COMPLEX WEATHERING EFFECTS ON THE DURABILITY OF BUILDING STONE

The weathering of building stone involves an often, complex progression from ‘fresh’ to ‘failed’ stone. Unfortunately, most standard durability tests only register the two end extremes in this progression i.e. stones are inserted as fresh blocks and only revisited upon disintegration. Durability tests (such as the sodium sulphate test) are therefore not designed for, or capable of, identifying the subtle, non-linear and invariably spatially specific changes that characterise stone decay. In addition to informing the selection of matching stone for conservation/replacement, the ability to predict ‘change during lifetime’ will greatly help in the selection of stone for specific purposes and environments and in the design of structures that will retain their architectural integrity as they age. Preliminary data are reported from laboratory simulation experiments where the effects of incorporating high magnitude, low frequency weathering cycles (freeze/thaw) into a regime of low magnitude, high frequency weathering (salt weathering) on Dumfries sandstone and Portland limestone samples are examined. Rates and patterns of decay for both stone types are reported with specific emphasis given to identification of extrinsic controls and lithologically-specific and non-specific intrinsic factors that determine the ability of stone to resolve both high and low magnitude stress events. Particular attention is also paid to the early stages of the decay sequence when degradation of structural and mineralogical properties is just beginning. These are the stages in stone weathering about which much is assumed but little is actually known.