the problem is the huge disparity between the stiffness of the rebar and the stiffness of the PS foam. I would add something to spread the force on the PS. A simple solution would be to use some mesh over the surface of the foam.

The bigger problem is that I think you are intending the have the rebar on the top surface rather than the bottom. This means that any force downwards will tend to pull the rebar away from the PS, and neither PS foam nor cement are very strong under tension. I think it would be better to have the rebar on the bottom and the foam on the top. Perhaps run some heavy gauge wire around the top side and put some thin wires through to put the foam under compression.

I'd even consider forgetting the rebar and just using fibreglass mesh.

Nathanhurst - wheeeee. Just using the mesh and forgetting the rebar is what I wanted to do anyway. It is a thrill to have you think it will work. Most people here seem to use other systems for supporting and reinforcing their benches and tables, so I thought I would try it their way.

I had intended the rebar to wind up on the underside of the tabletop - guess I didn't explain it clearly.

I have a question about your sentence:
"Perhaps run some heavy gauge wire around the top side and put some thin wires through to put the foam under compression."

I can imagine somehow putting enough pressure on the wire to cut or indent the styrofoam but not to compress it more. Or - was this idea going to mainly help attach the rebar to the foam?

If I can get the time I am going to make 2 identical tabletops - one with the rebar and one without - and compare the results.

I would be happy to hear additional suggestions from everyone about placement of the rebar for maximum effect.

Thanks,
Laura

Hi Laura,

Tango gave me a few pointers a while back. One of the things he pointed out was to not have the rebar joints meeting end-to-end, but rather overlapping and wired together. I used a lot more rebar in my current table, but it may have been overkill and I don't have foam in it. I made a rebar frame and sandwiched it between two pieces of lath.

Looks like you have a big project planned. Good luck!

Justin

You're probably right about the wire cutting through. However, if you put the mesh down then the wire, the mesh would support the wire. The idea is to use each material's optimum properties: wire is fairly stiff, but it really shines under tension. fibreglass is very stiff and bonds well to cement. cement is very hard, and protects the surface (and looks good). foam is strong under compression, weak under tension (and hence bends easily without a skin) and very light.

So you are trying to make a stressed skin frame using the fibreglass and PS. putting more compression on the foam will make it stronger (until you breached the crush point around 30lb/sqin). Perhaps just ties directly to the mesh?

Justin and Nathan; I appreciate the info you send. I just love trying to understand all this. It is great that people take the time to try to explain what they know.
Thanks!
Laura

Yes, I agree completely. I have learned tons here!!!

Oh, here's what it looks like when you get carried away with rebar. I suspect the correct amount is somewhere between your and mine :-)
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Fascinating ! Are there any pictures of the project in progress yet?
What is the basic size? Compared to the tool in the center, it looks like this would be a big table.
Thanks,
Laura

Hi Laura,

I'll try and post a few when I get a chance. I was thinking the plyers would serve as a size reference, but that wasn't the best choice. It's going to be big enough to be a pain for me to move ;-) I think the table top will be 4'x 2.5'. I'm aiming for it to look similar to the corner table I did last year, but we'll see. I'll post some picks later.

I really like your experiments with foam and fiberglass, and I've been following your progress closely! Keep up the good work.

Justin

Coupla' notes re: rebar...
As Justin noted, you definitely need to overlap & securely wire (or weld) all ends. Also, rebar must be more or less centered in the mix to add any strength. About 3/4's to an inch of concrete or 'Tufa above & below is the minimum. If you've ever seen a slab or driveway being poured over rebar, you may have noticed that the rebar was set up off the ground on little risers called "chairs". They are there to insure that the rebar is fully embedded and not sitting at the bottom where it provides no benefit at all.
And...I have to ask what the purpose of the styro might be? Are you trying to form a really "thick" table top and keep the weight down? If you have 4 inches of styro plus an inch & a half or so of mix on both sides you wind up around 7 inches. Thats a mighty chunky table top & twice as heavy as a single thickness over rebar & mesh would be. Just wondering as there are other ways to achieve the same "thick" look with less weight --- Tango

tango88
Thanks for your observations. I was planning on a 4'X 18" benchtop over 2" foam but I was going to cut grooves in the foam to "sink" the rebar. I was just planning to use 3 or 4 parallel lengths of rebar. Then cover the whole thing with, perhaps hardware cloth, perhaps mesh, perhaps both. Do you think that would work?

