Disaster waiting to happen? Using PVC for deck supports???

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"Thomas G. Marshall"
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Interesting concept. Never did that but with 2 or 3 rebar in it, maybe a #8 right up the middle, it seems like just a neat form that you don't have to remove.
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When I was in college, so many years ago. We learned that cement is strong for compressive loads (such as columns in the cellar). Steel is good for stretching loads (hanging a bridge from a frame).
If your friend's use of concrete is to support weight, it may work very well. Concrete often has reinforcing bar, or rebar. This steel helps to combine the compression strength of concrete with the stretching strength of steel.
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Fire protection of structural steel is placed on the exterior of the members, not within a pipe or tube.
To make it simple, take a look at a typical pipe pile, driven empty, then filled with concrete, with a circular cage reinforcing only in the upper section (top 20-30'), a vertical bar or two near the center does nothing.
As Matt and Rico have stated the concrete fill is to resist buckling.
Tom

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And I would submit that it does a lot more than just provide the buckling resistance. Concrete is widely used to form structural support to hold up buildings, bridges, etc. Even in this deck example, what are the footers made of? They are essentially cylindrical concrete pillars. So, while the concrete does keep the steel pipes in a lolly column from buckling, which is clearly important, the concrete also carries some of the weight directly. If instead of concrete, you had some other means to keep the column from buckling, say an internal criss cross web of little rods, I would say the concrete filled lolly would carry substantially more weight.
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For what?
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CWatters said something like:

I mentioned the deck in the subject and in the paragraph you snipped. But I was not as clear as I could be: he was using them as the posts.
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First, I'm no engineer, but a few thoughts did come to mind...
You didn't mention the height of the deck or the span between support posts, but in general there shouldn't be much of a load on a deck (no walls, ceilings, roofing, etc.). Assuming a typical residential situation, of course.
If the post footings are below the frostline in the area, they shouldn't heave anyway should they? But even so, PVC is fairly flexible, and even concrete flexes a small amount. With "typical" structural movements, I don't see this being an issue. They're not gonna "snap like twigs" at the first sign of stress.
If the PVC is filled with concrete, I don't see why it would be any different than a cardboard sonotube, except you would not need to remove the PVC after the pour. Even if the PVC doesn't offer any structural support, the concrete piers inside the PVC should support the weight of the deck.
If you're concerned about "flex" in the column, you should install rebar reinforcement whether you're using PVC, a sonotube, or even building a square box for the concrete pier.
Unless this is a second story deck, or one perched out over a hillside, "most" decks only sit a foot or two off the ground and are supported by multiple pier posts. I doubt there will be any significant sideways loads to buckle the columns.
A recent article in Fine Homebuilding showed a system that used PVC pipe to support a small shed (with no concrete). If it works for a shed, it's bound to be adequate for a deck.
Unlike a steel post, PVC won't rust. However, regular PVC will be damaged by ultraviolet light from the sun, and will get brittle. You could avoid this by using grey PVC conduit which has protection against ultraviolet light.
Around here, many decks are built with nothing more than a 4x4 post sitting in precast concrete pier blocks sitting on the ground and have lasted for decades. Also, many mobile homes sit on concrete blocks that are just dry stacked piers with no reinforcement (ours was setup that way for 13 years, with one end nearly four feet off the ground, and we survived two earthquakes with no problems). These are obviously UNDER engineered situations, but it's also possible to OVER engineer the support for a deck as well...
Anthony
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