title: Sensing and understanding the resilience of sandstone
Creators: name: Dassow, Jessica Maria Irmgard
Publisher: name: University of Glasgow
Year: 2019
Subjects: subject: Stone decay, weathering, sandstone, ultrasonic drilling, laser interferometry, micro-climate monitoring, non-destructive testing, Scottish building stones.

description: When stones are incorporated into buildings their mineralogical composition and texture alters to adapt to the new environmental conditions. One of the most common building stones in Scotland is sandstone. Due to its high porosity and mineralogical composition sandstones can be prone to weathering. Scotland's diverse stone built heritage is endangered by frequent changes in environmental conditions such as cyclic wetting and drying and temperature changes caused by insolation. Buildings also face increased pressure through climate change. In Scotland, the decay of sandstone is a serious problem that urgently requires new approaches to preserve built heritage and to reduce maintenance costs. It is therefore important to understand the extent and timescales of weathering processes. New approaches to assess stone decay are required for deciding on the best conservation strategy for historic buildings. These approaches or tools need to be minimally or non-invasive, portable and provide a consistent method to gain empirical data for evaluation of the progress of weathering. This project aims to develop two new in-situ techniques that can assess the state of decay of building stones beneath their outer surface. The developed techniques use a laser interferometer to measure dilation over time and an ultrasonic drilling tool to estimate the structural properties of the stones. These measurements are joint with micro-climate monitoring of sandstones to enable a combined assessment of stone decay. The laser interferometer can measure decay induced contraction or expansion of stones on the nanometre scale. Salt crystallisation in porous systems can be examined with very high precision under any temperature and humidity condition that enable salt growth. Characterising dilation events through precise measurement of displacement and its frequency can provide insight on the intensity of decay of the building material. The drying behaviour of different samples (unweathered, weathered and artificially weathered) was monitored to enable the differentiation of rock type and state of decay. The ultrasonic drilling tool allows identification of changes in the structure of a sample while drilling a hole up to 4 cm depth. By continuously measuring the power required to drill, porosity changes and/or the presence of salt at depth can be mapped. The use of ultrasonic tools enables penetration of the rock with less average force on the stone and a faster progress rate into hard materials without a significant wear effect on the drill bits. Operational settings can be kept constant for different physical properties of the rocks such as compressive strength. Monitoring of the power consumption enables to determine different stone types and the location of salt accumulation/damage in artificially weathered sandstones. Micro-climate monitoring includes the measurement of temperature and humidity changes at the surface of building stones. The analysis enables an identification of areas that suffer from increased stress caused by frequent and high rates of changes in temperature and humidity. A study at four historic buildings was conducted for nearly two years. The sites include the University of Glasgow, Dunkeld Cathedral, Jedburgh Abbey and Fort Charlotte (Shetland Islands). Seasonal changes, stone type and north-south elongation were evaluated for determining rocks with increased weathering risks. The developed techniques provide a more precise identification of stone weathering and allow for better prediction of the decay processes. The techniques allow up-scaling from the lab to the field, and can potentially be used in-situ on historical buildings under site conditions.