Jim Lux jimlux at earthlink.net
Tue Nov 1 16:25:20 PDT 2011

On 10/31/11 9:39 PM, Randy wrote:
> I don't think you could find any competent engineer that would say "fill
> it with concrete to make it stronger", and be willing to sign off on
> it...he or she would specify something that wouldn't fail without doing
> that.

In a compressive loading situation or for fire protection.  (although 
it's interesting that the earthquake retrofits they've been doing here 
in Southern California basically wrap the concrete column in a tight 
steel wrapper)

  I do have it, from somebody I respect, that dropping PVC pipe
> inside of the top section of TV-type "push-up poles" will contribute
> towards keeping the thing from collapsing. I'd be interested to know
> that, if you could magically seal both ends of a tube, having already
> packed it with magic dry sand, whether it wouldn't increase the
> strength, as far as kinking in a bending moment goes.

Sure that works. It's one way to smoothly bend thin wall copper tubing 
without kinking/buckling/flattening.

  I'd guess that it
> would help, and if that's true, then concrete's compressive strength
> wouldn't matter, even if it fractured so badly that it turned to dust.

mmmmmm maybe, maybe not. If the steel stretches, then the volume inside 
the tube has changed, and it might not be full.

(and the sand in the pipe technique for smooth bends fails sometimes, 
for just this kind of reason)

Ultimately, any of this "concrete inside the pipe" thing is more for 
changing the ultimate failure behavior rather than withstanding normal 
operating loads.  You might wind up with the pipe neatly bent at a 45 
degree angle instead of kinked and bent sharply.

> Concrete is caustic; I don't know how that relates to steel, but as far
> as water pipes, it *will* eventually corroded a hole in it, X decades
> later. Maybe it's more complicated than mere pH.. I dunno.

otoh, rebar is embedded in concrete and lasts a good long time.

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