Thomas Ring

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.

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.