Water seeping through joints and cracks in a bridge deck can rust steel reinforcing bars. Rusted bars expand, cracking concrete. Bearings, intended to allow bridge parts to expand and contract with temperature changes, can rust. This blocks movement. Pent-up stress leads to cracks.
Paint can chip, giving rust a place to form.
Even minor damage allows rust and rot to weaken bridge supports. Wave action, saltwater and floating debris can weaken or crack the substructure.