Two-dimensional resistivity surveys of the moisture content of historic limestone walls in Oxford, UK: implications for understanding catastrophic stone deterioration
O. Sass, H. A. Viles, 2010. "Two-dimensional resistivity surveys of the moisture content of historic limestone walls in Oxford, UK: implications for understanding catastrophic stone deterioration", Limestone in the Built Environment: Present-Day Challenges for the Preservation of the Past, B. J. Smith, M. Gomez-Heras, H. A. Viles, J. Cassar
Download citation file:
Catastrophic deterioration of limestone facades occurs where areas of stonework become rapidly hollowed out. It affects many historic buildings in Oxford, especially where soot-rich gypsum crusts have accumulated. In order to understand the processes of catastrophic deterioration we need to understand the microenvironmental conditions, especially the moisture distributions in the deteriorating walls. Geoelectric methods, in the form of two-dimensional (2D) resistivity surveys, have been used to study the distribution and amount of water stored in deteriorating limestone walls within the historic centre of Oxford. Fifteen vertical profiles, each 2–2.5 m in length, have been monitored at five sites using 50 medical electrodes and GeoTom equipment. Calculated moisture contents and distributions are presented for those profiles that extend up to 40 cm into the wall. The data indicate the diversity and complexity of moisture distributions within these often heterogeneous walls, which have also had long histories of decay and conservation. Replacement stone patches show consistently higher moisture conditions than the surrounding stone. Most profiles indicate the presence of wetter patches 5–10 cm behind the wall face under blackened crusts. Catastrophically decayed sections of profiles often exhibit wetter near-surface conditions than surrounding stonework, whilst areas with shallow but active decay are often much drier than surrounding crusted stone. In conclusion, the results give preliminary confirmation of a simple model of catastrophic decay and illustrate the complexity of moisture regimes in historic walls.
Figures & Tables
Limestone is a highly successful and widely used building material, found in many important historic buildings and new monuments around the world. Whilst its success reflects its durability under a wide range of environmental conditions, there are still important questions surrounding the selection, use and conservation of building limestones. In order to make best use of new limestone today, and to conserve old limestone most effectively, we need to bring modern research methods to bear on understanding the characteristics of different limestones, what mortars to use, and how key limestones have responded to polluted atmospheres. This volume brings together recent inter-disciplinary research on these issues, illustrating the diversity of innovative techniques that are now being applied to furthering our understanding of building limestones.