Concrete over foam board

[Jruhl]

I make precast concrete countertops in my spare time, and the one disadvantage I had was the weight of the tops was so much that I had difficulty moving them by myself. I tried an experiment by utilizing 3/4 inch foamboard in the lower half of the top. I put linoleum glue on the foam and sand over that. I tried using this glue to hold the rebar to the foam, but the glue never dried and wouldn't hold the rebar. The top looked great, I moved it around and finished it, then installed it in my own kitchen. About a week after installation, I noticed a crack running above the 2 long peices of rebar that got worse every day. Then 3 cracks appeared running front to back. I think I determined that the two long cracks appeared because the top was cantalevered and the weight put upward pressure on the rebar, which was too close to the surface (next time I am going to try recessing the rebar into the foam). Also, I think I need to try a faster and stronger setting glue to adhere the sand. I also thought about cutting the foam in half lengthwise and leaving a 2 inch gap between to place a reinforcing strip of concrete and a peice of rebar. Can anyone here think of anything else I can do to make this work this time?

[John Bridge]

Hi Jruhl,

We've done a couple threads on concrete tops. You might try a search. Also, there's a good book out there on the process. I've never tried it myself, but it seems to me you won't have enough mass of concrete over the rebar with it sitting on top of the foam. Rebar, in this case, is not the best reinforcing you could choose. 1X2 heavy mesh would probably work better.

And the cracks sound like stress fissures, and that may have something to do with the mix. Don't know. Hang around. It's a popular subject.

[Bud Cline]

Back in my days of doing concrete flatwork we always wanted the rebar 2" below the surface to eliminate surface stresses from the bar. I don't know why we did that though, I think it was a government standard.

Johns right, why not rewire?

[cx]

First, give us a better como-se-llama for yourownself. I can't spell jruhl, and I damn sure can't pronounce it, eh?

Rebar in concrete is of no value unless completely surrounded by concrete, and there is a formula for just how much concrete must surround it based upon the diameter of the rebar. It's something over 2x the diameter for smaller bars, as I recall, but don't quote me on that.

So, laying the rebar on the foam and covering it with concrete makes the rebar a negative rather than a positive feature; basically it's a long void in your pour. You would do better, as Mr. Bridge points out, with some smaller reinforcing material, or leaving out the foam and finding a better way to handle the slabs. Also not sure why we are gluing sand to the foam. If it's to let the concrete adhere to the foam, I think you're wasting effort there. Don't think there is any advantage at all.

I think you need to rethink the process. I don't think you can make a precast top less than 2" thick and hope to have it sound enough to be consistantly useful. The big guys what write books on the subject like 2 1/2 inches, and they use a little #4 bar and some 6" mesh centered in the pour. For ease of handling, they suggest you limit the size of each piece and be creative on the location and shape of the dividing lines.

You can get a little reduction in weight by using lightweight agregate, but probably not enough to outweigh (pun fully intended) the disadvantages.

So sayeth CX, who, when he finishes this top he is fixin' to pour soon, will have completed an impressive number of such creations.




Well, if you are impressed by the number one, that is. But I have poured a few yards of concrete in my day.

[John Bridge]

You know, it didn't occur to me that sticking concrete to styrofoam doesn't make sense. The foam is not strong enough to matter.

[tileguytodd]

Ah,But it is ze soundproof countertop vich fools ze mouse into believing it is not a real Mouse trap


I just read that Damn its been a long week