Long term natural ageing of marble: evaluation and analysis

Marble is a rock widely used in the world in civil structures both in ancient and in modern constructions. Mainly in the recent years, where, very often the esthetical criteria are predominant on the structural ones and the knowledge of the stone material is often considered negligible, it happens to use on a building façade a stone that after few years show structural problems. The decreasing of decay resistance, in fact, can show itself by means evident signals such as fractures and bowing that can compromise the building construction structural stability. In order to avoid these drawbacks, the forecast calculation of the stone decreasing in mechanical strength should be very useful and important. In this work six different kinds of marbles exposed for more than ten years have been studied. Petrographic characteristics were compared with the results of different measurements campaign on slabs exposed for in Turin. Their durability was detected by means of the following test methods: ultrasonic pulse velocity, total water absorption, water absorptions by means sponge contact on the surface of the specimens tested, flexural strength measurements. After that long period of time only one marble shows a very high decreasing in mechanical resistance (flexural strength decrease 50%), while two of them don’t suffer changes in mechanical resistance. From the comparison of the results obtained with the different techniques to detect the decay, it is possible to assert that all are complementary but the UPV measurement are best correlate with flexural strength. Besides, in order to obtain a provisional model for the natural ageing of rock, for two kind of marble a correlation with the results of the same tests after artificial ageing by means of freeze and thaw has been done. A correspondence between natural and artificial ageing have been find and a decay model was suggested.