In your picture I have circled sections where once can see the aggregate (i.e. stones) of the concrete slabs. I have poured thousands of cubic meters of concrete in my life and looking at this conglomerate of materials, i can see rebar and concrete. The rebar has rusted, which would be expected since it would have been exposed to water (to put out the fires) and air. And since New York City is a coastal city, I would expect the rebar to rust relatively quickly in such climates. My construction site is currently locate about a 1000 miles from the sea and if we have rebar that sits out in the open for longer than two weeks it begins to rust. Stress points in the rebar, such as bends, rust much more sooner. I remember my materials science lecturer gave us the reason for why stressed steel rusts quicker, but can't remember it to well. Something about displaced molecules.
The reddish areas are difficult to make out, because of the detail of the photograph. Anyone have a zoomed in photo of a red area? Two of my ellipses are in the red areas. Zooming in as best as I can, it appears one can make out the aggregate of the concrete. A better photograph concentrating on this area will be more definitive.
However, C7, nothing in that photo looks like molten metal. How could y12 or y16 (can't say for sure what the bar diameter is) maintain its round shape (cross-sectional), if all the metal around it was melted? Why did the rebar not melt as well? How could it stay round? Surely some cross-sectional deformation of the rebar would be expected, if it was immersed in a pool of molten metal? The rebar maintained its ridges, which enhance bond with the concrete. How could those ridges have survived? Any explanation to this? Or will my post be ignored?
Edit: I'm far too lazy to set post photo's properly, but you can see where I put my circles and refer back to your photo and look carefully at the areas I highlited.