RAPID, CATASTROPHIC DECAY OF BUILDING LIMESTONES: IMPLICATIONS FOR MASONRY SELECTION AND LIFETIME BEHAVIOUR

Stone is widely recognised as a sustainable construction material and as a store of much of the world's tangible cultural heritage. With this recognition has come an understanding that stone has a finite life that can be drastically curtailed when it is placed in the often-aggressive urban environments. In particular, many common building limestones experience seemingly unpredictable, episodic and sometimes catastrophic breakdown as stone strength is exceeded by gradual decay, the slow accumulation of internal stresses and/or subjection to extreme external stresses such as a severe frost. Episodes of rapid decay may be interspersed with periods of relative stability marked by, for example, the formation of pollution-derived calcium sulphate crusts. To control potential catastrophic decay it is therefore necessary to understand why rapid retreat is triggered, what allows it to continue, how it can be halted and how the causes can be avoided in the first place. This is particularly true where inappropriate conservation could accelerate decay, and where choices have to be made between possible replacement stone and stone selection in relation to new structures. To achieve this understanding four questions need to be asked.1. What processes are responsible for rapid retreat?2. What physical, chemical and mineralogical characteristics determine stone susceptibility to rapid retreat and how do these properties change during decay?3. How do microclimatic conditions at and beneath the stone surface change as stone retreats and how do these influence decay mechanisms?4. What permits continued weathering despite rapid loss, of weathered material in which, for example, damaging salts are concentrated?This interdisciplinary project will examine these questions through field studies of stone structures in Oxford and nearby areas built of oolitic limestone (e.g. Bath and Cotswold limestones) that is prone to rapid retreat. Linked to this will be the development of fibre optic sensors that will allow moisture and salt movement within individual blocks to be monitored in relation to environmental conditions, including temperature, relative humidity and surface wetting. These data, and the same sensor technology will be combined with analyses of weathered stone to design laboratory experiments using different varieties of Bath Stone to simulate breakdown patterns and the dynamics of salt and moisture movement as blocks retreat and are progressively sheltered. Results from field studies and controlled laboratory experiments will be combined to explain (model) the factors that determine overall susceptibility to either rapid retreat or stability and the operation of the processes responsible for decay. In particular, results will be used to determine what triggers positive and negative feedbacks that respectively accelerate and decelerate change within the stone decay system. This understanding will be used, in discussion with end-users, to develop protocols for limestone conservation and the selection of new and replacement stone matched to specific environmental conditions.