Ten years of natural ageing of calcareous stones

•Evaluation of natural and artificial decay of calcareous stones•Application of standardized methods for testing natural stones•Further investigation on petrographical features obtained by image analysis and AGA•Correlation between grain habit and durability

In the recent years, as aesthetic criteria are predominant over structural criteria and the knowledge of stone materials is considered negligible, sometimes a stone is used on a building façade that, after a few years, shows structural problems. Decreasing decay resistance, in fact, can show itself by means of evident signals such as fractures and bowing that can compromise a building's structural stability. In this work, nine different kinds of calcareous stone slabs exposed for ten years to natural weathering in Turin (Northern Italy) have been studied. Petrographic characteristics and an enhanced petrographic Analysis of Adjacent Grains (AGA) were compared with the physical and mechanical properties of slabs exposed. Stone durability was detected by means of the following test methods: Ultrasonic Pulse Velocity (UPV), water absorption, superficial water absorption by means of a contact sponge, flexural strength, bowing measurements and image analysis. From the comparison of the results obtained to detect the decay with the different techniques, it is possible to assert that the UPV measurement best correlates with flexural strength. After ten years of natural weathering, the decrease of mechanical resistance of calcitic stones with similar grain size is inversely proportional to the value of AGA. The same tests are performed after various thermal and moisture cycles, according the recent EN 16306 European Standard. Comparison of natural weathering and artificial ageing, by means of image analysis, suggests that the natural external factors cause different types and degrees of porosity than artificial ageing.

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