Hi,
steel is 205GPa while even good concrete is only 50GPa.

What makes you think that filling a rigid tube with concrete, which might as well be as stiff as a limp noodle, and have it be a great deal stiffer? Heavier, yes, maybe somewhat better damped, but
stiffer......NO. Damping and mass are useful....but way WAY down the list....with stiffness at the top, the second and third places!

The right answer is make the walls of the steel tube thicker and/or the outer dimensions of the tube bigger. If that increases the weight and the cost substantialy.....well yes that
is exactly what it means.

Craig