1,721,087 research outputs found
What controls algal greening of sandstone heritage? An experimental approach
Full text linkRecent observations have shown that many sandstone buildings, including important components of the UK’s cultural heritage, are becoming covered with green algal growths. This is likely to result from recent changes in air quality and the impacts of a changing climate. The northern regions of the UK in particular have an abundance of sandstone heritage and, given the likelihood of warmer, wetter winters here, algal growth on vulnerable monuments is likely to become a primary conservation concern over the next 50 years. Observations of sandstone monuments in the northern regions of the UK, in particular in Belfast (Northern Ireland), Sheffield and Edinburgh have highlighted that algal greening is notably patchy. This is likely due to the array of factors which affect the bioreceptivity of host substrates such as sandstone. The bioreceptivity of a substrate (its ability to become colonised by microbes such as green algae) is dependent on inherent, external and architectural factors. The role of these factors and the interrelationships between them requires further study.
This thesis aims to investigate the inherent, external and architectural factors which encourage colonisation of sandstone by green algae through an integrated programme of laboratory and field experimentation. The primary objectives of this study are: to develop improved laboratory experimental methods to control and monitor algal growth, to investigate the role of external, inherent and architectural factors and to explore the fundamental role of moisture in the development of algal greening.
In order to address these objectives, laboratory and field experiments have been linked within an integrated overall methodology. Short-term laboratory experiments have investigated the bioreceptivity of four British sandstones (Peak Moor, Dungannon, St Bees and ‘baluster stone’) to single and mixed green algal treatment with Stichococcus bacillaris, Chlorella vulgaris and Desmococcus olivaceus, under controlled conditions. Two field experiments have also been conducted. The first exposed unweathered blocks of Dungannon sandstone in the wet environment of Derrygonnelly, Northern Ireland for 30 months. The second exposed reclaimed sandstone balusters in a shaded and exposed site in central Oxford for 12 months. The laboratory and field experiments presented utlilise a range of simple and accessible methods to monitor biofilm development (for example novel methods to map biomass) and changes in substrate condition (such as monitoring surface moisture movements with weight change and hand-held moisture meters, and using light microscopy to help visualise the impact of green algal biofilms).
The results presented in this thesis confirm that moisture plays a fundamental role in the development of green algal biofilms. In laboratory experiments, colonisation often occurred within a consistent moisture zone and preferential greening in field experiments was observed in areas of frequent moisture movement. External factors have been shown to have a strong influence, in laboratory experiments where marine salts were applied, these were found to delay colonisation by around seven days. Furthermore, salts resulted in inhomogeneous patterns of colonisation, similar to those observed in scoping studies conducted in Sheffield. Laboratory experiments have also demonstrated that inherent substrate factors such as high porosity and presence of certain minerals (such as clay laminations in Dungannon) can increase the primary bioreceptivity of sandstone surfaces.
Field experiments have demonstrated that architectural factors such as aspect and geometry can increase the bioreceptivity of exposed samples. In particular, preferential greening was observed on the dynamically wetted south west facing blocks in Derrygonnelly and on exposed compared with shaded balusters in Oxford. Greening was also concentrated in areas of rainwater flows and stores.
Investigation of the role of external, inherent and architectural factors in the development of algal greening as provided by this project, supplies useful information for those managing our sandstone cultural heritage. This will enable more informed decisions to be made over appropriate management and conservation strategies for the future
Deterioration and conservation of rock-hewn sandstone cave-temples in Longdong area, China
Full text linkCave-temples are multivalent types of immovable cultural heritage which link spiritual values as places for Buddhism rituals, aesthetic values because of the Buddha sculptures, inscriptions and murals they contain, historic values as significant objective evidence of the history of the eastward spread of Buddhism, as well as economic values as tourist attractions. It is, therefore, highly necessary to preserve them in a sustainable manner and pass them on to the next generation.
The Longdong area refers to the area to the east of the Long mountains, i.e. the southern section of the Liupanshan mountain chain, in Gansu Province, China. This region was also the eastern section of the Silk Road connecting the Central Plain in China to western countries. A great number of cave-temples are hewn from outcrops of the widespread sedimentary sandstone units in the region and face severe deterioration problems endangering their values and integrity. Thus, study regarding deterioration process is required in order to understand the impacts of these deterioration risks.
