Many softwood panel fences are erected every year at considerable expense to the home owner. Unfortunately, a high percentage of these fences will need to have their posts replaced after as little as 4 years - sometimes less if the softwood that the fence and posts are made of is of low quality or they have not been pressure treated to the right standard.
A problem often arises when a fence post is set completely into concrete. This effectively forms a cup at the base where water, that runs down the outside of the post, collects around the end of the post and has no way to get out.
With the post sitting permanently in water the wood rots very very quickly leading to premature weakness. The post can break during strong winds which it would normally cope with easily.
Of course, if all of the posts are put up on the same day you could be faced with having to re-place the lot at the same time.
Click on the image and you will see that the post is allowed to extend through the concrete into the soil so that any water will never be trapped and can drain away before it becomes a problem.
If you have a clay soil that might make drainage harder then dig your post hole an extra 100mm deeper than you need, fill this with ballast (no cement) and then tamp down with the end of the post.
Now insert your post and concrete around the sides only. The ballast acts as a soak-away (to an extent) .
Normally, if the bottom of your posts - providing they are of good quality - should last 10-20 years.
One last tip regarding posts. After a year or so, the fence posts will often shrink in their concrete sockets ever so slightly. If you want to preserve the wood that little longer, trickle some water based preservative down the face of the post so that it finds it's way into the wood below ground thus helping to preserve the submerged wood that little longer.
Concrete mix - I am sure you have all heard that if you mix your ballast with your cement creating a dry mix and then back-fill your hole with this 'drylene' mixture the surrounding moisture will activate the mix and cause it to set 'concrete' hard?
I believe this method is flawed and all too often not strong enough to do a proper log term job. My advice would be to mix the all-in aggregate with ordinary Portland cement at a ratio 4:1 with enough water that will thoroughly mix to bind the material but will not slump or run. A fence post will stand up unaided until the concrete goes off leading to a much stronger job.