Modelling explosive spalling and stress induced thermal strains of HPC exposed to high temperature

Permeability of concrete without and with polypropylene fibres is experimentally measured at temperatures up to 300 °C by employing a new test setup. To investigate explosive spalling of concrete cover numerical modelling is carried out using coupled Thermo-Hygro-Mechanical model oriented towards multi-scale modelling approach. Load induced thermal strains are investigated at meso-scale and it is found that the most part of these strains can be captured by a meso-scale model

Microstructural response of polypropylene fibres at high temperature to protect concrete from spalling

The microstructural analysis of the interfacial transition zone (ITZ) and the interaction of polypropylene fibres (PPF) with mortar in the temperature region 170 to 300 °C have been considered. The results confirm a mixed mechanism: The presence of the PPF alters the ITZ and favours an initial cracking formation. After melting, PPF diffuses in the surrounding cement paste closing micropores while, simultaneously, macro pores are increased as consequence of the free space leaved and the growing of microcracks

Flexural Behaviour of Precast Aerated Concrete Panel (PACP) with Added Fibrous Material: An Overview

The usage of precast aerated concrete panel as an IBS system has become the main alternative to conventional construction system. The usage of this panel system contributes to a sustainable and environmental friendly construction. This paper presents an overview of the precast aerated concrete panel with added fibrous material (PACP). PACP is fabricated from aerated foamed concrete with added Polypropylene fibers (PP). The influence of PP on the mechanical properties of PACP are studied and reviewed from previous research. The structural behaviour of precast concrete panel subjected to flexure load is also reviewed. It is found that PP has significant affects on the concrete mixture’s compressive stregth, tensile strength and flexural strength. It is also found that PP manage to control the crack propagation in the concrete panel

Hydro-thermal model of concrete for the calculation of the pore pressure at high temperatures

En este trabajo se expone un modelo hidrotérmico simplificado para calcular la presión de los poros en hormigones expuestos a temperaturas elevadas, basado en el análisis de la transferencia de masa y energía en medios porosos multifase. El modelo ha sido estructurado de tal forma que permite, realizando una serie de asunciones, minimizar el número de parámetros de entrada sin comprometer los resultados de las predicciones. Se han validado los resultados de la simulación y se ha evidenciado que el modelo de cálculo es capaz de reproducir, con un alto grado de fiabilidad y precisión, la evolución de las temperaturas en el hormigón como resultado de la agresión térmica, tanto en la superficie expuesta como en el interior del elemento. Asimismo, las presiones máximas de los poros estimadas por el modelo ofrecen unos resultados adecuados si se comparan con los valores experimentales.In the present work, a simplified hydro-thermal model for pore pressure calculation in concrete exposed to high temperatures is presented. The model is based on heat and mass transfer analysis in multi-phase porous media. The model has been structured in such a way that it is possible, due to assumptions, to reduce the numbers of inputs without compromising the results of the predictions. The validation process has been performed, and it has shown that the computational model is capable to reproduce with a high reliability and accuracy the mass losses reported in the experimental tests and the evolution of temperatures on concrete due to thermal exposure, both at the exposed surface and inside the element. Likewise, pore pressure estimated by the model agrees with the experimental results.Subvención BIA2012-37890 otorgada por el proyecto «PYRODESIGN – Modelado de los parámetros térmicos y cinéticos para la caracterización de la reacción al fuego de materiales» financiado con fondos FEDER

Beton unter Brandbelastung - von der experimentellen Untersuchung des Abplatzverhaltens und der Transporteigenschaften zur Strukturanalyse von Tunnelschalen unter Feuerlast

