Keywords: thermal spalling, concrete structures, integrated modelling, tunnel fire.

In this paper a model for the analysis of the behaviour of concrete structures exposed to fire is presented with some relevant applications. In the model concrete is treated as a multiphase system with the voids of the skeleton filled partly with the liquid water and partly with the gas phase. The liquid phase consists of bound water and capillary water. The gas phase is a mixture of dry air and water vapour, and is assumed to be an ideal gas. Different physical mechanisms governing the liquid and gas transport in the pores of partially saturated concrete are clearly distinguished, i.e. capillary water and gas flows, adsorbed water surface diffusion, as well as air and vapour diffusion. All the important phase changes of water, i.e. adsorption-desorption, condensation-evaporation, and chemical reactions, e.g. hydration-dehydration, as well as the related heat and mass sources (or sinks) are considered. Changes of the material properties caused by temperature and pressure changes, concrete damage, as well as the coupling between thermal, hygral, chemical and mechanical phenomena are taken into account.

The model [1,2] can be usefully applied for the prediction of the occurrence of thermal spalling. This phenomenon is a very characteristic mode of concrete damage at high temperature, especially for the materials characterized by a low intrinsic permeability, like for example HPC or UHPC. Recently some simplified mechanisms of concrete damage and some spalling indexes have been proposed in [3] to give a quantitative assessment of the spalling risk in heated concrete structures. Five different spalling indexes have been defined, assuming different simplified models of the spalling mechanism or considering the main key factors influencing development of the phenomenon (e.g. gas overpressure, mechanical damage, elastic energy, traction strength, specific fracture energy, etc).

The model for the analysis of concrete behaviour at high temperature has been coupled with a computational fluid dynamics model allowing for the simulation of a fire development scenario in concrete tunnels. The integrated model starts the analysis from the combustion processes, taking into account the radiative heat fluxes and the convective heat transfer which develop during the fire, and finally the thermo-hygro-chemo-mechanical behaviour of concrete walls. The coupling between the interior of the tunnel, where combustion takes place and the tunnel vault where heat is dissipated is based on a Dirichlet/Neumann non-overlapping domain decomposition which enables us to face each part of the problem which is solved with different programs.

1

D. Gawin, F. Pesavento, B.A. Schrefler, "Modelling of hygro-thermal behaviour of concrete at high temperature with thermo-chemical and mechanical material degradation", Comput. Methods Appl. Mech. Engrg., 192, 1731-1771, 2003. doi:10.1016/S0045-7825(03)00200-7

2

D. Gawin, F. Pesavento, B.A. Schrefler, "Modelling of deformations of high strength concrete at elevated temperatures", Mat. Struct., 37(268), 218-236, 2004. doi:10.1617/14078

3

D. Gawin, F. Pesavento, B.A. Schrefler, "Towards prediction of the thermal spalling risk through a multi-phase porous media model of concrete", Comput. Methods Appl. Mech. Engrg., 195, 5707-5729, 2006. doi:10.1016/j.cma.2005.10.021

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