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Download Investigating stone decay in a façade
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Investigating stone decay in a façade Stone decay in a façade • Natural decay processes not switched off • Superimposition of new, additional, sets of conditions and factors that can interact in complex and unpredictable ways • Example - natural and anthropogenic salts in combination Stone decay in a façade • Location and geometry of a building can influence patterns of wetting/drying and heating/cooling, soiling, greening • Aspect can play a role in determining time-of-wetness • Although the influence of the cardinal compass points can be overridden by macro-environmental factors, for example, airflow around a building Stone decay in façade • Physical stresses caused by surrounding materials, for example, the mortar used • Blocks are sealed on 4 sides by mortar, leaving one exposed ‘front’ face though which moisture can penetrate and migrate Stone decay in a façade • • • • In essence, the underlying controls of stone decay (temperature and moisture) are forced to act through one face Dry conditions - moisture / soluble salt movement, periodic wetting of surface and drying Wet conditions - saturation? Biological growth keeps it wet? Seasonal salt weathering cycles become more important? Stone decay in a façade • Monitoring moisture movement • By instrumenting blocks from the back face with humidity sensors we can measure the movement of moisture from the front face • Can tie into wider meteorological data collected by weather station Moisture Distribution: Geotomography Permanent electrodes Resistivity low ≈ high moisture high ≈ low moisture GeoTom device + software Time series data e.g. of surface wetting Section through trial wall Resistivity ‘pseudosection’, indicating 2D moisture distribution How do salts behave when blocks are saturated? Ion diffusion • Ion diffusion studies neglected in sandstone, but important with changing environmental regimes • Diffusion in solution is the process whereby ionic or molecular constituents move from an area of high concentration to an area of low concentration • Diffusion occurs without any bulk water movement (important for saturated blocks) • Diffusion ceases when there is no concentration gradient (uniform concentration) Ion diffusion • Measuring diffusion - determining the diffusion coefficient • Developing methods used to study chloride diffusion through concrete Ion diffusion • Based on Fick’s first law: D = VL/CA x dc/dt (where V = volume of downstream cell, L = sample thickness, C = chloride concentration, A = transmission area) • The solutions must be well mixed • Concentration measurements of the solution are made using a minimum volume (5-10µL) for IC analysis of chloride and other anions (nitrate, sulphate) • The water in the core does not move (saturation) • Sorption is negligible Ion diffusion • Analysis of final solution • Not only anions of the input salts, but also silica aluminium and iron (colorimetric techniques) to see how ion diffusion impacts the mobility of these elements (detection limits as low as 0.005 and 0.02 mg/L for Al and Si, respectively) • Ties back into the biological side of the study by the potential of seeing how organic acid mixtures impact on diffusion and the mobility of elements • Cutting the stone up before it dries to see where salts are (nucleation points?)
