The “Chunnel” Fire. I: Chemoplastic Softening in Rapidly Heated Concrete

Abstract

This paper and its companion paper present the main results of an assessment of the fire in the Channel Tunel (the “Chunnel”), which destroyed a part of the concrete tunnel rings by thermal spalling. The study seeks (1) to evaluate the effect of thermal damage (loss of elastic stiffness) and thermal decohesion (loss of material strength) upon the stress state and cracking at a structural level; and (2) to check whether restrained thermal dilatation can explain the thermal spalling observed during the fire. In the present paper, a macroscopic material model for rapidly heated concrete is developed. It accounts explicitly for the dehydration of concrete and its cross-effects with deformation (chemomechanical couplings) and temperature (chemothermal couplings). The thermal decohesion is considered as chemoplastic softening within the theoretical framework of chemoplasticity. Furthermore, kinetics of dehydration, dimensional analysis, and thermodynamic equilibrium considerations show that a unique thermal dehydration function exists that relates the hydration degree to the temperature rise, provided that the characteristic time of dehydration is much inferior to the characteristic time of structural heat conduction. The experimental determination of the thermal dehydration function from in-situ measurements of the elastic modulus versus furnace temperature rise is shown from experimental data available from the chunnel concrete. Finally, by way of an example, the proposed constitutive model for rapidly heated concrete is combined with the three-parameter William-Warnke criterion extended to isotropic chemoplastic softening.