Moisture monitoring of stone masonry: A comparison of microwave and radar on a granite wall and a sandstone tower

Abstract Water is a fundamental control on the deterioration of historic stone masonry, of which wind-driven rain (WDR) is an important source in the UK. Non-destructive testing methods for moisture measurement can characterise the response of masonry to short (but intense) periods of wind-driven rain. An important part of this response is how masonry functions as a system of stone units and mortar joints, in which mortar can act as a conduit for moisture. While non-destructive techniques are common in moisture surveying of built heritage, there are no agreed best practice methods for collection, handling, and visual representation of data. This study explores the comparative advantages of microwave and radar measurements in two field experiments of exposure to short (but intense) simulated wind-driven rain exposure to demonstrate when and how they are most effectively employed. A novel method of representing data as percentiles is explored to facilitate effective communication of moisture measurements. In the case of the granite wall (e.g. with components of strongly contrasting hygric properties), microwave and radar provided similar information. The average travel time of the radar signal (from the back wall reflection) demonstrated that radar can non-destructively identify water penetration through mortar joints. In the sandstone tower, the microwave measurements were able to clearly identify four different moisture regimes as a result of different intensities of WDR exposure. The radar measurements were suited to identifying distinctions between localised moisture contents within masonry units and mortar joints, which characterised how the masonry was functioning as a holistic system. The measurements on both the granite wall and the sandstone tower demonstrated that the radar is influenced by environmental conditions which influence surface condensation and equilibrium moisture contents. Representing the measurements as percentiles improved visual representation of measurements with colour scales and minimised potential skewing of normalisation and scales from extreme values/outliers. This paper demonstrates that both microwave and radar techniques can be useful for monitoring moisture in stone masonry systems. Material characteristics of the masonry system and the objective(s) of the investigation should be considered during selection of the appropriate technique(s).