The exposure and deterioration of granitic building stone
The study of building stone decay is a field of increasing impOliance. Many previous studies have focused on limestone, and recently on sandstone. These studies can provide information on the spatial and temporal distribution of weathering forms and the degree of impact of polluted urban atmospheres. Several of these schemes rely on subjective visual appraisal by the examiner, which is unreliable and prone to measurement error. To counter these problems, and address the relative paucity of studies on granitic building stone, a novel semi-quantitative measure of rock quality was created. The creation of the Stone Deterioration Index (SDI) allowed the evaluation of longterm granitic building stone decay, while the short-term response of fresh granite to atmospheric exposure was examined through the use of microcatchment units (MCV' s). The measurement of run-off water and the physical response of fresh granite over a 91 week exposure period at three sites of varying pollution concentrations, allowed an assessment of the relative importance of surface roughness, composition and, dry & total deposition on deterioration rates. The exposure programme used MCV's containing granodiorite slabs with different degrees of surface roughness (finish) to collect run-off. This was measured for volume, pH, conductivity, and selected anion and metal concentrations. Run-off concentrations were measured by a Dionex 2000i ion chromatograph and a Perkin-Elmer ICP 40 inductively coupled plasma spectrometer respectively.Site was the most important factor in determining run-off concentrations showing significant variations between the urban and rural sites. Surprisingly, MCl) composition and surface roughness variations had little influence on component concentrations, and there were no systematic significant inter-finish variations. The MCV slabs showed no significant changes in weight, surface roughness or composition as a result of exposure. Granitic carousel tablets of varying composition, designed to assess the relative influence of dry and total deposition on granitic deterioration, also showed no systematic effects from exposure. The lack of significant variation between the pre- and post-exposure characteristics of the MCV slabs and carousel tablets could indicate that exposure length was not sufficient to generate measurable decay features. The assessment of long-term decay rates by the SDI, which was non-destructive and applied in situ, involved the measurement and rating of four parameters (surface strength, surface roughness, surface coatings and discolouration), to form a semi-quantifiable measure of rock quality. Eleven churches in Dartmoor, built between 1430AD-1896AD, were assessed by the SDI to provide a measure of long-term deterioration rates. Cross-church comparisons revealed no consistent effects on deterioration rates with increasing stone height. Aspect, however, did playa major role in influencing deterioration rates. The south facing walls (mean: 60 SDI points) showed the least amount of decay and had the slowest deterioration rate, while west (mean: 56 SDI) and north facing walls (mean: 53 SDI) deteriorated 6% and 14% faster than south facing walls respectively. Results suggest biological weathering was the major process in granite building stone deterioration in the Dartmoor region. These findings correspond to research by Robinson and Williams (1996) on sandstone churches. They ranked aspect by weathering scores derived from visual assessment of individual stone block, and found deterioration features were strongly affected by aspect, in the order; East>North>West>South. Although there is convergence in the decay pattern between aspects for the two stone types, the SDI index was formulated for coarse-grained igneous rocks, where it shows a strong statistical relationship between building age and rock quality (1=0.74). This allowed the formulation of an average linear deterioration rate for granite buildings in a clean, rural environment of 4.59 SDI points per hundred years.