The durability of Bo anov sandstone – simulation of Prague winter climatic conditions

Sandstones were often used mainly as building material in Prague. Weathering processes lead to deterioration of these dimension stones and to changes in their durability. The range of the deterioration depends on petrography, including internal structure, mineral and matrix composition, and also changes initiated. The stone’s properties are negatively influenced especially during the winter frost period, and the presence of salt can exacerbate this process. The national standardized testing methods for natural stone, such as determination of frost resistance and determination of resistance to salt crystallisation may not take into consideration the actual conditions in the stone’s locality. Road salts, mainly composed of sodium chloride, are used to de-ice streets in Prague during winter season. Samples of snow/water mixtures from Prague city centre were chemically analyzed and the average NaCl concentration was determined. To simulate the weathering processes typical in Prague’s winter climatic conditions, seasonal data from 1999 to 2008 were statistically analyzed. Based on this analysis, a simulation program consisting of 56 freeze/thaw cycles in the temperature range of -14°C to 14°C was developed. The weathering simulation program was divided into four stages, each consisting of 14 freeze/thaw cycles. The sandstone samples had been soaked for 24 hours in distilled water and in a 2.5% solution of NaCl before each stage. Each cycle lasted seven hours and the samples were maintained at minimal and maximal temperatures for two hours. The Bo anov Cretaceous sandstone was tested during this experiment. Changes developing in the internal structure were studied by Hg porosimetry. Physical-mechanical properties such as water absorption at atmospheric pressure, uniaxial compressive strength, apparent density and open porosity were also studied. The uniaxial compressive strength decreased after 56 freeze/thaw cycles in both combinations - with distilled water and NaCl solution. The decrease of open porosity measured by both methods corresponds to the decrease of water absorption at atmospheric pressure in both cases of loading.