J. Curran

Condition Assessment for building stone conservation: a staging system approach

Abstract Trofimov and Phillips (Geomorphology 5 (1992) 203) suggest that the ultimate goal of any science is to predict the behaviour of entire systems. With regard to the decay of building stone, making accurate predictions of stone behaviour remains an elusive goal but given our improved understanding of decay dynamics it should be possible to provide a forecast of likely system behaviour. However, forecasting system behaviour requires classification of the system state with the classification, whether formal or informal, founded on knowledge of the factors that control response. In the context of building stone decay these controlling factors include, structural properties, mineralogical properties, inheritance effects, contaminant loading and natural change. In trying to formalise building stone condition assessment and incorporate a forecast component, an analogy can be made between the requirements for classification and treatment determination of cancer patients and the approach to condition assessment and conservation of stone structures. In medicine, one of the most widely used and refined patient assessment schemes is the TNM Staging System. The rationale underpinning the TNM Staging System has many similarities with approaches to building stone assessment in that it seeks to impose a more formal structure on condition assessment that provides a commonality of approach, language and meaning and a procedure for forecasting the extent of remedial intervention required and outcome in terms of ‘life expectancy’.

Modelling the rapid retreat of building sandstones: a case study from a polluted maritime environment

Abstract Sandstones are widely used as building stones throughout NW Europe. Unlike limestone, sandstones tend to experience episodic and sometimes rapid surface retreat associated with the action of salts and often leading to the development of hollows/caverns in the stone. The unpredictability of these decay dynamics can present significant problems when planning conservation strategies. Consequently, successful conservation requires a better understanding of the factors that trigger decay and determine the subsequent decay pathway. An overview of results from previous studies provided the basis for simulation experiments aimed at identifying the factors that (a) initiate decay and (b) permit the continuance of salt weathering despite rapid loss of surface material. These simulation studies involve investigation of changes in micro-environmental conditions as surface hollows develop and examination of salt weathering dynamics within such hollows. These data combined with knowledge gained from previous work have allowed the refinement of a conceptual model of rapid sandstone retreat. In this model decay is linked to the establishment of positive feedback conditions through interactions between factors such as porosity, permeability, mineralogy and their effect on salt penetration.

Weathering of igneous rocks during shallow burial in an upland peat environment: observations from the Bronze Age Copney Stone Circle Complex, Northern Ireland

Abstract The stone circle complex at Copney, County Tyrone is a key Bronze Age site that forms part of the Mid-Ulster stone circle complex. This site was excavated in 1995 and since then, deterioration of the archaeological stonework has been a serious problem. Deterioration is visible as splitting/fracturing of stones, the development of a bleached outer margin, surface scaling and granular disintegration, and, in some cases, complete disintegration of individual stones to sandy regolithic material. Environmental conditions at this site exacerbate the deleterious action of weathering processes, with periodic waterlogging and sub-zero temperatures during winter months. The Copney stones comprise two igneous lithologies: quartz porphyry and porphyritic andesite, both Lower Ordovician (470 Ma) in age. Both rock types show extensive alteration by hydrothermal processes resulting in weakened stone fabrics exploited by weathering processes during burial and subsequent exposure. Mineralogy of buried and exposed material indicates that approximately 2500 years of burial in a peat bog has produced secondary porosity comprising extensive microfracture networks and dissolution voids, which permitted further ingress of moisture and acidic waters, thus, promoting chemical alteration of mineral constituents. The occurrence of completely grussified stones and arenisation of boulder surfaces at Copney indicates that the processes of grussification are not solely restricted to deep weathering environment but can be achieved by burial in a shallow, aggressive environment over periods measured in thousands of years.

Changing climate, changing process: implications for salt transportation and weathering within building sandstones in the UK

Salt weathering is a crucial process that brings about a change in stone, from the scale of landscapes to stone outcrops and natural building stone façades. It is acknowledged that salt weathering is controlled by fluctuations in temperature and moisture, where repeated oscillations in these parameters can cause re-crystallisation, hydration/de-hydration of salts, bringing about stone surface loss in the form of, for example, granular disaggregation, scaling, and multiple flaking. However, this ‘traditional’ view of how salt weathering proceeds may need to be re-evaluated in the light of current and future climatic trends. Indeed, there is considerable scope for the investigation of consequences of climate change on geomorphological processes in general. Building on contemporary research on the ‘deep wetting’ of natural building stones, it is proposed that (as stone may be wetter for longer), ion diffusion may become a more prominent mechanism for the mixing of molecular constituents, and a shift in focus from physical damage to chemical change is suggested. Data from ion diffusion cell experiments are presented for three different sandstone types, demonstrating that salts may diffuse through porous stone relatively rapidly (in comparison to, for example, dense concrete). Pore water from stones undergoing diffusion experiments was extracted and analysed. Factors controlling ion diffusion relating to ‘time of wetness’ within stones are discussed, (continued saturation, connectivity of pores, mineralogy, behaviour of salts, sedimentary structure), and potential changes in system dynamics as a result of climate change are addressed. System inputs may change in terms of increased moisture input, translating into a greater depth of wetting front. Salts are likely to be ‘stored’ differently in stones, with salt being in solution for longer periods (during prolonged winter wetness). This has myriad implications in terms of the movement of ions by diffusion and the potential for chemical change in the stone (especially in more mobile constituents), leading to a weakening of the stone matrix/grain boundary cementing. The ‘output’ may be mobilisation and precipitation of elements leading to, for example, uneven cementing in the stone. This reduced strength of the stone, or compromised ability of the stone to absorb stress, is likely to make crystallisation a more efficacious mechanism of decay when it does occur. Thus, a delay in the onset of crystallisation while stonework is wet does not preclude exaggerated or accelerated material loss when it finally happens.

Stone-built heritage inventory and ‘performance in use’ condition assessment of stonework

An academic–industrial partnership was formed with the aim of constructing a natural stone database for Northern Ireland that could be used by the public and practitioners to understand both the characteristics of the stone used in construction across Northern Ireland and how it has performed in use, and, through a linked database of historical quarries, explore the potential for obtaining locally sourced replacement stone. The aims were to improve the level of conservation specification by those with a duty of care for historical structures, and to enhance the quality of the conservation work undertaken by architects and contractors through their improved knowledge of stone and stone decay processes.

Wind-driven rain and future risk to built heritage in the United Kingdom: Novel metrics for characterising rain spells

Wind-driven rain (WDR) is rain given a horizontal velocity component by wind and falling obliquely. It is a prominent environmental risk to built heritage, as it contributes to the damage of porous building materials and building element failure. While predicted climate trends are well-established, how they will specifically manifest in future WDR is uncertain. This paper combines UKCP09 Weather Generator predictions with a probabilistic process to create hourly time series of climate parameters under a high-emissions scenario for 2070-2099 at eight UK sites. Exposure to WDR at these sites for baseline and future periods is calculated from semi-empirical models based on long-term hourly meteorological data using ISO 15927-3:2009. Towards the end of the twenty-first century, it is predicted that rain spells will have higher volumes, i.e. a higher quantity of water will impact façades, across all 8 sites. Although the average number of spells is predicted to remain constant, they will be shorter with longer of periods of time between them and more intense with wind-driven rain occurring for a greater proportion of hours within them. It is likely that in this scenario building element failure - such as moisture ingress through cracks and gutter over-spill - will occur more frequently. There will be higher rates of moisture cycling and enhanced deep-seated wetting. These predicted changes require new metrics for wind-driven rain to be developed, so that future impacts can be managed effectively and efficiently.