Improving non-destructive techniques for stone weathering research in situ

"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 &LT;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.

Advisor: Viles, H