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My neighbor and I are considering alternatives to wood-in-concrete fence posts for a privacy fence that can have significant wind load in an area that gets up to -30 degree frosts in winter. The biggest drawback with wood-in-concrete posts is that the wood rots in the concrete.

He has a metal post spike that looks like this:

enter image description here

He is thinking we can set them in concrete, as it should last longer and should be easy to level. The spike part has the advantage that the wood is not in contact with the ground (or concrete) and has a chance to dry. Also, it's easier to change the wooden post eventually.

The disadvantage with the spikes is that they normally are sledge-hammered into the ground, and according to this account, it's tough to get them leveled this way. The wooden posts are shorter, but the spikes cost more than the length of the post that's saved (overall it's more expensive with spikes).

What other pros/cons do you see in this approach?

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The main con is that since its designed to be driven down, the tapered tip will offer less torque, even set in concrete, than a post set to a similar depth.

Another consideration is frost heaving. The bare adapter should not heave, but the combined concrete/adapter might, unless the pour goes below the frost line. This can be mitigated with SONOtubes use.

I wonder if using 2 ez-mender, set in concrete, might not give a more stable base.

enter image description here

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Regarding heaving, is there such a thing as a conical SONOtube? That is, so that the concrete is slightly wider at the bottom than at the top? It's a tip for wooden posts set directly in the ground. –  Fuhrmanator Jun 4 '13 at 20:34
    
What your describing is making a small (spread) footing under the sonotube, a known technique for increasing weight bearing. If you're following the 1/3 in ground, 2/3 out and pack earth around tube (perhaps with 50% crushed gravel). The tube alone resists heaving by being slick (to the ice crystals) –  HerrBag Jun 5 '13 at 14:20
    
In my climate, the frost line is deep (1.2 m). I don't understand your 1/3 in ground 2/3 out comment. Also, what does "slickness" have to do with heaving? Compression from heaving is lateral from all directions. I can't see what friction (slickness) has to do with that. –  Fuhrmanator Jun 8 '13 at 16:05
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Actually: "The ice grows in the direction of heat loss (vertically toward the surface), starting at the freezing front or boundary in the soil. It requires a water supply to keep feeding the ice crystal growth; and the growing ice is restrained by overlying soil, which applies a load that limits its vertical growth and promotes the formation of a lens-shaped area of ice within the soil." wikipedia frost heaving –  HerrBag Jun 8 '13 at 18:10
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the 1/3 vs 2/3 is a rule of thumb for mechanical advantage and doesn't include frost considerations, which may require deeper embedding. –  HerrBag Jun 8 '13 at 18:13
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