Just keep in mind that the rebar absolutely must be centered within the mix or it does nothing. I would suggest you make a couple of lengths of rebar that are bent at right angles on each end ( sort of an"[" shape)and sized about an inch smaller all around than the finished piece will be. Then overlap the shorter ends & wire them together securely. Attach some lath or hardware cloth to that and if you put about 3/4's of an inch of concrete on each side it will wind up only about 1 1/2" thick but support quite a bit of weight. Still not understanding the purpose of the styro for a piece like this...???

I strongly recommend over-engineering your framework & using a strong mix like concrete for anything that will be supporting people. 'Tufa & stucco are great stuff, but they were never intended to be used for anything carrying a structural load.

All my best --- Tango

As I said earlier, I would just chuck the rebar completely and use foam + glass mesh. Then you'd only need half a cm of cement mix. I think you could get away with 40mm foam and 5mm thick layer on all surfaces (giving a table thickness of 50mm = 2 inches). I've found that 5mm of cement and mesh over foam will happily take a baseball bat on 2 foot centres. Perhaps the biggest problem will be the table blowing away in the wind.

I don't know why the table has to be able to hold people, here in .au we use tables for putting dishes and plates and what not on...

To give you an idea of the strength of cement + foam sandwitches, look at this ThermaSAVE site. There is a truck parked on a bridge made from a cement - foam - cement sandwitch. So it certainly looks sturdy enough for people :)

Why am I pushing this? Because cement over foam is clearly better for the environment, lighter, more flexible in use, cheaper, quicker and easier to fabricate. Making a table out of rebar and concrete seems rather pedestrian and '60s. And when you discover you don't like it any more, you basically have to bury it. You can recut cement over foam with a hand saw.

I was just sitting down to address the question of why I would want to use styrofoam covered with cement for a tabletop when I could make a form and fill it with rebar and 4 inches of cement and be done with it. I saw that NathanHurst had addressed the topic with his usual gentle wit and his own experience. I liked everything he said and I loved the site he sent us to.
For me there is an additional component to this desire to make a lightweight concrete table/bench/sphere/leaf/etc. It amazes me. I have been around a long time and am still thrilled when my assumptions are proven wrong. Last summer I saw some men making an EIFS system exterior wall and went over to see if they knew where to get fiberglass mesh you could use with concrete. While I watched them I wondered why I couldn't use the neat stuff they were smearing on the wall to make myself a thin concrete leaf. I couldn't buy any (you have to be trained in the EIFS system before a manufacturer will sell it to you) so I started researching the product and came upon Quickwall and fiber concrete and knowledgeable people on this list and before long I was making 3 pound leaves that seemed as strong as the 20 pounders I had started with. What a surprise!
Everything I have done with this mesh and fiber concrete has been a surprise to me. It always is harder/ stronger/ lighter than it "should be" according to my intuitive grasp on the world of physics . My personality wants to see how far this idea can be stretched.
If I make 100 pound or 200 pound tabletops I will not be able to move them. If I make 40 pound tabletops I can deal with them. Now all I have to do is make them strong enough to be durable and safe. I think I can do it with cheap materials and some experimentation.

Right now I have taken the foam in the pics and gotten rid of the rebar. I have given all sides of the tabletop one quarter inch thick coat of quickwall (fiber/portland/admix) and EISF mesh (alkali resistant fiberglass). My plan is to cure this for a few days and see what I think but probably add a second coat of everything . Then I will think of some way to test the strength of the piece and post the resulting success or failure photo here.
Any suggestions what test would be dramatic but realistic for this purpose????

