This is the 11631st most frequent Russian word.
железобетонные
Reinforced concrete.
Here, 'железобетонные' is used metaphorically as an adjective to describe 'нервы' (nerves), implying extreme steadiness or resilience.
Он проявил железобетонные нервы во время испытания.
He showed reinforced concrete nerves during the trial.
In this context, 'железобетонные' describes the noun 'мосты' (bridges), indicating the construction material and relating to durability.
Эти железобетонные мосты стоят уже десятилетия.
These reinforced concrete bridges have stood for decades.
In this sentence, the word 'железобетонные' (reinforced concrete) is used as an adjective describing the noun 'конструкции' (structures), emphasizing the material property of the items described.