Siegfried Siegesmund - Monument Future

All baluster and cylinder specimens exposed for heating in the fire container show macroscopically visible cracks (Fig. 6b). Moreover, they reveal heavy sooting on the surfaces. Discoloration of the sandstone or crack surfaces could not be detected by the naked eye.

The temperature curves for the small sandstone specimens (50 × 25 mm) are shown in Fig. 6c. They were heated at different temperature levels of 400, 500, 600, 700, 800 and 1,000 °C in the laboratory oven at the TU BAF. The set-point temperatures (dashed lines) and the actual temperatures measured (solid lines) show a good correlation. It is recognisable that the heating experiments in the laboratory oven are precisely reproducible.

In contrast, the temperature curves in the fire container at a height of 1.8 m (solid lines) illustrate that the temperature increased very fast (after approx. 3 minutes) to max. 900 °C (Fig. 6d). After a dwell time of about 15 minutes (= the time the wood crib takes to burn through), the temperature in the fire container decreased rapidly.

If one compares the slow and even heating in the laboratory oven to the dynamic heating in the fire container, the differences between both treatments become obvious. According to the temperature curves measured by the thermocouples in the experiment displayed in Fig. 6b, on the stone surfaces (dashed lines), even in one and the same experiment, the temperatures range between 400 and 600 °C (Fig. 6d). The maximum temperature of about 600 °C is reached after approx. 14 minutes. The heating of the air in the container is faster and reaches higher maximum temperatures than the stone surfaces. However, the stone surface is cooling down much slower than the 93surrounding air. The upper right diagram in Fig. 6d shows remarkable lower temperatures within the stone compared to the stone surfaces. The maximum temperature of about 230 °C is reached only after approx. 55 minutes, i. e. long after the rapid decrease of the temperature of the surrounding air. Although the absolute temperatures measured may differ between single experiments, the general patterns of temperature development in the air, on the stone surfaces, and within the stone are similar. That means that the direct fire impact results in very unequal spatial and temporal distribution of temperature in the specimen within a short time of heating. These differences in temperature may lead to material tension caused by different thermic dilatation between the outer and the inner parts of the objects, resulting in cracks (Gómez-Heras et al. 2009). Many authors refer to the transformation of α-quartz to β-quartz at around 573 °C and the related volume increase to explain deterioration and damage of quartz-rich building stones (e. g. Chakrabarti et al. 1996, Hajpál & Török 2004). In the presented example, heavy damages (cracks) occur, although this temperature is hardly reached on the sandstone surface (see Fig. 6d).

Figure 6: a) Small specimens (50 × 25 mm) of Posta and Cotta sandstone parallel (PS_P & CS_P) and normal (PS_N & CS_N) to bedding after heating at different temperature levels in the laboratory oven at the TU BAF, from left to right: 25, 400, 500, 600, 700, 800 and 1,000 °C b) significant cracks and heavy sooting on the Posta type sandstone cylinder after the fire test at the IBK in Heyrothsberge c) temperature curves of the small sandstone specimens (50 × 25 mm) heated in the laboratory oven at the TU BAF for different temperature levels d) temperature curves of the thermocouples in the fire container at a height of 1.8 m (solid lines) and at the stone surfaces (dashed lines). The small diagram shows the temperature curves of the thermocouples inside the Posta type sandstone cylinder.

In the oven-heated smaller specimens, tension due to temperature gradients does not occur due to 94slower, even heating. From this point of view, this kind of experiment does not reflect real, short-term fire scenarios on buildings. However, these testings give insight into effects of heat on mineral grains and intergranular matrix. In case of long-lasting fire events, these effects may additionally affect building stones and their material properties.

This study compares two different heat scenarios which are both necessary to investigate fire damages on sandstone objects or monuments.

The realistic fire scenario with the exposure of architectural sandstone elements to a burning wood crib within a fire container for a short time results in damages comparable to those observed on monuments which suffered from fire attack. The temperatures measured on stone surfaces and within the inner core of the objects indicate high gradients, resulting in material tension and subsequent cracking.

In contrast, smaller specimens of the same sandstone materials reveal no cracking even at higher temperatures when gradually heated in a laboratory oven. However, such tests and the respective test specimens which will be further investigated, may also be useful for enlightening the change of petrographic and material properties during heating.

The authors thank the Free State of Saxony (Sächsische Aufbaubank – Förderbank – SAB) for funding the work within the project WI631.

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