An idea that keeps popping up in my head, that I don't know enough structural/civil engineering to know if it's good from a technical point of view.
Take a piece of polyethelene (PE) piping, fill it with concrete, let the concrete bind. My understanding is that under compression, a concrete (or other) pillar will 'want' to shear apart in a plane 45° to the direction of the compression (assuming we don't bend our pillar). This is an outward movement. This shear force could be contained by the surrounding pipe, since PE is quite good in tension. Othe materials for the pipe might work the same, I say PE because it's quite corrosion resistant.
I know everyone is building concrete pillars with rebar inside, I don't claim my idea is superior. I'm not interested in understanding why everyone is building pillars the way everyone is building them, I want to understand the flaws and limitations in my idea. Some thoughts:
I think it's useful to think as pressure in the pipe, as we have that data easily available - if we use PN10, it can take 10 bar, etc., and we don't need to think about the thickness of the pipe and it's yield strength. But how would the compressive force translate into pressure on the pipe? The concrete is no liquid, so pressue will be less than compressive load / pipe area. How much? I think understanding this will tell us how much, or little, our pillar will carry.
Another possible issue is that PE is quite smooth. The pipe can't take forces via friction and in my mind that translates to point loads at the places where the concrete happens to deform most.