Originally Posted by
Mike Henderson
I've done concrete in situ (dry) and you can have problems with it. You get the strongest concrete by mixing the proper amount of clean water with the concrete and by agitating it to make sure the aggregate is well distributed through the mix. When you do concrete in place, you risk putting too much or too little water in the mix. Too little water results in weak concrete. And if you put too much water, the aggregate will work through the mix to the bottom, especially if you do anything to stir the mix. You can also get dirty water into the mix which will adversely affect the strength of the mix.
For most fence posts, you'll get a strong enough mix. But for gate posts, or for posts where the fence direction changes (such as corner posts) I recommend you mix outside the hole and then put a stiff mix into the hole.
Mike
[I've especially had problems doing it the way Norman Hitt describes above. If you put a lot of water into the hole first, then dump the concrete mix in, the concrete and aggregate separate, with the aggregate going to the bottom and the concrete to the top. Also, it's almost impossible to keep the water clean so you get dirt incorporated into the concrete.]