Ortho Acoustic WAW Part 3.

Time to mix some concrete and fill the form. You can mix your own concrete, add aggregate like Vermiculite to maker it lighter, plasticizer to make it flow better or pigment to color it. I went the easy route and bought ready mixed expanding concrete. It already contains plasticizer and the expansion counter the shrinkage and prevents cracks. Its twice the price of regular premixed concrete but still affordable. 

Start by mixing the concrete by hand if you don't have a powerful mixer. Do not add more water than specified by the manufacturer. Make sure to mix it for like five minutes, all of it.

Check that you form is level in all directions. Make sure any plugs are in place and that the reinforcement (chicken wire in this case) is at least 5 mm from all outer walls. Also make sure to lightly coat the melamine with a release agent, I used regular cooking oil.

Pour the mixed concrete into the form...

 Gently tap the form with a rubber mallet (be gentle to prevent the melamine from cracking on the inside). Tap it all around and watch for air bubbles to rise to the surface.

You can also put a drill into hammer mode, attach a bolt, and push it against the wall to wibrate the form. Or use an orbital sander without sanding paper etc.

The concrete will set a bit as bubbles rise to the surface. Add some additional concrete and trowel it until the top surface (that is going to be the bottom) is somewhat level and even.

You don't want the concrete to dry faster than necessary. It will take about 28 days for the concrete to fully cure and reach its full strength. Try not to remove the form for another 7 days if possible (the concrete has then reached about 70% of its strength).

Cover the top with some plastic to prevent it from drying. Keep adding some water to it to keep the surface damp until the form is removed. This will make the concrete stronger and prevent cracks.