Non-destructive moisture monitoring of historical load-bearing structures with Thermography, Ultrasound and Ground Penetrating Radar

Restoration or maintenance of historical monuments and buildings require profound knowledge of the history of the structure and its internal properties. This task requires interdisciplinary efforts that minimize destructive approaches.

Particularly moisture intrusions often pose significant threats for historic buildings. Thus, knowledge of the mechanisms of moisture ingress is of crucial importance for the realization of a sustainable and resource-saving restoration.  A usual way to get information about the inner conditions of the examined object is by removing material selectively by core drillings. The drillings are supplemented by a visual assessment and evaluation of the changes near the surface. However, this method is limited not only in the scale of the investigated area but also with respect to the depth of the investigated structure. Therefore, methods are needed allowing non-destructive and ideally rapid assessment of larger parts of the structure. An integrative approach is essential, not only to remedy acute damage, but also to take preventive measures and ensure the long-term preservation of historic buildings. The aim of this contribution is to demonstrate the feasibility of non-destructive geophysical measurements techniques for moisture detection during the renovation process, including Infrared Thermography (IR), Ultrasound (US) and Ground-Penetrating-Radar (GPR).  The focus is on adapting measurement principles from the field of geophysics to transfer them into practicable measurement and evaluation methods for historic buildings and detection of moisture.         
We report on results obtained by geophysical investigations of objects in Northern Germany. Results show that it is possible to identify moisture and classify the non-visible internal properties of both brick masonry and timber structures non-destructively.  The combination of Ultrasound, GPR and Thermography allow to quantify and monitor material properties during restoration by repeated measurements.