In the last years, the tunnel fires have shown that the tunnel support structure is severely damaged at an extremely high fire impact. “In some tunnels, up to two thirds of the tunnel inner shell are shattered by explosion. The leftover concrete suffers a severe thermal damage. This combination can lead to structure collapse in the case of one-shell tunnels that are close to upper areas,” clarifies Matthias Zeiml of the Institute for Mechanics of Materials and Structures (IMWS) of the Vienna Universisty of Technology.
He and his colleague from Munich, Roman Lackner (lecturer at IMWS), analyzed, as part of a three-year FWF-project, the “transportation processes in concrete at high temperatures.” “The blowup of the concrete sticks is a consequence of the thermal wedging and of the steam pressure, which develops in the heated concrete and which cannot escape.
These flattenings sometimes reach far behind the reinforced steel,” explains Zeiml. At the same time, University of Technology Professor Ulrich Schneider of the Institute for Building Construction and Technology and the Research Institute of the Austrian Cement Industry (VÖZFI) analyzed the effect of minuscule polypropylene fibers (carpet fibers) which are blended into the concrete. When the concrete is warmed up, adding a few millimeter-long fibers produces channels through which the water steam can escape. This way, flatennings can be effectively prevented.
The results of this fundamental research are now useful to researchers for the KIRAS-Project (Austrian Support Program for Safety Research) of the BMVIT, which received a grant in June. This research project benefits from the participation of a consortium that consists of University Institutes of the Vienna University of Technology and the Vienna University of Natural Resources and Applied Life Sciences, infrastructure construction developers (ÖBB, ASFiNAG, Wiener Linien) as well as engineering companies and research laboratories. At the forefront of research there is the development of a new evaluation pattern which for the first time facilitates the prognosis of the vital processes which are influenced by the structure support behavior. “Our project partners - ÖBB, ASFiNAG, and Wiener Linien – are interested in a close to reality prognosis of the tunnel safety level under fire impact. Moreover, we have to answer questions regarding issues such as the need for a temporary support and the extent of the necessary restructuring measures for different fire scenarios,” adds Lackner.
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