corner pieces being set up. Helianthus tuberosus, Jerusalem artichoke, that bracket the spot will grow tall and relatively straight; it should be nice.

cheers and thanks,


    Bookmark   May 21, 2006 at 4:00PM
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DebZone8(S.Puget Sound)

Very nice! And these have styrofoam encased inside? This is my kind of project--my little blocks, even made of tufa, are pretty heavy. How tall are they?


    Bookmark   May 21, 2006 at 8:03PM
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Thanks Deb, and yes, it's a hypertufa mixture on top of styrofoam. Standing about 12 inches high.

I'm not sure if they're durable enough really to be in a garden. With shovels crashing around etc. But .. I guess we'll see. Certainly very light.

If I continue putting hypertufa on styrofoam a solution will be needed for the slumping. The bottoms were done, and then the sides after many hours of setting up, but it was a pain. The material was sagging down. I've done three other troughs like this and they've all wound up with a sort of a pot-bellied aspect to them. A case, perhaps, of the trough taking after it's owner.



    Bookmark   May 22, 2006 at 1:42AM
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nathanhurst(VIC Aust)

I'm not sure about this delaminating theory, here in .au most houses are constructed with PS paneled walls with cement-acrylic base coat with fibreglass reinforcement. We've just redone our garage this way, and the company providing it guarantees their product for 20 years (when applied professionally etc), and claim to have had it in common use in QLD for 20 years already. I guess that delam might be a problem with poor basecoat choices.

    Bookmark   May 30, 2006 at 2:44AM
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nathanhurst(VIC Aust)

incidently, I have thousands of these cement over foam boxes for plants - I made a retaining wall out of the self locking ones filled with subsoil. I found the best strategy is to stack the boxes and fill with dirt, then render.

My rendering technique was to make a sticky mix from subsoil and 10% cement then flick handfuls at the surface. You get a nice granite look (particularly as the subsoil I used had chunks of quartz and feldspars in it), and once it has partly cured, I attacked with a high pressure cleaner to get more sparkle. After a year I had mosses in all the right places.

We've moved though, so I don't know how long it would have lasted.

    Bookmark   May 30, 2006 at 4:43AM
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Hi Nathan,

I'm not sure what you mean by 'the self-locking ones'. Interested to hear what you're saying about how you covered them though. Is that the entire recipe? Subsoil and cement? It must be at least moderately durable to hold up to a pressure washing. I don't think that mix I used, and/or the way I used it is sufficiently durable for the garden. I'm already seeing some dings on the pieces that I did. I wouldn't have much hesitation about many other settings, though. Small wall hangings, and so on.



    Bookmark   May 30, 2006 at 6:01PM
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nathanhurst(VIC Aust)

by self locking I mean that many foam boxes, if you stay in the one design, will stack together with little bumps and grooves and crenulations and whatnot so that when they are stacked on a truck they don't move around.

My preferred box is called a 'brocolli box' here and is 1'x1'x2' without holes and with a lid. They have 8 pegs moulded into the tops and 8 matching dimples in the base, and you can stack vertically, or any kind of stretcher bond (because the pegs make two squares). They are strong enough that you can fill them with water and stack them 5 high without collapsing. They're getting hard to find though, as growers seem to have switched to some kind of reuseable polyethylene or polypropylene crates.

The subsoil I use is clay with granity chunks in it, and it sets fairly hard without cement, but slumps when wet. Adding 10% cement powder makes it rock hard. Trying to remove the surface often resulted in the polystyrene pulling apart rather than the surface. (When we moved I had to dismantle the wall, and I ended up using a hammer to break the surface up, even then I only removed enough to get them apart) Part of the magic is the technique of splatting the mix on rather than troweling. The force drives the cementy mixture into the cracks and forms a strong bond with the foam. You also get a far more natural finish than trowelling approaches. This process could be automated with a concrete sprayer. I found it helpful to rub over the surface with a cloth with some mixture on it. Originally I thought this was to wet the surface, but I now think that what was happening was that the gritty mixture scratched the surface giving a better key.

I think polystyrene and a tensile element is probably one of the best structural materials out there for this kind of work - PS and mylar film is still the material of choice for ultralight mirrors used in cloud chasing. Look up 'tensegrity structure'. My prefered method is PS with fibreglass mesh and acrylic-cement basecoat. Then you can go nuts with whatever tufa you like.

They make buildings out of PS foam with plywood coatings now
and cementboard coatings

    Bookmark   May 30, 2006 at 8:25PM
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Thanks for posting that. I've not given up on the outdoor styrofoam items, but I'll want to try a different mix,



    Bookmark   May 31, 2006 at 12:34AM
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