If concrete is poored around a pressure treated post, will it rot?
Question:I've been told that you should poor the conrete first and then set the post on top. Why not poor the concrete around the post itself?
Answers:
Your question is will the post rot if you pour concrete around it.
99% of the time you will be fine. Occasionally, a post will rot if concrete is poured around it instead of below it. It has nothing to do with the chemicals in the products, but rather water retention.
Concrete shrinks slightly as it cures. A treated post will also shrink just a bit as it dries. If you pour concrete around a post, the end result is you may end up with the pocket in the concrete being slightly larger than the post, creating a place for water to collect.
Standard treated posts are not rated nor intended for immersion in water. Over a period of time, if the post is immersed in water many times it will develop some problems.
Another problem with casing the post in concrete also revolves around the pocket of water. If the concrete pocket is above the frost line, the trapped water can freeze. Water expands when it freezes, and it can literally bust the concrete around the post making it loose.
Both of these issues vary with soil type and climate. It may or may not cause a problem where you are, but if you are in an area that experiences deep freezes from time to time or if you have soil that tends to retain water anyway I would not encase the post in concrete.
If you are building a deck, barn or some other structure, there is zero reason to concrete around the post. You really need the concrete in the bottom to distribute the weight into the ground. The structure will minimize lateral movement. How many houses have you seen built with the foundation backfilled with concrete?
A fence is the one application where pouring concrete around the post has some value. You have no significant weight to distribute, only the lateral force of wind and other pressure on the fence. In most soils, you can do just as well by compacting the soil back around the post without concrete in the bottom or around the post if you have 1/3 of the length of the post buried. Corners and line ends may be an exception, there is extra force there so it may be a good idea to concrete around those, but only below the frost line.
no it will not rot this is done all the time.
My deck is over 25 years old and I poured conrete in the very same hole the PT posts were in. The deck is as solid as ever. So to answer your question, no. PT is not affected by the portland in concrete. Once the concrete sets and dries, it does nothing to the posts.
Concrete is okay. But, as you might expect, people still advocate gravel back fill, which in my opinion is not as good. I've attached a link so you can read the debate and decide for yourself.
But the issue is, which backfill keeps water away from the post. The gravel champions say gravel because of its nature drains the water away. The concrete people say the concrete forms a seal preventing the water from contact with the wood post. I go with the concrete people.
However, there is an argument to be made that you don't need to bury your posts at all. You site them on the concrete footings, even using a metal bracket that sets them onto and away from any contact with the concrete.
So, you might ask, what about post supporting a deck on a hill. Well, assuming you build a level deck (except for the slight incline for drainage), that deck doesn't know its on a hill. So, with the ends of the footings properly level, the deck will be as well supported as if it were on level ground.
The acid in the concrete does counteract the preservative in the wood, but this can't be helped. The best thing to do is to slope the concrete away from the post so that water does not pool at the bottom of the post keeping it wet and eventally rotten.
When this can't be done i.e. porch posts on slab then a metal bracket is used to raies the bottom of the wood above the wet concrete.
water is the woods enemy not concrete.
It also depends on what you using the post for, if it is a post to hold a deck then having the concrete below the post helps transfer the load across a greater portion of soil.
If it is a fence post it won't matter what is under the post more important is how much is beside the post to help with the lateral wind load.
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