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.