Accordingly, the research presented in this thesis focuses on studying and evaluating the rock deterioration and its impacts aiming to clarify the major weathering mechanisms of these cave-temples. The North Grotto Temple (NGT), situated near Qingyang, is the oldest site representing the highest artistic value of grotto art in the Longdong area, hence, was selected as the study site. The overall research comprised three phases. The first phase involved field investigation and survey of weathering features; the second phase focused on in situ detection, modelling prediction and laboratory characterization of deteriorating salts, and the third phase comprised laboratory simulation of salt weathering at North Grotto Temple and how it affects the rock properties. A range of portable non-destructive devices (including Karsten tube and surface hardness tester (Proceq Equotip® 3 and 550), ultrasonic instrument (Proceq PunditLab) were used in the research, along with a non-invasive method of sampling salts (fiber paper pulp poultices), a range of laboratory analysis and experimental methods (including ion chromatography and environment cabinet) and modelling software (thermodynamic model - ECOS-RUNSALT).
According to the research, several deterioration patterns, such as granular disintegration, efflorescence, alveolar, were identified on the NGT sandstone façade, which illustrate that salt weathering is probably the leading weathering agent at the site. A mixture of salts was predicted to form in the site, i.e. aphthitalite, bloedite, picromerite, darapskite, mirabilite, hexahydrite, starkeyite, nitromagnesite, halite, niter, and sylvite. Large diurnal variation of humidity in the site enhances the likelihood of salt weathering. Laboratory simulation showed experimentally that the salt mixtures can cause material loss, appearance changes and modify the petrophysical properties of sandstones, which badly affects the integrity of rock.
In sum, this study illustrates that the sandstone cave-temples in the Longdong area are under cumulative damage from salt weathering. Environmental control is a necessary measure in order to manage the risks threatening the on-going preservation of the sandstone cave-temples
A holistic approach to evaluating the performance of consolidants on sandstone
Full text linkSandstone, one of the most ubiquitous building materials around the globe and across time presents a complex range of characteristics and responses to environmental and physical erosion over time. The range of sandstone sub-types that have been used for structure and decorative carving through history was highly variable but is diminishing as a resource as more and more quarry sites are worked out or closed. This process has led to the use of ‘replacement’ stones in conservation work which are sourced from a small number of known sites and are similar to historic types in appearance if not age or internal characteristics. This project examines one of these replacement stones, Locharbriggs sandstone from Dumfries in Scotland, and couples it with two of the most commonly used silane consolidants in order to improve the understanding of the performance of both in lab and field conditions.
The project begins by taking observations on site at Kenilworth Castle (Warwickshire, UK) shaping the research through the identification of areas of concern for the conservation of historic sites, such as erosion driven by visitor footfall and the effects of prior treatment applications. These are used to formulate an experimental programme which combines lab and field experiments, and begins with the development of a method for artificially weathering the stone substrate using the controlled application of heat, in preparation for consolidation.
The field-based experimental work, hosted at the Wytham Woods research site near Oxford UK, examines the uses and limitations of two forms of field trial; the exposure of laboratory-prepared stone samples on a purpose-built exposure rack, and the incorporation of stone samples of variable size and treatment within a purpose-built test structure. Both experiments trial a range of assessment metrics for the detection and monitoring of consolidants applied to the samples, within the context of exposure to local weather conditions. In addition to these experiments, a further trial was hosted by SATRA Technologies (Kettering, UK) which utilises a mechanical prosthetic leg to simulate physical abrasion analogous to that observed on site, modelling the type of wear driven by footfall across exposed stone surfaces.
These experiments trial the use of a range of systems for the assessment of stone condition and consolidant performance, combining those which are understood in other heritage contexts with the novel combination of handheld optical and laser scanners for the detection of change on samples. These are formalised into a ‘toolkit’ of methods and instruments that can be applied to sandstone- consolidant experiments, including novel methods of visualising and analysing data.
