The chunnel fire. I: Chemoplastic softening in rapidly heated concrete

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.

[1]  T. L. Brownyard,et al.  Studies of the Physical Properties of Hardened Portland Cement Paste , 1946 .

[2]  T. Harmathy,et al.  Effect of Moisture on the Fire Endurance of Building Elements , 1965 .

[3]  E. P. Warnke,et al.  CONSTITUTIVE MODEL FOR THE TRIAXIAL BEHAVIOR OF CONCRETE , 1975 .

[4]  Zdenek P. Bazant,et al.  Pore Pressure and Drying of Concrete at High Temperature , 1978 .

[5]  S. Mindess,et al.  Creep and drying shrinkage of calcium silicate pastes I. Specimen preparation and mechanical properties , 1978 .

[6]  Zdeněk P. Bažant,et al.  Pore Pressure in Heated Concrete Walls: Theoretical Prediction , 1979 .

[7]  Jan Byfors,et al.  Plain concrete at early ages , 1980 .

[8]  Michael Ortiz,et al.  An analysis of a new class of integration algorithms for elastoplastic constitutive relations , 1986 .

[9]  Wai-Fah Chen,et al.  Plasticity for Structural Engineers , 1988 .

[10]  Amar Khennane,et al.  Plasticity model for the biaxial behaviour of concrete at elevated temperatures, part I: failure criterion , 1992 .

[11]  Gamal N. Ahmed,et al.  Modeling the thermal behavior of concrete slabs subjected to the ASTM E119 standard fire condition , 1995 .

[12]  Olivier Coussy,et al.  Modeling of Thermochemomechanical Couplings of Concrete at Early Ages , 1995 .

[13]  F. Cohen Tenoudji,et al.  Mechanical properties of cement pastes and mortars at early ages: Evolution with time and degree of hydration , 1996 .

[14]  Olivier Coussy,et al.  Strength growth as chemo-plastic hardening in early age concrete , 1996 .

[15]  Wei-Ming Lin,et al.  Microstructures of Fire-Damaged Concrete , 1996 .

[16]  K.F. Schoch,et al.  MECHANICS OF POROUS CONTINUA , 1996, IEEE Electrical Insulation Magazine.

[17]  Z. Bažant,et al.  Concrete at High Temperatures: Material Properties and Mathematical Models , 1996 .

[18]  F. Ulm,et al.  Creep and plasticity due to chemo-mechanical couplings , 1996 .

[19]  Long T. Phan,et al.  International Workshop on Fire Performance of High-Strength Concrete, NIST, Gaithersburg, MD, February 13-14, 1997, Proceedings | NIST , 1997 .

[20]  Zdenek P. Bazant,et al.  Analysis of Pore Pressure, Thermal Stress and Fracture in Rapidly Heated Concrete , 1997 .

[21]  Franz-Josef Ulm,et al.  THE "CHUNNEL" FIRE. II: ANALYSIS OF CONCRETE DAMAGE , 1999 .