Wetting and drying of masonry walls: 2D-resistivity monitoring of driving rain experiments on historic stonework in Oxford, UK

摘要

2D resistivity methods can provide useful information about moisture distributions within porous historic stonework, especially when used in conjunction with simulated driving rain. Moisture is known to be a highly important factor in the deterioration of porous stone. We report here on a series of experiments on historic walls within the centre of Oxford, UK, which illustrate varying degrees of deterioration (including the formation of gypsum-rich black crusts and exfoliating hollows). Using medical electrodes we have been able to carry out non-invasive and non-destructive 2D resistivity surveys before and after simulated rainfall to investigate the progress of wetting and drying. We have applied tentative calibrations to convert resistivity values to moisture contents. Our results show that rainfall penetrates in predictable ways, with clearly defined wetting and drying fronts. Stone blocks with highly weathered surfaces exhibit the most rapid and high levels of water uptake, but also dry out more quickly than crusted or sound blocks. Thus, positive feedbacks may be encouraged whereby more water is cycled through damaged blocks, thereby enhancing the potential for further damage.