The thesis concludes that the range of assessment techniques trialled here have a range of useful applications in the detection of consolidants in heritage sandstone and for the detection of change on site and in laboratory conditions, and that there is significant scope to employ them in a range of conservation applications
Developing relations between heritage conservation and urban revitalization: lessons from China
Full text linkThe overall aim of this thesis is to further understanding of the developing relations between heritage conservation and urban revitalization, by following the on-going World Heritage List nomination process for the Grand Canal in China. A review of the history and current situation of heritage conservation and urban revitalization establishes several key gaps in knowledge, for example, the field of systematic heritage conservation is still left blank. Taking advantage of the opportunity to work with this on-going nomination, the project examines the nature and challenges of heritage conservation research and practice for a heritage property of large scale and complexity, in the form of four linked studies. A critique of the national level nomination process to date illustrates the complexity of the task, and concludes that systematic heritage conservation has not been established as a concentrated and nation-wide heritage conservation activity in China. Focusing on the municipal level nomination process to date through a case study of the city of Zhenjiang (and Yangzhou as a comparator) establishes key reasons why one city has been more successful than the other in its participation in the World Heritage List nomination. A second case study, of the Xi Jin Ferry area in Zhenjiang (Jiangsu Province), provides a fuller account of the ways in which heritage conservation can contribute to, and work with, urban revitalization. Drawing on the case of Xi Jin Ferry, a theoretically-informed, but practically-viable approach to linking heritage conservation and urban revitalization is developed which learns from the past and looks to the future. Building on the empirical research projects, a critique of existing approaches to heritage conservation of large, complex sites leads to a proposal for a 'Heritage System' framework which would facilitate future evaluations and improve the prospects for on-going management of the Grand Canal
The effects of open shelters on the preservation of limestone remains at archaeological sites
Full text linkShelters, as preventive conservation methods, have traditionally been considered a better option than leaving the site exposed. However, there has been limited research on their effect on the preservation of heritage materials and, as a result, there is no clear scientific evidence to support sheltering. This study aims to provide the first rigorous scientific assessment of the effect of lightweight, open shelters on limestone deterioration at archaeological sites. A method based on the use of low-cost environmental monitoring equipment and limestone blocks and tablets (as indicators of decay) has been developed to determine the degree of protection provided by the shelters at the Bishop’ Palace (Witney, England) and Hagar Qim (Malta). Preliminary visual assessments of the field sites were followed by 12-18 month exposure trials. Temperature extremes and fluctuations, frost events, relative humidity extremes and fluctuations, NaCl crystallisation events, solar radiation, wetting events, salt content, atmospheric pollutants and dust deposition were monitored. In addition, stone decay was studied by analysing changes in weight, elasticity, surface hardness, ultrasonic pulse velocity, surface colour, moisture content and general appearance (microscopic and macroscopic pictures) in stone samples. An exhaustive assessment of the shelter at the Bishop’s Palace was carried out using Chalk, Cotswold and Portland limestone blocks as well as Portland limestone tablets (specifically for studying dissolution, soiling and biological growth). Additionally, a comparative assessment of the effects of the two shelters in contrasting climatic environments, the Bishop’s Palace (temperate maritime) and Hagar Qim (Mediterranean), was undertaken by monitoring Globigerina and Coralline limestone blocks simultaneously at both sites. The research has shown that lightweight, open shelters do not exclude decay completely but minimise it. However, there are some areas at higher risk of decay, i.e. top parts of the walls and the periphery. In addition, problems with the shelter design can enhance some decay mechanisms, such as biocolonisation on the periphery at the Bishop’s Palace and dust deposition under the shelter at Hagar Qim. Therefore, the effectiveness of shelters should not be assumed
A holistic approach to diagnose deterioration of rock-cut structures in Lalibela, Ethiopia
Full text linkRock-cut architecture is made through a subtractive process of removing material from rock outcrops or cliff faces to create spaces. Rock-cut sites are at the intersection of the natural and built environments, producing a unique semi-natural setting in which both naturally existing geological features and human activities contribute to their deterioration. This thesis develops a holistic approach to diagnosing deterioration of rock-cut structures by focusing on the rock-cut church complex at Lalibela, Ethiopia. The diversity of rock-cut church types in the UNESCO world heritage site of Lalibela makes it a suitable case study to investigate the factors that influence the deterioration of rock-cut structures.
In semi-natural settings, key drivers of weathering can be broadly classified into climate, lithology, architectural form and history of conservation. These drivers were investigated in Lalibela using field studies, laboratory-based experiments and archival resources to develop a comprehensive understanding of weathering of rock-cut sites.
Rock-cut sites share lithological characteristics; many of them are built in low-strength rock outcrops. This inherent characteristic is correlated with the presence of secondary minerals such as swelling clays. The churches at Lalibela are carved from basaltic scoria that contains > 30 \% swelling clays. The climate at Lalibela is characterised by intense seasonal rainfall (main rains season) and high rock-surface temperature fluctuations (dry season). High, but fluctuating relative humidity (RH) and intense rainfall during the main rains season are favourable conditions for surface and deep-seated wetness and for frequent wetting-drying cycles to occur. To determine the impact of wetting-drying cycles on clay swelling/shrinkage and salt crystallisation/dissolution cycles, dilatation was measured in salt-free and salt- contaminated basaltic scoria samples in simulated wetting-drying cycles. Results showed that the introduction of salt (NaCl) to clay-rich basaltic scoria can substantially accelerate decay and that the range of RH fluctuations at Lalibela can cause significant dilatation.
