Non-destructive measurements of moisture content across concrete walls of a U-boat bunker from WWII (Trondheim, Norway)

Historic concrete buildings worldwide face severe weathering that threatens the durability, appearance, and structural integrity of concrete and its metal reinforcement. In general, moisture and soluble salts are the main responsible of concrete deterioration, and knowledge of their effects is crucial in suggesting possible solutions. Non-destructive techniques can be used for early detection of damage and proactive maintenance. Here, non-destructive testing (NDT) techniques consisting of capacitance and microwave moisture meters, are employed to investigate moisture content (MC) distribution on the external concrete walls of a WWII U-boat bunker named Dora I (Trondheim, Norway) that is affected by significant visible decay, in the attempt of differentiating electrical signals based on the different factors that generate them. Positive and negative anomalies in moisture content readings by electrical devices were confirmed to be influenced by interfering elements such as thick salt layers on the surface, internal metal reinforcement, and surface-exposed fractures (SEFs). The study highlights the importance of accurately interpreting moisture content values to avoid incorrect conclusions about the conservation status of historic materials.