Keywords: concrete, fire, experiment, thermal strain, micromechanical modelling.

Starting with an experimental program comprising concrete and cement-paste samples, the main mechanisms in heated concrete are identified. The tested specimens are subjected to constant uniaxial (mechanical) loading and heated to 800° with a heating rate of 1°/min. During the experiment, both the radial and axial deformation of the specimens were monitored, yielding the axial and radial strains of heated concrete. Moreover, the elastic properties of the heated concrete and cement paste under various load levels were determined, applying an oscillating mechanical loading.

In order to consider the complex chemical and physical processes in heated concrete, a micromechanical model taking the composite nature of concrete (comprising aggregates, cement paste and pores) into account, is proposed, using the Mori-Tanaka scheme for homogenization. Based on the aforementioned experimental data, the macroscopic behaviour of concrete is determined. Comparing the model response with the collected experimental data shows good agreement of the proposed material model.

Finally, the combined thermo-mechanical behaviour of thermal strains (load induced thermal strains (LITS)) is modelled using an approach based on the one proposed by Thelandersson [1] [2]. Additionally, this model together with the micromechanically determined material properties is implemented into a finite-element program [3]. With the developed tool at hand, validation simulations are performed, comparing the numerical results with experimental data from large-scale fire tests on concrete frames [4,5]. Consideration of LITS within the material model is found to reduce thermal restraint and give improved agreement with experimental results, providing the basis for realistic structural safety assessment and, therefore, for a more economic design of concrete structures subjected to fire loading.

1

S. Thelandersson, "Modeling of combined thermal and mechanical action in concrete", Journal of Engineering Mechanics (ASCE), 113(6), 893-906, 1987. doi:10.1061/(ASCE)0733-9399(1987)113:6(893)

2

T. Ring, M. Zeiml, R. Lackner, "Thermo-mechanical behavior of concrete at high temperature: From micromechanical modeling towards tunnel safety assessment", 2012. (In preperation)

3

K. Savov, R. Lackner, H.A. Mang, "Stability assessment of shallow tunnels subjected to fire load", Fire Safety Journal, 40, 745-763, 2005. doi:10.1016/j.firesaf.2005.07.004

4

T. Ring, M. Zeiml, R. Lackner, "Underground concrete frame structures subjected to fire loading: Part I - Large scale fire tests", Engineering Structures, 2012. (Submitted for publication)

5

T. Ring, M. Zeiml, R. Lackner, "Underground concrete frame structures subjected to fire loading: Part II - re-analysis of large-scale fire tests", Engineering Structures, 2012. (Submitted for publication)

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