The architectural form influences the degree of exposure of rock-cut structures to climatic variables such as rainfall and insolation. Moisture dynamics assessments done in the main rains and dry seasons demonstrated that the saturation state of the walls is influenced by exposure of the roofs to rainfall. Salt distribution assessments showed that the thickness of the walls (which lack foundations to retard the flow of moisture) influences salt loading. Conservation at Lalibela has mainly focused on protecting the exposed roofs from water ingress and has often been destructive or altered the cultural landscape. Revisiting traditional conservation techniques that mimic soft capping may present a new opportunity to preserve the sites in a manner that is in keeping with the intangible heritage values of the site.
A holistic methodology to understand the processes that lead to the deterioration of rock-cut architecture should be based on a semi-natural site framework, as has been attempted in this thesis. Two scales of approach are recommended to assess the state of and threats to the preservation of rock-cut structures: architectural form and landform. The architectural form will influence micro-climate, moisture dynamics and salt distribution, and overall, the post-exposure history of transformation and conservation of rock-cut sites. At the landform scale, inherited properties (pre-exposure history) such as topography and geological discontinuities can influence weathering.
By identifying and assessing the key drivers of weathering of rock-cut structures, this thesis lays the foundation for a scientific approach to understanding the deterioration of rock-cut architecture, which can be applied elsewhere beyond the seasonal tropical environments of northern Ethiopia
Methodologies for evaluating exposure and response of stone masonry to wind-driven rain
Full text linkWind-driven rain (WDR) is a main moisture source and weathering factor for monumental and vernacular stone masonry in the UK. To conserve and manage these structures, especially as weather events are predicted to become more intense during the 21st century, methodologies are needed that: (a) characterise environmental WDR exposure, and (b) non-destructively monitor the response of moisture regimes within stone masonry.
This thesis aims to address exposure and response between WDR and stone masonry, integrating characterisation and methodological development with an emphasis on data handling and visualisation. Semi-empirical approaches are employed to characterise current and future WDR exposure in the UK to evaluate existing standards and metrics. The use of non-destructive electromagnetic techniques for moisture measurement is explored for comparative advantages when applied for stone masonry.
Extreme value analysis (EVA) is used to evaluate severe WDR exposure in the UK at eight sites. While reinforcing established trends (e.g. prevailing wind directions) this research highlighted the impact of wall orientation on the volume of water and consistency within WDR spells and their quantity and duration. The EVA demonstrated that current standards (ISO 15927-3 and BS 8104) underestimate extreme exposure. A combination of UKCP09 Weather Generator output with probabilistic processes demonstrated that existing contrasts between sites will be magnified by predicted climatic changes and become more seasonally polarised, providing an impetus to improve current standards by incorporating extreme value analysis and temporal metrics.
A novel, cost- and time-effective method of laboratory gravimetric calibration using 'isolated diffusion' was validated, which produced calibrations of radar and microwave techniques for three UK building stones that matched modelled behaviour. The combined use of microwave and radar techniques in field studies on two stone masonry constructions characterised localised moisture regimes within stone masonry systems (stone units and mortar joints), demonstrating that technique selection is optimised with consideration for material properties and the investigation objective. Innovative data handling and visualisation strategies demonstrated their utility for these scenarios of stone masonry composed of different materials.
By developing methodologies for semi-empirical evaluation and non-destructive techniques, as well as characterising environmental and hygric properties/behaviour of stones and stone masonry, this thesis has contributed to both progress in scientific research and practical aspects of heritage conservation in the context of a changing 21st century climate.</p
Improving non-destructive techniques for stone weathering research in situ
Full text link"In time, and with water, everything changes" (Leonardo da Vinci in Kemp, 2006). In the field of cultural built heritage these changes often eventually result in loss of unique irreplaceable sites. This loss is considered to have an effect on societies as heritage is an important part of cultural identity and future development. In order to prolong the life of built heritage structures and preserve the collective memory they represent the weathering behaviour of the materials needs to be understood. Stone is a very common component of built heritage, especially limestone which is the focus of this thesis.
Stone weathering behaviour can be investigated under controlled laboratory conditions, but results do not entirely reflect its behaviour under real world conditions (because of complex weathering histories and spatial heterogeneity found on real built heritage). Therefore, it is necessary to complement the laboratory approach with in situ investigations. For in situ investigation a variety of methods is available ranging from destructive to non-destructive (NDT) and sophisticated and expensive to more simple and economical. This thesis is based on the key principle of built heritage conservation i.e. to preserve as much original fabric as possible and keep destructive sampling to a minimum. Furthermore, to allow for wider application on a bigger scale and more frequently, the focus has been on non-destructive, portable, and economical methods. However, standards and good practice guides for these methods have not yet been developed. Thus, the overall aim of this thesis was to develop reliable methodologies for these methods in order to quantify the extent and rate of limestone heritage decay in situ under real world conditions.
