Today I build the lower formwork. I used exactly the same technique that I used for the garden shed. I was also able to reuse most of the edge supports that I built for the shed. These supports were 800mm long boards laid flat on the ground, with a 300 – 400mm vertical. This keeps the formwork in place and stops it from bowing out. These supports were placed about every 1200mm. They were held in place by a 300mm tent peg driven through a hole in the flat board. There was also an occasional wooden stake into the ground and screwed to the vertical piece when the tent peg didn’t prove adequate.
I also used a 90×35 block screwed to the vertical piece to give something more substantial to nail the formwork to.
Once I had built 3 edges I spend some time to make sure they were square. Pythagoras came in handy.
On the southerly long edge the timbers were quite bowed, so I had to plane them down quite a bit. I also made the mistake of joining two long 5.4m pieces together and then setting them up. This proved to be a bad idea, as I did not cater for the bowed timber very well. Also, the 1.2m level I am using is not long enough to get a good feel for the levelness of a long edge, so I need to get a longer level.
All the corners were held together with a 100x100x100 4mm bracket bolted to the formwork (with the nuts on the outside).
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In the end I think I am within about 10mm of being perfectly square.
Notes for next time:
After seeing the finished slab, I noticed some sections had bowed out slightly due to the weight of the concrete. Maybe more supports are required.