Читать книгу Monument Future - Siegfried Siegesmund - Страница 114

Heat-induced short-term decay of dimension stone on buildings and monuments caused by fire is a well-known phenomenon. Most of the scientific studies about thermal behavior and thermal changes of building stones are carried out in laboratory ovens by stepwise heating of stone samples to different stages of temperature. However, real conditions of fire attack on stone elements of buildings might differ considerably from the relatively slow, even heating of small samples in ovens. Therefore, more realistic fire scenarios were designed to test the behavior of sandstone specimens such as cylinders and balusters (height 58 cm and max. diameter 19 cm). The samples comprise the Cotta and Posta type of the Cretaceous Elbe sandstone. They were exposed to a real scale fire test, based on the standard ISO 9705 (room corner test). The specimens were mounted in a fire container at a height of 170 cm above the fire source, a wood crib in accordance to DIN EN 3–7. The standard defines a known theoretical heat release rate, producing a maximum air temperature of approx. 900 °C for about 15 minutes. The temperature in the container as well as on the surface and within the stone specimens was monitored by thermocouples during the tests. The measured surface temperatures vary between 350 and 600 °C, whereas the temperatures at some 4.5–9.5 cm below surface vary only between 200 and 350 °C, depending on the shape of the samples. After the fire tests, different crack patterns were observed. In contrast, smaller specimens heated in a laboratory oven did not reveal any macroscopic cracks, although they were exposed to the same or even markedly higher temperatures (1,000 °C in the sample core). However, both treatments are needed for a better understandig of fire damages on stone buildings since the material behavior of sandstone on grain size scale (fabric and mineralogy) triggers macroscopic crack patterns such as fragmentation and scaling.