Over the course of my career as a building inspector, I’ve noticed that PT posts embedded in concrete tend to last a lot longer when the concrete is sloped away from the base of the post. Not only does this allow water to drain away from the post, it helps prevent leaves, dirt, and other organic matter from building up around the post base, where that sponge-like layer will absorb moisture and create a rot-friendly environment. A bit of formwork - made from empty 5-gallon drywall buckets - around the post will make it easier to neatly shape the concrete.
Start by slitting the side of the bucket from top to bottom with a jigsaw, then cut out the bottom. This creates a flexible plastic form that can be slipped around the post even after the post has been installed in the excavated hole. The form can be adjusted to fit holes of different circumference, and moved up or down so that the lip is slightly above grade, even on a sloped site.
Before pouring any concrete, wrap the part of the post that will be embedded in concrete with plastic to prevent the base of the post from absorbing moisture and swelling as the wet concrete cures. That will help prevent a gap from forming between the post and the cured concrete as the post dries out and shrinks back to its original size.
After filling the excavation with concrete to a little beyond the top of the form, use a trowel to mold the top surface into a sloping mound, using the rim of the bucket as a guide. After the concrete cures, you could probably remove the plastic form with a little effort, but you don’t have to. The post shown in the photo was installed 23 years ago following this method, and it looks as good now as the day it was installed.
