Monument Future. Siegfried Siegesmund

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      Figure 4: The AE amplitude versus the rate of temperature change (RTC) of the rock samples and its frequency by RTC. A: granite, B: marble, C: sandstone.

      In the case of the granite, the maximum AE amplitude was recorded when the RTC = 1.83 °C/min and the frequency of the RTC = 1.5–2.0 °C/min was approximately 20 %. In the granite, a relatively large AE amplitude is generated as the RTC increases. However, in the marble, the maximum AE amplitude occurred when the temperature decreased. The frequency of the RTC < −1.5 °C/min accounts for approximately 45 % of the whole. In the case of the sandstone, the maximum amplitude is generated at RTC = 1.5 °C/min, though the frequency of the AE during the temperature decrease is high.

      Thus, the AE signal occurred when the temperature increased above RTC = 1.5 °C/min in the case of the granite and sandstone and when the temperature decreased below RTC = −1.5 °C/min in the case of the marble.

      There have been many field observations of the RTC, but during recent years, it has been reported that large temperature changes have instantaneously occurred. McKay et al. (2009) measured the surface temperature of basalt using thermocouple sheets in the Atacama Desert and the cold deserts 184of Antarctica. It was found that the RTC of ≥ 2 °C/min appeared approximately 8 % on average, and the RTC of ≥ 8 °C/min appeared 0.02 % on average. In addition, Molaro & McKay (2010) measured the surface temperature of basalt and sandstone samples using a 0.375-s interval in Death Valley (USA) during April 2009. As a result, the RTC at 2 °C/min or higher accounted for 71.6 % of the basalt and 66.3 % of the sandstone, respectively.

      These studies suggest that rocks subjected to rapid temperature changes due to solar radiation may form microcracks and fracture via thermal shock. In this study, it was presumed that microcracks occurred in the rock samples at an RTC above ±1.5 °C/ min. For this reason, it is believed that long-term continuous temperature change due to radiation, which is effective for microcrack generation, leads to stone deterioration.

       Conclusions

      AE amplitude was generated in three rock types resulting from a temperature change of from 4 °C to 84 °C, which is probably observed in the field. The generation of AE signals the formation of microcracks due to thermal stress acting on grain boundaries. A large AE amplitude occurs when the temperature gradient is > 1.5 °C/min. This means that microcracks are generated inside the stone even with normal temperature changes in the field. As these microcracks grow, the bond strength between minerals eventually weakens, leading to stone degradation.

      The generation, frequency, and pattern of AE differ depending on the rock type. These depend on the mineral composition, structure, and physical and mechanical properties of the rock itself. Granite containing a large amount of quartz has the highest AE generation, followed by that of marble with a uniform mineral composition; the lowest AE generation occurred in high-porosity sandstone.

      For cultural properties composed of these three rock types, maintaining a small temperature change will prevent microcrack occurrence, but it is difficult to do so. The expected temperature increase in the future due to global warming is likely to further increase the potential for thermal weathering of cultural properties.

       References

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       ACOUSTIC EMISSIONS – INSIGHTS INTO THE DECAY MECHANISMS OF THERMALLY TREATED MARBLES

Stephan Pirskawetz¹, Johanna Menningen², Siegfried Siegesmund²

      IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

       – PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION СКАЧАТЬ