ABSTRACTThe material degradation of concrete subjected to fire events has a severe influence on the load‐carrying capacity of support structures. Spalling of concrete layers, exposing the reinforcement bars and degradation of the material properties (Young's modulus, compressive strength) may lead to significant damage of the reduced cross‐section and, therefore, cause failure of the structure. In order to understand the stress build‐up at the heated surface caused by thermal expansion due to fire loading, finally leading to damage and spalling of concrete, the strain behaviour of cement paste and concrete exposed to combined thermo‐mechanical loading is the focus of this work. Hereby, the evolution of thermal strains, Young's modulus and Poisson's ratio with increasing temperature are investigated experimentally. For this purpose, the specimens are loaded uniaxially while the temperature is increased up to 800 °C. The obtained results provide the proper basis for the development of realistic material models, allowing more sophisticated simulations of structures exposed to fire.

Tunnelbraende sowie Brandversuche an Tragstrukturen haben aufgezeigt, dass Betontragwerke extremen physikalischen Prozessen (zum Beispiel Abplatzungen, Abnahme des Elastizitaetsmoduls beziehungsweise Druckfestigkeit, etc.) ausgesetzt sind, welche das Gesamttragverhalten wesentlich beeinflussen. In dieser Arbeit wird das Strukturverhalten von verschiedenen Tunnelgeometrien (Rechteck- und Gewoelbequerschnitt der offenen Bauweise) mit Hilfe eines FE-basierten Simulationstools untersucht. Bei diesen Studien werden verschiedene Berechnungs- und Materialmodelle miteinander verglichen, begonnen mit einer linear-elastischen Simulation in Kombination mit einer linearen Temperaturverteilung ueber die Schalendicke (so genannte aequivalente Temperatur). In einem Zwischenschritt wird elasto-plastisches Materialverhalten gemeinsam mit der linearen Temperaturverteilung (aequivalente Temperatur) angesetzt. Abschliessend wird nichtlineares (elasto-plastisches) Materialverhalten und die reale, nichtlineare Temperaturverteilung beruecksichtigt. Neben dem Einfluss der Berechnungsmethode wird der Einfluss der temperaturabhaengigen Materialparameter (E, fc) auf die Entwicklung des thermischen Zwangs untersucht. Hierbei werden die Designkurven fuer E und fc gemaess Eurocode 2 beziehungsweise CEB angesetzt. Der Vergleich der Ergebnisse zeigt einen grossen Einfluss der angesetzten, normgemaessen Materialparameter auf die Simulation. Des Weiteren wurden grosse Unterschiede zwischen den Rechenmodellen hinsichtlich der Schnittgroessen und Verformungen beobachtet. Im Falle eines stark vereinfachten Rechenmodells (das heisst linear-elastisches Materialverhalten und lineare Temperaturverteilung) wurden die Schnittgroessen zufolge Zwang stark ueberschaetzt und die Verformungen unterschaetzt. Fuer die Prognose der Verformungen der Betonstruktur ist es unerlaesslich, ein realistisches Modell (das heisst elasto-plastisches Materialverhalten und nichtlineare Temperaturverteilung) zu waehlen. Die realistischere Bestimmung der Schnittgroessen fuehrt in der Praxis zu einer wirtschaftlich optimierten Bemessung von Tragstrukturen im Brandfall. Der Vergleich zwischen den einzelnen Tunnelgeometrien zeigt die Unterschiede zwischen den einzelnen Querschnitten (Rechteck- beziehungsweise Gewoelbequerschnitt) unter Brandbeanspruchung auf. (A) ABSTRACT IN ENGLISH: Recent tunnel fires as well as fire experiments have revealed various physical processes (such as spalling, material degradation) which significantly reduce the load-bearing capacity of reinforced concrete structures under fire. Within this contribution, the structural behavior of different tunnel geometries (rectangular and arched cross-section) is analyzed using a numerical FE-based tool. Different numerical and material models are compared, beginning with a linear-elastic material model together with a linear temperature distribution across the lining thickness (so-called equivalent temperature). In an intermediate step, elasto-plastic material behavior together with the linear temperature distribution (equivalent temperature) is applied. Finally, these simplified models are compared to a more sophisticated model, assuming elasto-plastic material behavior together with the real nonlinear temperature distribution. In addition to the influence of the numerical model, the influence of degradation of material parameters (E, fc) using different standards (Eurocode 2, CEB) on the magnitude of the thermal restraint is investigated, showing a large influence of the used design curves for E and fc. Moreover, a large influence of the underlying model on internal forces as well as deformations was observed. Considering a simplified numerical model (linear-elastic material behavior and linear temperature distribution) leads to an overestimation of the thermal restraint and an underestimation of the deformations. In order to obtain realistic deformations of concrete structures subjected to fire loading, the use of a realistic model (elasto-plastic material behavior and nonlinear temperature distribution) is required. The more realistic determination of internal stress resultants leads to a more economical design of load-carrying structures subjected to fire loading. The comparison of different tunnel geometries highlights the differences between the investigated cross-sections (rectangular, arched) under fire loading. (A)

