Concrete vs crushed gravel for fence posts in clay soil?
In clay soil, the water just sits in the hole, especially where the water table is close to the surface.  Gravel lets the water get right up against the post, which will rot it out fast.  Using concrete will help the post to last longer, if you create a dome of concrete above soil level and make sure it is smooth and tight against the post.  This will ensure that no water gets down into the post.  Also, make sure the bottom of the post is not below the concrete level or the post will soak water up from the bottom (pour a little concrete in the hole, then set the post, then fill the rest of the way)