Many pillars, especially in the type of fancy buildings you see in a downtown environment, tend to have a mostly hollow, decorative outter-shell around the workhorse center of the pillar. Case in point, at the building I used to work at one would think that the pillars outside the lobby
would be solid through and through. But when they renovated the lobby and demo'd the pillars, it turns out that the outter surface is actually a shell of concrete about two or three inches thick, with the inner "guts" of the pillar, the actual concrete/steel beam that supports the building, being a few inches behind the outter shell.
That said, shattering the outer layer of that pillar with a casual swing was still an impressive strength feat for Bane.