A nonlinear analysis tool to assess the structural safety of underground frame structures under fire loading is presented. The material and numerical model is validated by comparison of the numerical results with experimental data from large-scale fire experiments. Benchmark examples (real tunnel cross-sections) are analyzed, illustrating the advantages of the nonlinear over a linear-elastic analysis regarding an economic reinforcement design as well as the realistic prediction of the deformation behavior. Future work focuses on introduction of the realistic, nonlinear analysis tool in engineering practice as well as in design guidelines.

Constituents of holistic analysis of tunnels subjected to fire are addressed. This includes simulation of the combustion process by means of computational fluid dynamics, high-temperature permeability tests on cement-based materials, and structural analyses of two different tunnels exposed to high temperature loading. These analyses show that the resistance of circular tunnels to fire loading is greater than the one of tunnels with rectangular cross-sections.

The realistic description of the behavior of concrete under high temperature and mechanical loading is of great importance, especially in terms of the safety assessment of concrete structures subjected to extreme events like tunnel fires. In order to captur e the complex chemical and physical processes in heated concrete, a micromechanical model, taking the composite nature of concrete into account, is presented in this paper. Based on experimental results obtained from specimens subjected to combined thermo-mechanical loading, a two-scale model formulated within the framework of continuum micromechanic is developed, giving access to the elastic properties as a function of temperature and load level.

When concrete structures are subjected to fire loading, temperature-dependent degradation of the material properties as well as spalling of near-surface concrete layers has a considerable effect on the load-carrying capacity and, hence, the safety of these structures. Spalling is caused by interacting thermo-hydro-chemo-mechanical processes with both mechanical and transport properties playing an important role. Within experimental research activities at the IMWS, these properties are subject of investigation, i.e., (i) the strain behavior of concrete under combined thermal and mechanical loading and (ii) the permeability increase of temperature-loaded concrete and cement paste.

Computational Civil Engineering Mechanics - Elucidating General Features of Materials and Structures

Fire accidents in tunnels considerably influence their structural safety. During fire exposure, the concrete lining is damaged due to various processes (i.e., thermal degradation, spalling etc.), eventually leading to failure of the tunnel structure. Simulation of the structural behavior of tunnels under fire loading can only capture the real structural behavior when all governing processes are taken into account with sufficient accuracy. This work concentrates on the influence of various parameters on the quality of prediction of the structural behavior of tunnels under fire loading (temperature distribution, material model, spalling).

Tunnel cross-sections are analyzed applying different material models (linear-elastic and linear-elastic/ideal-plastic) and modes to consider fire loading (equivalent temperature loading and nonlinear temperature distribution). The influence of spalling and the effect of combined thermal and mechanical loading (by consideration of Load Induced Thermal Strains — LITS) on the numerical results is investigated.

Computational Civil Engineering Mechanics - Elucidating General Features of Materials and Structures