PT wood underground, how to preserve longer?


I am building a garden gate and frame out of PT lumber. The gate frame is two 4x4s that hold a 4x4 cross member as so:
| | | | | |_____ground level_______ |----| | |
How can preserve the underground members so that they will last longer? I will be putting a layer of gravel and some cement. Is there a product such as cresote or tar that I can apply before I install the frame?
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when I install posts into the ground I wont use concrete. if you use just a good gravel mix to pack in around the pole and you go a good foot lower than the bottom of the post you will keep it from ever getting water logged. The gravel provides drainage. Contrete will wick moisture into the wood. so even if there is only damp earth around the post the post will be wet all the time and rot becomes a problem. you can wrap your posts in 6mil poly too just like vapour barrier. I dont bother. If you have a good mix of Pea Gravel that is used over weeping tiles that will keep the post solid and dry. Just tamp the gravel after every 6inches or so as you put it in. And just as a side note I usually make sure I have 4 feet of post in the ground. it just wont tip then.
Doug

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Phisherman wrote:

It's probably not a big concern. This from the Southern Pine web site: "How long does pressure-treated wood last? Currently available research shows that wood that has been properly treated and installed for its intended use can be expected to last for many decades.
Ongoing tests sponsored and monitored by the USDA forest Service's Forest Products Laboratory confirm this finding. Test stakes of treated wood have been buried in the ground at various locations, stretching from the Mississippi Delta to the Canadian border. Data analysis indicated that CCA-treated Southern Pine stakes in place since 1938 have shown no failures at chemical retention levels of 0.29 pounds of preservative per cubic foot of wood, or higher."
If you use the typical .40 retention ACQ (CCA replacement), commonly called ground contact, and don't bury a cut end, it should last decades without further treatment. If you want higher levels of protection you can purchase higher retention level treated wood.
If you're a belt and suspenders guy, and you want some added assurance, you've answered your own question. A coating of roofing or flashing cement will inexpensively waterproof the buried end of the post.
R
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How do you keep from burying a "cut end"?
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Nevermind... lol... obviously they mean an end cut after treatment has been applied.
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You may or may not know that there are "different" grades of SYP pressure treated wood ???
Be sure to buy pt wood that is graded for "direct burial".
Not all Home Centers carry that grade.
http://www.southernpine.com/grade.shtml
They also recommend using a water proofing after the project is completed, to help prevent checking.
Phisherman wrote:

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