I'm curious whether anyone here has actually compared the flex of a pole before and after adding concrete. Intuitively it sounds like it should make the pole more rigid, but I really wonder just how much improvement you can expect. I also wonder how much of the observed "flex" in these poles is actually the whole pole moving a bit.
But yeah, filling a 21' x 2" pole seems like a waste of time.
I assume the plan was already like Rainman said, ie to attach to a building. I've only seen one dish mounted on a 21' pole free standing, and it was a heavy ~11' dish at that, and it boggles my mind how that can work, even if filled with concrete. Plus it would really make routine alignment an adventure.
And re the heavy Sched 40 pipe being sold by ID, and the OD really being 2.5", as mentioned above. That's what I used to think, but it's actually more complicated. It's actually sold by something called "Nominal Size", which ends up with the ID being close to that size for sched 40, but not exactly. The OD of 2" pipe is actually 2.375", and the wall thickness varies for different schedules. For sched 40, the ID is 2.067, ie very close to the 2", but not exactly, and for other schedules it varies between 1" and 2.345". So it isn't really sold by ID, it's just that the ID of sched 40 is usually close to the nominal size. The only fixed size of these scheduled pipes is the OD.... I have no idea of the logic behind any of this.