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I'm planning on making an L-shaped concrete kitchen countertop (2100mm x 2500mm) with a single large piece of concrete (Yes, I know, see below!). As the melamine sheets only come in a max size of 1220x2440, I'm trying to figure out how to best join the base sheets together for a seamless mold without leaving a depression in the finished surface.

I'm considering using Silicone on the exposed edges and then clamping the sheets together and bracing it underneath and on the sides with strips of wood. The big concern is ensuring that the seal keeps the liquid inside the mold and doesn't soak in to the chipboard.

I'm aware that it will end up being a very heavy piece of worktop, and may even break under its own weight when I (and 5 other people!) move it in to position, but I really want to give this a shot. I've planned and experimented with hollowing out half the depth of the counter with foam and am using glass fibres to add strength to the whole piece. I'll also be using rebar / mesh around the sink cutout to keep that together. A bit of research and math has told me the full weight will come in somewhere around 150-180Kg.

Someone asked a similar question before (How do I make this large mold for concrete countertop? ) but the suggestions came back with where to divide up the mold rather than how to make a single one.

Edit: I'll be making the mold in the same room as where it will be placed (so, thank you for the warnings on movement and structural strength, but that's not my query). I'm willing to accept breakage and retries. My end goal is to produce a single piece of polished concrete from a mold, and don't want to pour in place as the skill and labour involved is beyond my mere DIY determination. Poured molds have worked really well for me so far, I want to figure out how to make a bigger one.

  • You're going to use rebar/mesh for the whole thing right? Not just around the sink? – JPhi1618 Jan 19 '18 at 16:19
  • Have you considered lining the inside of the mold with plastic sheeting, available in long rolls? And in general maybe a little gap in the mold is OK as you can grind the resulting bump down later. – Shimon Rura Jan 19 '18 at 16:57
  • Wouldn't plastic sheeting end up folding/creasing when it comes to the corners? I think you're right about leaving a bump though, easily ground down as long as it doesn't destroy the mold before the concrete sets – beirtipol Jan 19 '18 at 17:04
  • quality packaging tape (thin) will help buy time to stop leaks on large seams. – dandavis Jan 19 '18 at 19:53
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    I just viewed a YouTube video where he just used a bit of caulk in the corners. Otherwise concrete will not penetrate the crack much, and any "flash" would break off when unmoulding. Go look at some of the video-ed techniques. – DaveM Mar 17 at 21:03
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Any seam with a void in the mold will result in a raise bead on the concrete top which could be polished down after curing.

I doubt 5 people are going to be sufficient to lift and move a counter of that size. 5 may be able to lift it but moving it through doors and setting it place will be much easier with more people. I think your weight est is a bit low.

I have the book "Concrete Countertops" by Fu-Tung Cheng and they specify clearly how to make molds for tops that are in two parts to be joined at the installation process but they do not talk at all about a mold longer than 8'. I recommend you get it. You may want to consider doing a pour in place counter top instead or come up with a design that incorporates the seam into it.

Idea: If you fill the seam with something, a resin or epoxy? and sand flat you can minimize the void but you need to research how the filler will work with or react with concrete. You would absolutely need it to be flat or you will create a void in the top and it would be more work to fill the void than to polish a raised bead. ANYTHING IN THE MOLD WILL BE TRANSFER TO THE TOP.

  • I'm making and pouring the mold in the same room as the units so will only have to lift it a few feet at the same level as where it eventually rests. – beirtipol Jan 21 '18 at 0:04
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I think the melamine would be a good mold material. BTW poured masonry counter tops have been around for some time. I recommend you consider pouring in place or nearly in place. Then only minor adjustments need to be make for final set. One thing that is done to lighten the load is to add, during mixing beads of expanded polystyrene.

  • If you add styrofoam beads to the concrete mix, won't they show through the surface when the top is polished? – Sparky Oct 25 '18 at 16:35

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