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I've been wanting to make a concrete counter-top for a long time but there's one section that is giving me trouble. The mold would have to be bigger than would fit on a standard 4' x 8' sheet of melamine. Hoping someone might have encountered this before.

The dimensions in the diagram aren't exact but give a rough idea of the size and shape.

L Shaped counter-top with corner sink

As I see it I have 2 options.

Connect 2 pieces of plywood to create one large mold

Not sure if this is a good idea or not. Might create an uneven surface where the melamine is joined or create the appearance of a seam? Need to reinforce it to make sure it doesn't come apart with the weight of the cement mix. Also will have one very heavy piece to move.

Make 2 molds

I can split the counter-top into 2 molds and then seam the two pieces with epoxy. It should blend in pretty good plus the sink hole will remove a lot of the seem from view. I'll also have 2 more manageable slabs to move around.

3 options for where to put the seem. Yellow line, red line and blue line.

Red and blue lines I'm worried might be too close to the sink opening and compromise strength? Anyone know how close it's recommended to have a sink hole to the edge? Blue

Blue line is pointless now that I look at it. No benefit and just makes the weight distribution between the 2 parts weird.

Yellow line is probably the least conspicuous location but I think might make the mold a little more difficult. Especially getting the sink hole edges to line up. I'd like to use an undermount sink.

Make 3 Molds

After reading the helpful input and better understanding the challenges I'm now leaning towards making 3 molds with a seam at the red and blue lines.

I think it will make it easier to create the molds, make it easier to get a good sink hole and overall be stronger. Three slabs will be easier to move around than 2 also.

Open to any opinions.

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Plywood or melamine is regularly sold by special order in 5X10 sheets and ordered in custom widths and lengths too. –  Jack Dec 13 '13 at 5:46
    
Are you pouring on-site or off (I assume off). If so, I don't think you'll want to do this in one monolithic pour simply due to how difficult it will be to transport and install without it breaking along the sink area. –  DA01 Dec 13 '13 at 5:58
    
@DA01: I have seen slabs more challenging than this transported by building a rigid frame clamped on to relieve strain near weak areas like the sink. –  wallyk Dec 13 '13 at 6:41
    
@Jack a 5x10 sheet is still going to be too small by about 6-7 inches. –  OrganicLawnDIY Dec 13 '13 at 14:20
    
@DAO1 I'll be pouring it in my garage when the weather gets better. It won't have far to go but still would be nice to have lighter slabs so I need less people to move it. –  OrganicLawnDIY Dec 13 '13 at 14:21
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Yellow line is where you need to go, make the form slightly longer on the meeting edges so the ends can be recut to a clean joint. the reason being, moisture loss in concrete will change the look of the slab were it looses it rapidly. This means at any joint in the form where water can leak out will force the concrete to cure differently. Caulking the corners will help fix the problem an will also radius the edges a little or more if you choose, when done in the right spots. This should be done on the meeting joint, but of course you don't want a radius where they meet, hence the recut. Have some 1/4" stainless steel rod to drop into the wet mix at the sink front to strengthen those ends. If you put any on the back edge, be sure you steer clear of the area where the faucet is drilled.

You may be aware of this already, but I need to mention this. Concrete slab tops are honed/polished like natural stone is. A stain can be added to the surface prior to honing to add a variation in color too. Melamine will give a good start, but the honing and polishing will give a fine finish.

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I was leaning more towards the yellow line. Thanks for the tips on making it slightly longer. How much would you say? I won't caulk that joint but maybe caulk the butt joint on the form before screwing it in to help seal it? I was planning on cutting foam to form the faucet holes instead of drilling but I guess with the seam it would be hard. I was hoping to do all the cutting/polishing outside but 2 or 3 holes shouldn't be too bad. Do you have any recommendations for colorants, stains and sealers? I like a more honed look instead of super glossy. –  OrganicLawnDIY Dec 13 '13 at 14:41
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The added length only needs to be no more than 1/2". Just to confirm, the caulk I speak of is only in the form to seal the water in the concrete pour IN. The slower the concrete cure, slower the water leaves, the stronger the concrete. Cure time/water loss affects the color of concrete too. Controlling this is important. The recut on the meeting portion is to make square where the addition of caulk will round the joint. Styrofoam plugs are good for the holes, even for the half holes. Just figure where your cut will be and place accordingly. Honed finish is 400 grit, anything finer is polished. –  Jack Dec 13 '13 at 17:18
    
Thank you so much for your insight. I had one more thought. What if I make three molds with seams at the blue and red lines. It would be much easier to build and carry plus I could have rebar going across the whole sink opening. More seams but I think makes everything easier (less chance of screwing up) and stronger. What do you think? –  OrganicLawnDIY Dec 13 '13 at 18:08
    
It has it ups and downs. Pros, yes it will be stronger in a sense since three slabs will be supported independently instead of a longer slab going over spans. (Shims and silicone fix that though)Easier to handle, less chance for chipping when handling. Any discrepancies in the flatness can be divided between slabs to help "flatten" the whole top. Cons, more chance for chipping at the joints when butting to slabs together, 2 long joints to match rather 2 short ones. –  Jack Dec 13 '13 at 20:01
    
Yes the sink area will be stronger in 3 pieces, but in my opinion an experience, so will the joint at the sink, rod will go there too. I cut my own stone slabs for my kitchen, and rodded the "wings" at either side of the sink, and it holds up under my pendulous belly. Though can be tricky to line up. What I did, and though you have concrete, and I have stone, the principles are the same. I laid out my slabs and polished the edges before I brought them into the house. you can do the same with your template. CAREFUL how much grind to finish your tops, you will not want to exposed your aggregate. –  Jack Dec 13 '13 at 20:12
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