Matthias Zeiml

Mechanical and transport properties of concrete at high temperatures

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.

A Coupled Thermo-Hygro-Chemo-Mechanical Model for the Simulation of Spalling of Concrete Subjected to Fire Loading

During spalling of fire-loaded concrete, the cross-sectional area of the concrete member is reduced, seriously affecting the integrity of the structure. Spalling is mainly attributed to two types of processes: thermo-hygral and thermo-mechanical. Thermo-hygral processes refer to the build-up of vapor pressure inside the concrete pores. Thermo-mechanical processes refer to the thermally-induced, restrained deformation of concrete. Both types of processes are closely connected to the physical and chemical behavior of concrete as a porous material. This contribution aims at 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. This requires modeling of the two above-mentioned types of processes via a coupled thermo-hygro-chemo-mechanical model. Based on a coupled thermo-hygro-chemical model, the authors adopted a formulation of the effective-stress theory by combining the respective model with a multiscale homogenization approach (with the latter considering dehydration as the reverse of hydration), which gave access to the Biots coefficent as a function of temperature. This led to a coupled thermo-hygro-chemo-mechanical code simulating the stress state in fire-loaded concrete as a consequence of both thermo-hygral and thermo-mechanical processes. In this coupled code, an embedded strong-discontinuity model is to be implemented, which is capable of capturing and tracking the propagation of a crack evolving in concrete as a quasi-brittle material. The aim is to attain the crack path as well as the width of the crack with the latter being closely connected to permeation of gas and water through the crack. With the resulting coupled model, it will be possible to take into account all major couplings, allowing to realistically simulate the spalling process.

Underground Structures under Fire – From Material Modeling of Concrete Under Combined Thermal and Mechanical Loading to Structural Safety Assessment

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.

Concrete Structures Subjected to Fire Loading: From Thermo-Mechanical Modeling of Strain Behavior of Concrete Towards Structural Safety Assessment

In this chapter, results obtained within a 4-year research project on the safety of underground structures subjected to fire loading are presented. For this project, a consortium consisting of three scientific partners (Vienna University of Technology, University of Innsbruck, University of Natural Resources and Life Sciences, Vienna) and eight industrial partners (OBB-Infrastruktur AG, ASFINAG, Wiener Linien, Arge Bautech, VOZFI, Buro Dr. Lindlbauer, Schimetta Consult, ZT Reissmann) was established. Whereas the mentioned research project followed a holistic approach, covering simulation of the fire event, experimental investigation of concrete and concrete structures at high temperatures, and modeling and simulation work at both the material and the structural scale (Amouzandeh, Development and application of a computational fluid dynamics code to predict the thermal impact of underground structures in case of fire, Ph.D. thesis, Vienna University of Technology, Vienna, 2012; Ring et al. Brandversuche zum Abplatz- und Strukturverhalten von Tunnel mit Rechtecksquerschnitt [Fire experiments investigating the spalling and structural behavior of rectangular tunnels], Technical Report, Vienna University of Technology and Vereinigung der osterreichischen Zementindustrie (VOZFI), Vienna, 2012; Ring, Experimental characterization and modeling of concrete at high temperatures: Structural safety assessment of different tunnel cross-sections subjected to fire loading, Ph.D. thesis, Vienna University of Technology, Vienna, 2012; Zhang, Simulations for durability assessment of concrete structures: multifield framework and strong discontinuity embedded approach, Ph.D. thesis, Vienna University of Technology, Vienna, 2013), this chapter focuses on one aspect of the project, namely modeling and simulation of the behavior of concrete and concrete structures under combined thermal and mechanical loading: 1. First, a micromechanical model taking the composite nature of concrete into account is presented. Based on experimental results obtained for cement paste and aggregate subjected to thermal/mechanical loading, a two-scale model formulated within the framework of continuum micromechanics is developed, giving access to the effective elastic and thermal-dilation properties of concrete as a function of temperature. 2. In a second step, these model-based properties are considered within a differential formulation of the underlying stress–strain law, accounting for the influence of mechanical loading on the thermal-strain evolution. The proposed micromechanical approach and its implementation are validated by experimental results obtained from concrete specimens subjected to combined thermo-mechanical loading. 3. Finally, the effect of the underlying model assumptions at the structural scale is illustrated by means of the safety assessment of underground support structures under fire attack.

Assessing the Structural Safety of Underground Frame Structures Subjected to Fire Loading

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.