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When the wooden fence was built, they used metal posts but only filled the 2' hole with one bag of concrete. The posts are leaning at about a 25 degree angle. I want to dig out the hole again, then add another bag of concrete to each post.
My question is, will this work? or do I need to drill holes in the existing concrete and put in a few pieces for rebar to "marry" the new concrete to the old?
I live in a "high wind area" and you don't need to go with deeper holes.
I think your 30" depth is adequate and the size of the hole is more than adequate for a 6' fence. The problem is that you need to pour a concrete "collar" at the top of the hole.
I guess you could fill the post hole up with concrete, but you have rather large holes and that could be expensive. I'd dig out the dirt backfill down to the concrete at the bottom of the hole. (I'm assuming the metal fence post is partially encased in the concrete at the bottom of the post hole.) Then, I'd fill the hole up with crushed rock to the point that about 1 bag of concrete would fill the balance of the hole...creating a collar at the top of the post hole.
25 degrees, that's huge!
If you go with the concrete method, no need to drill holes, however ...
Quicker and more robust than pouring more cement:
dig out the top 12" inches around each post
attach a 2' length of ground-contact pressure-treated 2x8 to the post a couple inches below grade, using, for example, 5/16" U-bolts if the post is pipe. The 2x8 needs to be eyeball parallel to the fence.
back fill a couple inches at a time, compacting it as you go. Gravel would help, particularly on the lee side relative to prevailing winds or current lean, but is not necessary.
Going deeper per @BMitch would help if done right (it'd certainly help on a new install), but again not necessary assuming they are currently deeper than the frostline.
if the hole is mostly empty, but there is in fact still hole above the concrete, you could just use fence foam.
http://can.sika.com/en/group/News/Sika_Post_Fix.html
it works really well and its moron proof
