Pressure treated posts set in concrete?
I have inspected some 20 odd houses damaged by hurricane flooding that were supported on wooden piles. Some of the piles were directly embedded in sand, some were embedded in concrete, and some were driven into the ground and packed around with gravel or crushed stone.
The only thing the houses had in common was that the house above the piles had sustained massive damage from flooding. In a few cases, part or most of the foundation had been washed away as well, although in most cases the foundation was intact, while the house was mostly destroyed. I would post pictures, which are really pretty interesting, however it would violate my contractual agreement with the companies I worked for, so unfortunately I cannot.
I can say that the wooden piles exposed to salt water were damaged primarily in the tidal zone, and the majority of damage was from marine insects. The dry zone above the high water line was generally in excellent condition, even for untreated timber piles that were 50 or more years old. In many cases, portions of the piles well below the normal sand line were exposed due to loss of shoreline, and the originally buried portions of the piles were in generally good condition. The portion of the pile subject to wetting and drying was always the worst, and in many cases there was 50 percent loss of section. In a few cases, I wondered how the house even stood.
Based on my inspections, and literature I have reviewed, I have come to the belief that treated or untreated wood performs very well if it is kept either completely dry, or is constantly saturated, either with fresh or salt water. I have inspected numerous dams built on untreated wooden piles that are over a hundred years old, and in several cases we cored the wood, and the conclusion is the same. If the wood is always wet or always dry, it performs very well. If it is subject to alternate wetting and drying, it rots relatively quickly.
I have heard all sorts of stories purporting to have evidence that embedding wood in concrete causes the wood to rot quickly. I have never personally seen any evidence of this, and like PoleBarnGuru, I would very much like to see a well developed report on this topic. I believe that the portion of the pile just above the ground is likely to be the most at risk for rot, hence must be the best protected, whether by use of waterproofing agents, or perhaps wrapping in fiberglass or similar material.
Several houses I inspected which had severe foundation damage were being rebuilt, using precast, prestressed concrete piles, which can be driven directly into the ground, and provide outstanding resistance to rot. A few houses were being rebuilt using steel piles, which also can be directly driven, and will last a very long time under normal service conditions. There is still a thriving wood pile industry, as wood is less expensive than either concrete or steel, and not every building needs to last a hundred years or more.
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