I've made dozens of concrete countertops and seats with the technique I used in this project, and each time I do, I'm amazed by the quality of surface I achieve. Slabs this size require no additional strengthening, so you don't have to add rebar or wire mesh.
Once the melamine-coated particleboard form is complete, place it on a sturdy, flat surface and level it using a spirit level and shims slid under the corners as needed. Getting the form level is crucial for a slab of even thickness. Next, dump a 20-kg bag of sand-mix concrete (no gravel) into a clean plastic garbage can and add two litres of cool, clean water and two litres of an acrylic bonding agent. You could use just water, but, in my experience, the bonding agent helps the concrete settle in the form and leaves you with a smoother finished slab. Mix the concrete thoroughly using a paddle mixer chucked into a 1/2" corded drill until there are no lumps or dry pockets.
Like buttering a cake pan, you should lubricate the inside surfaces of the form. Before adding the wet concrete, I sprayed a thin film of Bostik's Top Cote onto all inner surfaces of the form. Top Cote is a lubricant and sealant normally used for protecting tool surfaces. Alternatively, you can wipe a thin layer of vegetable oil onto the inside of the form to serve as a release agent. Whatever you use, the idea here is to prevent the cured concrete from sticking to the form.
Pour the mixed concrete into the form using a small container to scoop it out of the garbage can; I prefer the plastic jugs used to hold milk bags. This is far easier than trying to hoist up all that wet concrete to dump it. By adding the concrete slowly, you can direct it with greater accuracy. Tap the sides of the form all around using a rubber mallet. The concrete should settle out level below the top of the sides of the form. Continue tapping the form until small bubbles no longer rise to the surface. Leave the concrete alone for a few days in the shade or covered with a wet cloth, then remove the sides of the form to release the slab. If you have a small grinder, ease the sharp edges of the concrete. If you don't have a grinder, an abrasive stone rubbed on the edges does a good job-it just works a little slower!
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