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Cardboard probably will not work as the wet concrete may destroy it's integrity and it could deform or even collapse...if you do go that way paint the inside and outside of the tube to make it water proof...a couple of times
If you go for cardboard do not use wet concrete..put it in dry and let the thing draw in moisture from the ground..that's what I do and days later give them a hit with a squirt bottle..not much mind you...then more the next day...there is a case to suggest it is stronger doing it dry and slowly letting it gain moisture..look it up.
What you need is a PVC pipe...sometimes you see ones that telecom have abandoned...but go for PVC..or steel...my triple piers..one has steel piers and the other has PVC...
You can also make a box section pier using ply ...Actually I have made a pier out of just ply and fibre glass which worked well.
Bite the bullet and ask Bunnings..I think 225 mm is what I last used.
Alex
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