He had noticed that the lolly-columns in his basement had been filled
with cement prior to being used. He assumed that the strength of the
support was in the cement, and therefore concluded it was a good idea
to try filling PVC tubes and using those as posts (note-not as
sonno-tubes, but as fully structural support posts).

I was concerned that the first major frost heave under his deck that
is able to stress the ledger enough to pull outward a small amount
would cause his pvc+cement "posts" to break.

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