Siegfried Siegesmund - Monument Future

The maximum AE amplitude of the granite is at 20 °C and 60 °C, but the frequency is the highest at 70–80 °C. However, in the case of the marble and sandstone, the maximum AE is observed at approximately 20 °C, and its frequency is large. The appearance patterns for these rocks are different, as shown in Figure 2. The AE amplitude is observed when the temperature decreases in the marble and mainly when the temperature increases in the sandstone.

Figure 3: The AE amplitude versus the rock surface temperatures and its frequency by temperature. A: granite, B: marble, C: sandstone.

Thermal stress that causes AE is a result of the anisotropy in the thermal expansion properties of different minerals (Sirdesai et al. 2017) and the amount of certain minerals such as quartz. According to Kinoshita et al. (1995), in the case of granitic rock, even when heated at a slow heating rate that does not cause a temperature gradient inside the rock, due to the mismatch in the thermal expansion coefficient of the mineral particles, 183AE signals occur when the temperature reaches from approximately 60 °C to 70 °C, and its amplitude increases with heating. In other words, the reason why the frequency of the AE in the granite increased toward 70–80 °C is probably due to the difference in the thermal expansion coefficient of the constituent minerals. The sandstone is also composed of aggregates of various mineral grains. Although the porosity of the sandstone is high as previously described, the reason that the AE of the sandstone mainly occurred at the time of the temperature increase is thought to be due to the mismatch of the thermal expansion coefficients of the constituent minerals. The main constituent mineral of the sandstone and granite is quartz. Quartz thermally expands more than other minerals: quartz shows a thermal expansion of 0.14 % (⊥ c) and 0.08 % (||c); however, plagioclase shows an expansion of only 0.09 % (||a) and 0.03 % ( ⊥010), from 20 °C to 100 °C, respectively (Skinner 1966). Therefore, AE is generated at a relatively low temperature in rocks containing quartz. In addition, in such rocks, the increase in the AE with a subsequent temperature increase is remarkable.

Because marble is composed of only a single mineral, microcrack occurrence due to inconsistency in the thermal expansion coefficients of the minerals is difficult to recognize. However, calcite shows the thermal anisotropy of 0.189 % ( ⊥c) and −0.042 % (||c) from 20 °C to 100 °C, respectively (Skinner 1966).

Figure 4 shows the relationship between the RTC of the rocks and the AE amplitude as well as frequency of the AE for each RTC. In Figure 4, the rate of the temperature increase is shown as a plus, and the rate of the temperature decrease is shown as a minus.

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.

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.

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