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 standard as fire source.
92
Figure 5: a/b) Views inside the fire container b) positions of video and infrared camera c) record of video monitoring d) record of infrared monitoring.
Results and Discussion
The macroscopic results for the stone samples after heating are shown in Fig. 6a/b. There are significant differences between heating in the laboratory oven and in the fire container. In Fig. 6a the small Posta and Cotta type sandstone specimens with axis parallel (PS_P & CS_P) and normal (PS_N & CS_N) to bedding are displayed. They are arranged according to their temperature treatment levels (from left to right: 25, 400, 500, 600, 700, 800 and 1,000 °C). The specimens appear more reddish with higher temperatures. These color changes are related to mineral transformations, namely of iron-bearing minerals (cf. Hajpál & Török 2004).
Within the brownish to yellowish Elbe sandstones, mainly limonite changes to the red hematite at elevated temperatures (Fig. 6a). Slight color changes to red can be detected for all sandstone types already at 400 °C (cf. Gómez-Heras et al. 2009). In Cotta sandstone also glauconite transformations might contribute to discoloration.
The cylinders treated in the oven did not reveal any macroscopic cracks. The treated and untreated specimens were stored in plastic bags after cooling down. After moving these sample bags for further investigations, loose single sand grains, increasing in number with temperature, were detected in the bags for those samples heated above 500 °C. They indicate decreased cohesion of sand grains in the respective sandstones. In case of Cotta type sandstone this effect was less developed than for Posta type sandstone.
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.
