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Eingeladener Vortrag
- nein (6)
Bauteile bzw. Tragwerke aus Hochleistungsbetonen müssen in der Regel gegen brandinduzierte Abplatzungen mit geeigneten Maßnahmen geschützt werden, um einen ausreichenden Feuerwiderstand im Brandfall zu gewährleisten. Die bisher wirtschaftlich und technologisch sinnvollste Methode zur Verhinderung von explosionsartigen Betonabplatzungen im Brandfall ist die Zugabe von Polypropylen-Fasern. Die Wirksamkeit der Fasern konnte zwar empirisch gezeigt werden, es stellt sich allerdings die Frage, welche Mechanismen zur Verhinderung der Abplatzungen führen. Der vorliegende Beitrag fasst bisherige Theorien zur Wirkungsweise von Polypropylen-Fasern in brandbeanspruchten Hochleistungsbetonen zusammen und stellt eine innovative Methodologie zur Erforschung der mikrostrukturellen Prozesse vor. Die Ergebnisse zeigen, dass die nach dem Schmelzen und Zersetzen der Polypropylen-Fasern frei werdenden Mikrokanäle durch eine gleichzeitig einsetzende Mikrorissbildung netzartig verbunden werden. Die Mikrorissbildung ermöglicht somit den Abbau von Eigen- und Zwangsspannungen im Beton (mechanischer Effekt) und die Entstehung eines Transportwegesystems für den ausströmenden Wasserdampf (Permeationseffekt).--------------------------------------------------------------------------- Structural members and bearing structures of high performance concrete generally have to be protected against explosive spalling due to fire exposure to guarantee a sufficient fire resistance. Up to now, the economically and technologically most worthwhile method to prevent explosive spalling is the addition of polypropylene fibres. Though the effectiveness of the fibres could be shown empirically, the mechanisms preventing explosive spalling are still debatable. The present article summarizes the existing theories concerning the mode of action of polypropylene fibres in fire exposed high performance concretes and presents an innovative methodology for analysing the micro structural processes. The results show that due to the thermal decomposition of the polypropylene fibres micro channels are created and simultaneously connected due to a netlike micro crack formation. This enables the relief of internal stresses (mechanical effect) and the formation of a permeable transport system for the escaping water vapour (permeation effect).
Monitoring of fire damage processes in concrete by pore pressure and acoustic emission measurements
(2011)
It has been shown in fire tests that polypropylene fibres reduce or avoid explosive spalling of high performance concrete. In the critical temperature ränge up to 300 °C the permeability of HPC increases by using polypropylene fibre. Due to this the water vapour, which is the main reason for explosive spalling, can escape. There exist different theories in the literature conceming the micro structural mechanisms, which cause an increase in the permeability. Within the framework of an internal research project at BAM an innovative methodology was developed for experimental verifying of existing theories and to get new insights into this problem The methodology used is unique and has been undertaken here for the first time. This consists of the combination of acoustic emission and ultrasonic measurement during temperature loading and the non-destructive micro structural analysis of cooled down samples with the aid of micro X-ray computed tomography. For the validation of the nondestructive test methods scanning electron microscopic images of prepared samples were undertaken. The results show that due to the thermal decomposition of the polypropylene fibres micro canals emerge. These are connected due to a simultaneous micro cack formation.
Monitoring of fire damage processes in concrete by pore pressure and acoustic emission measurements
(2011)
This paper presents the combined use of acoustic emission (AE), gas pore pressure and temperature measurements (PT). The simultaneous application of both techniques represents a new methodology in the context of fire spalling and contributes to a better understanding of the mechanisms of fire spalling, particular with regard to the interaction of micro cracking and pore pressure evolution. The study presents fire tests on normal strength concrete specimens with and without reinforcement at ISO Standard fire and at hydrocarbon (HC) fire. Supported by AE-analysis, it can be shown that due to the higher heating rate of the HC-fire in comparison to the ISO Standard fire the damage processes inside the concrete during the exposure are increased combined with augmenting the concrete permeability. As a consequence lower pore pressures were measured. However, despite the lower pore pressures, explosive spalling was observed.