The thesis has three objectives. Objective 1 improved the application of selected NDT methods under laboratory conditions, focusing on sampling protocols (e.g. sample sizes) and reliability of data generated. Innovative aspects of research for this objective include extending their application (converting some drawbacks into advantages), combining them and applying modern statistical methods to the data evaluation. With this approach information on stone surface and subsurface properties was gained. This assists to capture stone weathering behaviour trajectory more holistically by investigating processes preceding total stone mass loss (erosion). Objective 2 applied the improved NDT methods to a time series of dated Portland limestone gravestones covering 1 to 248 years of exposure in order to evaluate the changing rate of surface property changes. The method proposed here provides a novel application of surface hardness data for quantifying stone deterioration rates over short- and long-term. Further, QC50 (the regression coefficient for 0.50 quantile regression) is introduced as novel robust measure for surfaces property changes. Is was found that depending on the time scale of investigation weathering behaviour is either defined as non-linear (whole period of 248 years) or linear (periods <100 years). It was found that stone weathering behaviour in cases needs to be investigated below block scale due to spatial variances. Objective 3 applied the NDT methods to diagnose the nature and causes of catastrophic limestone deterioration observed after a harsh winter at the archaeological site of Dülük Baba Tepesi, South Turkey. The cause for catastrophic stone decay in situ were reconstructed using NDT techniques and past climate data reports. This provides a novel application to infer the cause of catastrophic decay in situ by combining moisture uptake characteristics with robust data evaluation for surface and subsurface hardness data with past meteorological data. It was concluded that the Hellenistic-Roman structures are too vulnerable to be exposed to the prevalent environment without any further preservation measures.
Similar to the 'scientific toolkit' recommended by Meneely et al. (2009) for more sophisticated methods (e.g. 3D laser scan, ground penetrating radar etc.) the methods evaluated in this thesis are seen as a contribution to a potential 'scientific toolkit of low-cost methods' which could be complemented with other methods like ultrasound velocity measurements, drilling resistance etc. Thus, this study shows that the improved methods may assist in both 1) understanding heritage stone weathering under real world conditions (without damaging them by sample taking, whilst capturing surface/subsurface changes); and 2) more frequent investigation of the state of preservation/deterioration of stone heritage on-site in order to detect ongoing deterioration at an early stage.</p
Designing and evaluating repointing lime mortar for the conservation of historic buildings in highly exposed environments
Full text linkMany historic buildings are under threat due to their severe exposure to wind, rain and high humidity. One of the main conservation challenges for historic and traditional buildings is to ensure water ingress is mitigated and moisture that has entered a wall evaporates, contributing to drier and less damp conditions. These are some of the main roles of mortar joints in a masonry: absorbing moisture from the surrounding stones and contributing to its evaporation. When mortar deteriorates or when inappropriate mortar, that traps moisture inside the wall and encourages evaporation through the porous stone, has been used, it has to be replaced by a new mortar called repointing mortar. This thesis focuses on repointing in a common UK situation of high exposure to wind and rain and masonry with low permeability, as found in the south-west of England. It aims to investigate the role of repointing mortars in the conservation of historic buildings in exposed environments: What would be the most effective mortar to repoint dense stone masonry in exposed locations? How do the environmental conditions found on-site affect these mortars? Can these repointing mortars mitigate driving-rain ingress? This thesis addresses these research questions with a multi-method and multi-scale methodological approach, from laboratory samples to test walls, to optimise, evaluate and validate twenty-three compositions of lime mortars. This work is therefore relevant to other high humidity environments and to a multitude of structures. Key scientific findings are made on the effect of specific materials in lime mortars, such as the use of wood ash, porous aggregates and quicklime, qualifying them as suitable materials to optimise repointing mortar. Wood ash contributes to increase the capillary absorption capacity of mortar while giving a potential pozzolanic activity, a positive outcome when applied under humid conditions. The thesis brings further understanding of the impact of humid environmental conditions on the properties of lime mortars, and showed that laboratory evaluation should be made on samples cured under realistic conditions if information on the early to medium-term (up to 90 days) characteristics of NHL mortar is required. Findings from experiments on test walls demonstrate that repointing mortar, by being more compacted, helps mitigate rain ingress after a short intense rainfall event. The novel pilot scoring system developed could have considerable implications for more effective and durable repointing interventions. Together, these findings have significant implications for decision-making on repair interventions for the conservation of historic buildings, especially in building resilience in a context of climate change.</p
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