I'll have to go back and watch it again. I can't remember if David said how thick the steel was....Funny- though David knows he was able to break through the pressure chamber (carbonized steel) in a matter of minutes, why would he figure that he'd never be able to break through the concrete and lead (which is actually softer than steel). Maybe he figures that , while his Hulk persona is powerful enough to damage steel, his determination and endurance is not limitless and that he'd enventually just give up.