Definition of "unreinforced"

Two [viaducts] in particular sit on the West Highland Line extension from Fort William to Mallaig, which opened in 1901. They are Loch Nam Uamh and Glenfinnan. Both use concrete, but unreinforced, which gives them strength in compression rather than tension.

2023 May 3, Philip Haigh, “The art and science of building bridges”, in RAIL, number 982, page 41