I agree. Concrete is porous and will absorb water just as easily as does the soil, so it does not add any benefit to rot resistance. You can however treat the wood with a preservative that will extend its resistance. There are lots of no/low VOC, non-toxic wood sealers on the market and you should be able to find at least a couple for sale at Lowe's or Home depot.

Thank you for the information! Yes, it is real redwood. I will look at water sealers--that's definitely an easier option than framing out more concrete.

If I brush it with a sealer, I suppose I should coat the entire post? I've read that if I go the lazy route and just seal the bottom half, moisture will get in the unsealed parts and have no way of getting out. Or am I mistaken?

loribee-

The problem is that it is too late to do anything. Rot typically occurs with the section of the post in the ground/concrete. Sealer will do nothing to prevent that. Typically, in a situation like this, one would put gravel in the bottom of the hole before pouring in concrete, which allows some water to drain out the bottom rather than be contained by the concrete and contribute to faster rotting. Also, a pressure treated post would be superior to natural, untreated wood. Having said that, however, since you have a redwood post it will be a number of years before you have to worry about anything.

Concrete in contact with wooden posts promotes rot, wicking moisture to the post. Posts should be set in gravel or crushed stone.

If you like, you can put some boric acid rods in the wood to prevent rot. But you would periodically have to add new ones as they wear out.

https://www.amazon.com/System-Three-4-Inch-12-Pack-14112/dp/B0098WFHTU/ref=sr_1_1?ie=UTF8&qid=1501736829&sr=8-1&keywords=bor8+rods

I use post anchors set in concrete so wood is above ground. My clothesline is 30 years old and the post is solid. I had to replace one gate post maybe 20 years ago and did the same, it's still solid and I went ahead and used them for the rest of the fence posts a few years ago.

Post anchors are not allowed by code unless the fence is connected to something like the garage and/or house to give it stability. People do this all the time, but it's not acceptable if you need to get a permit. The reason is that fence posts connected at only the bottom by post anchors are poor at resisting rotational and wind loads. You've been lucky...

The clothesline is a single post so is balanced both in use and empty. The fence is 3' tall pickets with 50% coverage with 2.5' tall posts so very little worry about wind damage with either structure. Not worried and also not really surprised that it's not up to code - thanks so much for the info.

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