the 28 holding area is a steal platform. There are a few operator's who think that sitting on the metal at the hold for elongated periods is bad news for the nav kit on some craft.
Reinforced concrete of the strength required for aircraft taxiways and runways is up to about 40% steel, as mesh and bar embedded in the concrete. You can't see it but it is in there. If it wasn't the concrete would be cracked in a day.