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I am thinking about using Sonotubes and the adjustable post support shown below. The idea is to rest the square with the screws on the sonotube to make sure the screws will be set with their heads level at the same depth and in vertical position.

Should I use epoxy? How do I apply it ? I want to make sure the screws do not rust a lot in time

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    I wouldn't do that. If the screw heads ovestress the concrete and start to turn, you are done. I would embed U-bolts and add a re-rod. – Harper - Reinstate Monica Jul 11 '17 at 14:39
  • That came too late...:-) but it was good idea. If that happens I will have to cut the screws and drill holes for anchor bolts. I will not tighten them excessively – MiniMe Jul 11 '17 at 17:31
  • You can always try the epoxy after they spin... give em lots of time to cure and hopefully they won't. – Harper - Reinstate Monica Jul 11 '17 at 17:39
  • Well I was thinking about the epoxy before I put them in concrete, in the end I need to make sure that what stays in cement does not rust. I bought an expoxy spray for ~11 CAD, that should do it for 20 screws. – MiniMe Jul 11 '17 at 18:17
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Setting bolts in wet concrete is a standard practice, and using a board to hold them in place is a good idea.

You absolutely do not need any epoxy when setting into wet concrete. I don't know what effect that would have, but its certainly redundant. Epoxy is used when setting anchors into preexisting hard concrete which has been drilled.

Generally you wouldn't use regular hex-head bolts for this, you would use an actual anchor which would look something like this:

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The bend prevents both pull-out & rotation once the concrete cures. These come in varying lengths depending on your application (eg 8, 10 inches) - but they are inexpensive so you can just err on the side of "bigger is better" assuming you have enough concrete depth (which in a post should not be a problem).

Where I live I think these are sometimes referred to as "J-bolts".

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