Zsfassung in dt. SpracheTunnel fires of the past have demonstrated that the load-bearing capacity of the supporting structure is reduced by temperature-induced degradation of stiffness and strength of concrete and reinforcing steel as well as by spalling of near-surface concrete layers, eventually causing collapse of the tunnel. In this work, the following questions are addressed:1. What are the main processes responsible for spalling and what is their individual influence? High-speed camera images allow determination of the size, shape, and velocity of the spalled-off pieces. With this information at hand, the released energy associated with spalling is computed and compared to the energies associated with pore-pressure and thermal-stress spalling. This comparison provides new insight into the impact of various thermal, mechanical, and hydral processes as well as the main influencing parameters controlling explosive spalling of concrete. In addition, permeability experiments are presented, where specimens made of in-situ as well as laboratory-cast concrete with or without additional polypropylene (PP) fibers after cooling from high temperature were tested. The obtained permeability values are related to the pore structure, accessible via mercury-intrusion-porosimetry (MIP) tests, highlighting the effect of the PP-fibers as well as of additives and the production process on transport properties of concrete under fire attack.2. Which phenomena shall be considered for realistic predictions of temperature distributions within heated concrete? Results from finite-element analyses, taking the coupling between heat and mass transport into account, are compared to results from analyses considering heat transport only regarding their agreement with experimental values. In addition, the obtained gas-pressure distributions within the tunnel lining provide insight into the risk of spalling of concrete for varying amount of PP-fibers, agreeing well with experimental observations.3. How is the structural behavior of concrete tunnel linings affected by fire loading? A structural analysis tool, considering spalling of near-surface concrete layers as well as the effect of temperature and gas pressure on the mechanical properties of the heated lining concrete, is employed to investigate the structural performance of a cross-section of the Lainzer tunnel (Austria) characterized by low overburden (shallow tunnel).Tunnelbrände haben gezeigt, dass die Tragfähigkeit der Struktur durch die temperaturbedingte Verminderung von Steifigkeit und Festigkeit des Betons und des Bewehrungsstahls sowie das Abplatzen oberflächennaher Betonschichten reduziert wird, was im äußersten Fall zum Versagen der Tragstruktur führen kann. In dieser Arbeit werden folgende Fragestellungen behandelt:1. Welche Prozesse sind für das Abplatzen des Betons hauptverantwortlich und welchen Einfluss haben sie? Aufnahmen einer Hochgeschwindigkeitskamera ermöglichen die Bestimmung von Größe, Form und Geschwindigkeit der abgeplatzten Betonfragmente.Darauf aufbauend kann die während des Abplatzens freiwerdende Energie abgeschätzt und mit numerischen Ergebnissen verglichen werden. Dieser Vergleich gibt Aufschluss über den Einfluss verschiedener mechanischer, thermischer und hygrischer Prozesse und lässt auf die Haupteinflussfaktoren für Abplatzungen schließen. Weiters werden Permeabilitätsexperimente vorgestellt, bei welchen sowohl unter Baustellenbedingungen (in-situ) gefertigte Probekörper als auch Laborprobekörper, mit bzw. ohne Zugabe von Polypropylen-Fasern (PP-Fasern), bei Raumtemperatur getestet wurden, nachdem sie unterschiedlichen Temperaturregimen unterzogen wurden. Die Ergebnisse werden mit der Porenstruktur von thermisch geschädigtem Beton (untersucht mittels Quecksilberporosimetrie) in Beziehung gesetzt.Dadurch kann der Einfluss der PP-Fasern sowie von Zusatzmitteln und des Fertigungsprozesses (in-situ vs. Labor) auf die Transporteigenschaften von brandbelastetem Beton bestimmt werden.2. Die Berücksichtigung welcher Phänomene ist für eine realistische Abschätzung der Temperaturverteilung im brandbelasteten Beton erforderlich? Ergebnisse aus Finite-Elemente-Analysen der wesentlichen Kopplungen zwischen Wärme- und Massetransport werden mit Ergebnissen aus einer rein thermischen Berechnung hinsichtlich ihrer Übereinstimmung mit experimentellen Werten verglichen. Weiters geben die erhaltenen Gasdruckverteilungen im Beton Aufschluss über das Abplatzrisiko von Beton mit unterschiedlichem Fasergehalt, die erhaltenen Trends stimmen mit experimentellen Beobachtungen überein.3. Wie wird das Tragverhalten von Tunnelinnenschalen aus Beton durch die Temperaturbelastung während eines Brandes beeinflusst? Mit Hilfe eines Strukturprogramms, das sowohl Abplatzungen von Betonschichten als auch die Abnahme der Festigkeit des Betons aufgrund von Temperatur- und Gasdruckbelastung berücksichtigt, wird ein Querschnitt des Lainzertunnels mit geringer Überdeckung (oberflächennaher Tunnel) untersucht.17

A coupled thermo-hygro-chemo-mechanical model for the simulation of spalling of concrete subjected to fire loading

The presented research work contributes to the realistic simulation of the stress state within fire-loaded concrete in order to attain insight into the development and occurrence of the critical state right before and during the event of spalling. A coupled thermo-hygro-chemo-mechanical code simulating the stress state as a consequence of both thermo-hygral and thermo-mechanical processes is presented together with an embedded strong-discontinuity model which is capable of capturing and tracking the propagation of a crack evolving in concrete as a quasi-brittle material. Combination of the two mentioned models is currently under way. With the resulting coupled model, it will be possible to take into account all major couplings, allowing to realistically simulate the spalling process

On the effect of pore-space properties and water saturation on explosive spalling of fire-loaded concrete

In this paper, the influence of concrete properties such as permeability and environmental conditions (water content) on the spalling behavior of concrete subjected to fire loading is investigated. For this purpose, a special fire-test setup is presented, allowing an improved reproduction of temperature histories observed, e.g., in tunnel fires. Moreover, the developed test setup enables a continuous monitoring of spalling, giving access to the spalling history and the final level of damage of concrete specimens. The obtained results, considering different water/cement-ratios and saturation degrees, are related to the required initial tensile strength determined by means of a numerical assessment tool. Finally, correlations between the identified parameters governing fire-spalling were established and allowed an evaluation of the spalling risk of concrete mixtures. These correlations revealed that – in contrast to requirements given in national and international standards – the combination of water content and permeability may serve as proper key and design parameter, defining the risk of spalling