FINITE ELEMENT MODELLING OF CONCRETE AT HIGH TEMPERATURE

摘要

The present work deals with a new model to analyse the behaviour of concrete at high temperature. For the safety evaluation of concrete structures exposed to elevated temperatures, for instance during a fire or in the case of nuclear accident, the prediction of concrete performance is of great practical importance. In such conditions concrete may be subject to spalling phenomena which jeopardize the integrity of the construction. Spalling involves thermal, hygral and mechanical processes. It may be explosive because of high values of pore pressure reached during the heating of the material. This is true especially for high performance and ultra high performance concrete because of their greater compactness and lower porosity. Such complex phenomena require a numerical simulation based on new mathematical models. In the model here presented concrete is considered as a multiphase material consisting of a solid phase, two gas phases and three water phases. The effect of damage on permeability and non-linearities due to temperature and pressures are included and phase changes such as hydration-dehydration, evaporation-condensation, adsorption-desorption, are considered. A 1-D case concerning a wall and a 2-D case regarding a column, both subjected to ISO-fire conditions, will be shown. In the first one the effects of long term heating on an ordinary concrete will be presented, in particular as far as the hygro-thermal behaviour is concerned. The second one represents a simulation of a real scale structure under fire with consequent damaging of the column and loss of its bearing capacity.