Monument Future - Siegfried Siegesmund

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VERLAG 2020.

      3 Technische Universität Dresden, Institute of Geotechnical Engineering, 01062 Dresden, Germany, [email protected]

       Abstract

      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.

       Introduction

      Due to firestorms caused by heavy bombardments during the Second World War, lots of buildings and objects made of sandstone were massively damaged. These damages became probably more severe by extinguishing fire by water, leading to another short-term temperature change. The typical damage observed is fragmentation, thus reducing the load-bearing capacity of architectural elements, as shown by the examples of the Church of Our Lady (Frauenkirche) and the altar in the Church of the 90Three Kings (Dreikönigskirche) in Dresden, Germany (Fig. 1).

      Figure 1: Fire damages of historical buildings in Dresden a) Church of Our Lady (Frauenkirche) 1971 (Wikimedia Commons, CC BY-SA 3.0, Lencse Zoltán) b/c) altar in the Church of the Three Kings (Dreikönigskirche).

      Numerous studies (e. g. Chakrabarti et al. 1996, Hajpál & Török 2004, Gómez-Heras et al. 2006, Hager 2014, or Lintao et al. 2017) deal with methods to record material changes of different sandstones caused by high temperatures. However, most of these studies investigate the thermal behavior of small samples with laboratory heating regimes in high temperature ovens. In contrast, there are only few studies dealing with small scale real fire scenarios of sandstones, e. g. Koser & Althaus (1999), Ehling & Köhler (2000), Pohle & Jäger (2003), McCabe et al. (2007), or Smith & Pells (2008). Obviously, the damage patterns of heat-treated laboratory samples and fire-affected objects and buildings (cf. Fig. 1) are different. This study on Elbe sandstones compares the behavior of oven-heated with flame-treated samples, the latter corresponding to a more realistic fire scenario.

       Materials and testing procedures

      The investigated material comprises sandstone of Cotta and Posta type which are the two main varieties of the Upper Cretaceous Elbe sandstone, occurring south of Dresden (Saxony, Germany). The Cotta type is a grey to yellowish-brownish sandstone with clay-bearing, organic and ferritic flakes parallel to bedding. It is a fine-grained and siliceous quartz arenite (> 90 % quartz). In addition, K-feldspar, kaolinite and few illite, glauconite, and rare organic components occur. The color of the Posta type varies between light grey and yellowish-brownish. It is a fineto medium-grained, occasionally coarse-grained, porous and siliceous quartz arenite (quartz nearly 100 %) without organic matter and with very small amounts of kaolinite (Grunert 2007, Grunert & Szilagyi 2010).

      For the laboratory heating experiments cylindrical specimens of both sandstone types with different dimensions of 50 × 25 mm and 50 × 100 mm (Fig. 2a) were used. They were orientated normal and parallel to bedding. The dimensions of the specimens for the real-scale fire exposure tests were significantly larger with heights of 58 cm and an approx. diameter of 19 cm. To imitate real shapes of architectural elements such as pillars, balusters and cylinders were carved from Cotta and Posta type sandstone blocks (Fig. 2b/c).

      The small sandstone specimens (cf. Fig. 2a) were treated in a laboratory oven (Nabertherm LT24/12) at the Institute of Geotechnics, Technische Universität Bergakademie Freiberg (TU BAF) at 6 different temperature levels (400, 500, 600, 700, 800, 1,000 °C) with a heating rate of 10 K/min and a cooling rate of 1 K/min after a holding time of 6 hours at each target temperature level.

      The cylinders and balusters (Fig. 2b/c) were marked for drilling boreholes to mount thermocouples (Fig. 3a/b) which monitored the temperatures on the stone surfaces and within the stones during fire exposure over time. In the cylinder samples, 5 boreholes with a diameter of 8 mm were drilled 91to a depth of 9.5 cm. In accordance to the specific shape of the balusters, 7 boreholes with a diameter of 8 mm were drilled to depths between 4.5 and 9.5 cm. Flowable mortar was used to fix the thermocouples in the boreholes and to guarantee undisturbed heat transfer.

      Figure 2: Investigated specimens of Posta and Cotta type Elbe sandstone a) small cylinders (50 × 100 mm) b) cylinders (58 × 19 cm) c) balusters (58 cm in length and max. diameter of 19 cm).

      Figure 3: a) Scheme of drill holes on a baluster and a cylinder specimen b) cylinders with mounted thermocouples.

      For the real scale room fire tests a fire container (height: 2.40 m, width: 2.35 m, depth: 4.13 m) was used based on the standard ISO 9705 (room corner test at the Institute of Fire Protection and Disaster Control (IBK) in Heyrothsberge (Fig. 4). In the fire container, the cylinder and baluster sandstone specimens were placed at a height of 1.7 m above the fire source (Fig. 5a), achieving a direct flame treatment. The fire source consisted of a wood crib according with DIN EN 3-7 which provided a known theoretical heat release rate with a maximum temperature of approx. 900 °C for about 15 minutes. N-heptane acted as a fire accelerant which was ignited in a pan below the wood crib.

      The temperatures in the container were monitored by thermocouples over time. An infrared and a video camera (Fig. 5b) recorded the heat distribution and the fire behavior in the container which could be followed in real-time on a monitor in the nearby laboratory (Fig. 5c/d).

      Figure 4: Sketch of the fire container (top view) with sample and thermocouple positions; red: wood crib in accordance to the DIN 3-7

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