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).

Drying response of lime-mortar joints in granite masonry after an intense rainfall and after repointing

“…The MI calculated from microwave moisture data allows comparison between granite and mortar. Other electromagnetic devices (such as ground penetrating radar) were used in this study to measure the moisture level of the different building materials (not presented here) and found similar results in the comparative behaviour of mortar joints and granite units [65]. In addition, the drying curves found in laboratory experiments ( Fig.…”

Determination of the hygric properties of the heritage stone of the Cathedral of Cuenca through the water absorption by capillarity test

“…To a large part, heritage stone (HS) pathology is caused by phenomena generated by the transport of water inside the material [2][3][4][5][6]. Therefore, the hygric characterization of this material is essential to carry out preservation and rehabilitation actions [7,8]. In Spain, this has resulted in the standard UNE 41810 [9].…”

A review on freeze-thaw action and weathering of rocks

“…Direct determination of moisture content is done by analysing the weight of hammered samples or drill powders using carbid meters or gravimetrical methods before and after drying (Sandrolini and Franzoni, 2006;Sass, 2005a). Novel handhold sensors are based on microwave propagation (Orr et al, 2019a(Orr et al, , 2019b. Another technique is measuring the humidity by placing moisture-sensitive sensors in a drilled hole within the material.…”