I worked on bridges as a youngster - ISTR that if there would be water
pressure on the concrete it was coated with bitumen prior to
backfilling. There was a mimimum coverage on the steel (50mm?) and of
course the mix was carefully controlled and pokered to remove voids.
Also strict rules about the steel being rust free.

(Some of the guys I worked with had built the nuclear bombproof
aircraft hangars at the US airbases. Steel went up, followed by the
peace protestors delaying work, followed by a strike. 9 months later
they then attempted to clean up the steel with wire brushes before
pouring concrete. The guys assured me they'd be lucky to just stay up,
let alone take a direct hit).