By Scott Allan Orr
In celebration of British Science Week (11–20 March 2016), we are highlighting some heritage science projects, news, and topics that may be of interest.
Many people are interested in the interplay between moisture and historical buildings to understand the governing physical principles and inform conservation policy. This is especially pertinent in the context of a changing climate, in which many locations are predicted to experience more frequent and extreme weather events. Due to the sensitive nature of built heritage, modelling is a crucial tool to further understanding of the material interactions with water.
The interactions between water and historical constructions are remarkably complex, regardless of the scale in consideration. This post highlights work that is being done to understand how water is, and may in future be, affecting historical buildings.
Marzeion, B., and Levermann, A. (2014). Loss of cultural world heritage and currently inhabited places to sea-level rise. Environmental Research Letters, 9.3, 034001. DOI: 10.1088/1748-9326/9/3/034001
In 2014, there were 720 cultural and mixed sites inscribed on the World Heritage List (N.B. there are currently 834). A study produced in 2014 found that 136 of these would be impacted if the current trend continues and temperatures rise to 3°C above pre-industrial levels in the next two millennia. According to the researchers, this is something that is likely and not a particularly extreme scenario.
Some of the sites that would be affected include historic city centres such as Brugge, Venice, Havana, Macao, Corfu, and St. Petersburg. Other sites under threat include the Tower of London, the Sydney Opera House, and the Statue of Liberty.
The threat of rising sea levels call into question the mandate and capabilities of managers and stakeholders to preserve the integrity of these sites for posterity.
Boinas, R., and Guimarães, A.S., and Delgado J.M.P.Q. (2016). “Rising damp in Portuguese cultural heritage–a flood risk map.” Structural Survey, 34.1, online pre-print. DOI: 10.1108/SS-07-2015-0034
Researchers in Portugal have developed a hydrological model of the country in order to produce a risk map to cultural sites of national importance. The model primarily considers the location of monuments in relation to a waterway and the material properties of building materials. For historical building materials such as stone, mortar, and wood, the properties of chief interest are the water absorption coefficient and hydric expansion. They found that the height difference between a cultural site and waterways was the most influential factor in the definition of risk flood of a building.
470 sites classified as National Heritage Monuments are in mainland Portugal and are enclosed spaces (i.e. buildings). Of these, 40% were at medium or high-risk of flooding. This demonstrates the importance of developing a national action plan to respond to regional-scale weather events such as flooding.
Hall, C., Hamilton, A., Hoff, W. D., Viles, H. A., and Eklund, J. A. (2011). Moisture dynamics in walls: Response to micro-environment and climate change. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 467(2125), 194–211. DOI: 10.1098/rspa.2010.0131
Water ingress into historical buildings is a very localised activity that depends on meteorology, microclimates, and groundwater. Porous building materials, typically used in historical buildings, absorb groundwater. If there is a sufficient quantity of water in the ground, and a constant supply of water to replenish it, an consistent height of water often results in masonry constructions (e.g. Figure 2).
Rising damp is deceptively dynamic: although the height of water rise may not change drastically, there is ongoing evaporation from the masonry surface. This water is replenished with water uptake from the ground. This can be damaging to the stone masonry as the cycling water can dissolve soluble salts from the stone and mortar, thus removing from the infrastructure and lowering the integrity and strength of the materials over time.
A UK-based research team (see aforementioned paper published by the Royal Society) demonstrated that, in European climates, the net flows of water through stone masonry can be up to 10 litres per day, reaching hundreds of litres per annum. This was done by combining a sharp-front model, which assumes a constant concentration of water in the area affected by water, with a model of evaporation.
The sheer quantities of water that can move through historical masonry demonstrates the need for tools and methodologies that can rapidly analyse moisture contents in historic buildings, including the use of non-destructive moisture sensors.
Roels, S., and Carmeliet, J. (2006). Analysis of moisture flow in porous materials using microfocus X-ray radiography. International Journal of Heat and Mass Transfer, 49.25, 4762–4772. DOI: 10.1016/j.ijheatmasstransfer.2006.06.035
The movement of water through the open volume of porous building materials (typically less than a few micrometres wide) is very complex. Developing theories and equations that characterise this behaviour has significant ramifications for understanding the interaction between water and porous materials in historical buildings. Novel techniques have been appropriated from medical imaging and studies of fundamental physics, such as magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR). These tools are uniquely applicable for studying small-scale interactions in laboratory environments, but could not easily be scaled up to a building survey or employed in situ.
Another example of micro-scale techniques used to study moisture movement is x-ray radiography, that can produce 3D images of moisture in a small material sample. This is done by creating a subtractive model that compares the ‘images’ of moisture over the process of drying to a completely dry sample. In this way, the material of the sample is removed, and only moisture contents are left remaining.