Laura

ps I have a horse. Would it be impressive to get her to stand on the tabletop (it would be resting on the ground) --- or would this prove anything??
She weighs about 1200 pounds. It isn't like she is going to "fall thru" styrofoam but she might crack off the concrete covering. I think I could get her to put her front feet on it if it was propped up on a couple of concrete blocks..... would that be better?

I would have to lug the whole project 30 miles to the place I keep her so this may not be the best idea....

(for anyone worried about the safety of my horse during this testing.... be assured I value her more than I do most humans. I would do a lot of pretesting to convince myself whether or not it made sense to ask Ellie to be the crash dummy.)

I am quite familiar with the new generation of laminated EPS materials which is precisely why I would still recommend the pedestrian rebar & concrete approach. Unless you have access to the full range of technology & controls involved, you are never going to approach the kind of strength these remarkable materials can offer. You could also build a remarkably strong & lightweight bench using carbon fiber & graphite technology...but not in your backyard.

I build both custom concrete furniture and sculpture for a living and have employed both technologies. For lightly loaded sculpture, high density polystyrene is great stuff. And, while I have indeed built benches that utilize "foam" internally, I could never recommend it for use by anyone other than a very experienced professional with a sound understanding of the engineering requirements involved. I certainly encourage any & all crafters to experiment and push the limits...but please...unless your knowledge AND your shop are equal to the brains & facilities at ThermaSave...don't try this at home unless you are prepared to conduct the rigorous testing necessary to confirm your systems long-term viability. There's a little more to it than slapping stucco onto an ice chest.

As Always, All My Best --- Tango

Tango, the extent of your knowledge and experience makes you such an exciting resource that I can't resist prevailing on your good will a little more. I am probably at that slapping stucco onto an ice chest stage but willing to learn. My thought is that a tabletop's safety and long term viability are pretty dependant on whatever structure supports it. With that in mind, I would like to test the 30 by 40 slab when I think it is really strong. Before I invest the time to make it beautiful, I want to reassure myself that it won't disentigrate or have chunks break off if I sit a 20 pound flower pot on it.... in other words - that it will act like a table should. Can you think of a practical test that makes sense? ( I will never sell this table - I am not developing a prototype for production... no one is going to sit on the table - I am just satisfying myself)
Seems to me that a good test would be to support the slab down the center on a row of concrete blocks - then put 10 gal water containers on each corner (out at the edges - not over the row of blocks) and fill them. If 80 pounds per corner caused no problems I would leave the weight there for a few hours and see what happened. If all went well I think I would be satisfied the slab could function as a table and I would finish it and use it.
Does this seem like a reasonable test considering my intended use?
Laura

Laura --- Sounds like a reasonable approach to me. Coordinating the weight addition from corner to corner might be a little tricky, but if you go around and add slowly you should be able to manage it. I am guessing from your central support idea that the actual table top will also have a central support, it that correct? If so, will the top & base be seperate or a single unit? The structural design makes a big difference in how a load is transferred to the ground. Also, consider testing with more than 80 pounds. That's really not much of a static load. If a 180 pound guy plunks his butt down on a corner that will be more like 300+ pounds (I haven't done the math, but you get the idea).

As far as other approaches, I'd have to know what the rest of the structure was like to make any worthwhile suggestions.

You are definitely right to question and test any assumptions or suggestions (including mine). Potential for failure exists in all we do. Ask NASA. All you can do is try and think through the consequences and address the more perilous ones. People sit both at...and on, tables. Some people are lightweights, others big & burly. Kids climb, jump & play on them (mostly when you aren't looking). Those are some of the things I try to take into account in my designs. Once they leave my studio...Lord only knows what they will be subjected to, so I need to prepare them to handle almost any eventuality.

As my father told me when I was a kid..."Prepare for the worst, hope for the best, and learn something from what you get".

Tango