An application of a damage constitutive model to concrete at high temperature and prediction of spalling

Abstract

A characteristic feature of concrete under uniaxial compression is the development of cracks parallel to the loading direction. A damage constitutive model proposed by Ortiz [Ortiz, M., 1985. A constitutive theory for the inelastic behaviour of concrete. Mech. Mater. 4, can predict the transverse tensile stress responsible for these cracks by considering the interaction between the aggregate and the mortar and the development of damage in the latter. When concrete is exposed to high temperature, as is the case during fire, the failure mode is thermal spalling. In order to improve the prediction of the stresses involved in this failure OrtizÕs model is extended to account for the effects of high temperature. Published experimental results for uniaxial and biaxial compression at high temperatures are used to calibrate the temperature dependence of some of the material properties. The transient creep strain is accounted for by modifying the constrained thermal strain. The stress analysis is coupled with hygro-thermal analysis of heat, mass transfer and pore pressure build-up. The effect of pore pressure on the damage evolution is modeled by applying a body force in the stress analysis module proportional to the pressure gradient. A numerical example of concrete under fire is solved and the computed results are discussed. Spalling is predicted when the damage variable reaches its maximum value of unity. The predicted depth and time of spalling for a range of variation of permeability and initial liquid water content are presented. They are in good agreement with published experimental results.

Concrete spalling: Interaction between tensile behaviour and pore pressure during heating

“…In the present work, both the real material strength decay (for the investigated temperatures) and the effect of the thermal stress in case of slow heating rate proved to be negligible. The effect of pore pressure seems to confirm the approach based on the effective stress assuming p s = p gas for any porosity and neglecting the role of the capillary pressure [10]. One possible interpretation is based on fracture mechanics and on the stability of the inherent material defects.…”

Behaviour of high-performance concrete at high temperatures: some highlights

“…It appears during the non-stationary heating of concrete under sustained mechanical load [5,[33][34][35][36][37]. It was shown that the presence of this phenomenon could relax the compressive stresses in a column subjected to temperature [39,40] and that this strain component can reach a high magnitude. Extensive studies of this phenomenon have been conducted for ordinary concretes by [35][36][37][38].…”

Retained properties of concrete exposed to high temperatures: Size effect

“…Furthermore, the explosive thermal spalling of concrete during fire occurs mostly without advance notice [9,10,18,24]. It should also be noted that the presence of loading during heating influences the strength of concrete since the failure mode is generally thermal spalling when concrete is exposed to high temperature [25]. …”