John
It is a double entendre (two possible meanings).
I would prefer that the building and/or bridge is designed / fabricated / erected as a steel structure with concrete in its proper place...the foundation!
But, if the owners decide to go with concrete for their structure, well, the only thing that is going to keep the concrete in its place is STEEL...rebar, embeds, etc.
So, STEEL is keeping conrete in its place!
ziggy