Well I did not learn this building my coop since I have yet to start construction. Maybe this weekend?????
Anyway, when planting posts in the ground with Quickcrete, concrete, etc, don't encapsulate the post on the sides and the bottom.
One option is to pour the concrete in the bottom of the hole and let it set up. Then set the post on top of it and pack the hole tight with gravel and dirt.
The other option is to put some gravel in the bottom of the hole, set the post, maybe add a little more gravel, then pour the concrete to fill the rest of the hole. You may or may not pour all the way to the top depending on how deep the hole is and what the post is doing. (Holding the corner of a coop, mailbox post, etc.)
If you pour concrete in the bottom of the hole and set the post in the wet mix, then continue to pour up the sides of the post, you will have problems later on. As the wood shrinks back away from the post it creates a gap. When it rains, water runs down in the gap, but has no where to go. So the post then sets in water. Even a treated post will rot. This speeds up the process.
So if you have concrete on the sides of the post, you want the bottom to be open so that it can drain. If you want the concrete under the post to prevent sinking, then set the post on hardened concrete and don't put any on the sides so that it can drain.