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I did a bit of research to find suitable materials for this project, so here's a quick overview that others can hopefully find useful.
Here's how the tabletop looks after coming out of the form and a quick sand.
To make the form, I used this melamine https://www.bunnings.com.au/2400-x-1200mm-16mm-melamine-particleboard-white-matt_p0097412
The concrete I used is this one: https://www.bunnings.com.au/easy-mix-20kg-55mpa-super-strength-concrete_p0760460
This reomesh https://www.bunnings.com.au/jack-1800-x-1050-x-5mm-light-duty-reo-mesh_p1060321
Silicone spray to ensure no sticking to the form https://www.bunnings.com.au/wd-40-300g-specialist-lawn-garden-water-protective-silicone_p3410102
Silicone for rounding the corners of the form https://www.bunnings.com.au/parfix-roof-and-gutter-silicone-300g-black_p1232632
Paint scraper set for rounding the silicone in the form https://www.bunnings.com.au/paint-partner-4-piece-silicone-scraper-set_p1660196
Cut the pieces of melamine, I made it so the concrete will be 40mm thick. The size was 2m x 1m. I needed 8 bags of concrete for this size.
Then screw the sides on.
Then run a thin bead of the silicone around the edges and round them using the scraper set.
This will mean the corners of the tabletop will be nicely rounded.
Prior to adding the concrete, ensure the melamine is clean, then spray all over using the silicone spray.
Mix up the concrete according to the instructions on the bags, I had to add an extra couple hundred mils of water to what it said to get it a nice consistency.
Fill the form up half to 3/4 with concrete.
I left the edges a bit longer so it was easy to grab onto to pull them off later.
Then you'll want to vibrate it a lot on the sides and bottom. I used a reciprocating saw with no blade in it, and ran it around the sides and underneath for about half an hour. You will see all the bubbles coming up. But otherwise you can use a rubber mallet, or even a hammer, you may have to do it longer to get good results.
Then add the reomesh. I needed slightly more than just the one sheet for the length, so I cut a bit off another sheet I already had.
Then fill up with the remainder of the concrete.
After that, vibrate again for as long as there's still bubbles coming up. I smoothed the surface a few times and vibrated again. Probably 45 mins or so of vibrating.
Then that's it for the casting, just smooth the surface as much as you want (it will be the bottom once you flip it, so it doesn't matter too much).
Cover it with plastic so it dries slowly. Might need a light spray each day for a couple of days. Then plastic back on.
Then remove the sides and flip it and remove the top. Sides and top came off super easy.
Then I wet it down and used 240 grit wet and dry to sand it, then 400 grit for final wet sanding.
After that I sealed it using penetrating sealer.
I'm super happy with how it turned out. It's really strong, doesn't have any flex when you lift it from the two ends.
Hello @lens
Welcome to the Bunnings Workshop community. It's sensational to have you join us.
Let me tag @markw to make them aware of your kind words and questions. It would be interesting to find out how the concrete top is holding up. Has it lost its shine? Have cracks started to appear? Having used high strength concrete I suspect it is still in good condition.
In regards to your project, I think it's possible to build your proposed concrete top, but I would suggest adding Rio support bars just to make sure that it can carry the weight of the BBQ and anything else that you might be putting on top.
If you have any other questions we can help with, please let us know.
Eric
I’ve attempted to pour a concrete bench top in place for an outdoor kitchen.
it’s being curing for the past week and it’s time to get polishing. Or so I thought.
I’ve gone to hit it with some 80 grit sanding pads on my orbital sander to smooth some rookie troweling marks but it seems to be tearing at the surface. It’s not smoothing it’s making very gritty.
I used a pre mix sand and aggregate mix from my local landscaping supply, and dingoes Cement, Davao Lanko 311 Plasticser and Dulux Avista white oxide. All based on a 3,2,1 mix.
any thoughts or advice?
Hello @Mitch31
I suspect that the surface of the concrete is too soft and can't withstand the low grit count of the sandpaper. I suggest using 240grit sandpaper to see if the surface can withstand the sanding process. If it still strips the surface, it might be necessary to render the surface with high strength concrete.
Let me call on our experienced members @Dave-1 and @Nailbag for their recommendations.
If you need further assistance, lease let us know.
Eric
Evening @Mitch31
I havnt had a lot of experience with sanding/grinding concrete yet 
I think when I did the concrete repair on my patio it was going to be weeks before it had reached full strength. Id be tempted to wait a few more weeks for it to harden. Other then that The render suggestion of @EricL sounds like a good plan B
Dave
