Vic,
Check out the epoxy's at Marine Tek. I've used them, and they are very good,
especially for outdoor applications with lots of moisture.
As far as bolt depth, go as deep as you practically can. When a bolt pulls
free from being embeded in concrete, it can pull a cone of concrete with it.
Assuming bolt diameter between 1/2 and 1", I would go a foot deep, because
you really don't know the tensile strength of the concrete.
Fortunately, for tower bases, the loads are usually more shear than tensile
on the bolts.(Do NOT overtighten upon assembly.) The location of the holes,
relative to the edges of the "pad" also plays into the equation. Err on the
safe side, and drill as deep as your patience or drill bit will allow, but I
would make it at leat a foot deep in "unknown" concrete. Rent a BIG hammer
drill, and wait for a cool day. Keep the hole clean as you drill. Also,
epoxy embedding can actually be better than direct casting of the stud in
concrete when poured, especially in high shear/vibration loads, which can chip
away at the high stress points at the surface of the concrete where the stud
enters. The epoxy can stabilize that interface.
73,
Chris-AI4MI
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