Mathematical Model And Numerical Method For Simulation Of Coupled Chemo-thermo-hygro-mechanical Process In Concrete At High Temperature And Failure Analysis

In recent years, high performance concrete (HPC, or high strength concrete-HSC) has been widely used in engineering practices such as bridges, runnels, tall buildings, offshore oil platforms and nuclear engineering applications due to its intrinsic properties of high strength, high durability etc. Hence, the prediction of HPC structures performance is of great practical importance, in particular for safety evaluation of HPC structures subjected to fire or other thermal hazard situations. In such conditions, increases in concrete strength and durability have been accompanied by an increasing tendency towards explosive thermal spalling of HPC structures, which leads to the total structure collapse and causes more casualties and damage to the surrounding environment. Especially after the 911 event, the research on the mechanism of the failure phenomena which occur in the concrete at fire, has become a new focus.The emphases of this thesis are focused on the three aspects of the subject. Firstly, a mathematical model to quantitatively describe the Coupled chemo-thermo-hygro-mechanical behavior in concretes at high temperature is proposed. Secondly, a finite element method for the numerical solution of the initial and boundary value problem of the mathematical model is formulated. Finally, a coupled elastoplastic-damage constitutive model taking into account chemo-induced material elastoplastic-damage effects for the simulation of coupled chemo-thermo-hygro-mechanical behavior in concretes at high temperature is developed.Modelled as the solid-liquid or solid-liquid-gas mixture, saturated or unsaturated concretes possess non-homogeneous structures in space. A fundamental assumption is to model the real non-homogeneous multiphase system as a porous-continuum in the macroscopic level, in which eachphase is assumed to fill up the entire domain, i.e. in the domain all phases are supposed present at every point at the same time (overlapping continuum). Meanwhile, at this level we usually deal with homogeneous media. Thanks to the assumption, the theoretical frame of the continuum can be applied to porous media as two or three phase mixtures.A hierarchical chemo-thermo-hygro-mechanical (CTHM) model for concrete at high temperature is proposed in the present work on the basis of the existing coupled thermo-hydro-mechanical (THM) model. The hierarchical macroscopic model with two immiscible-miscible levels is proposed for concrete as a hygroscopic and porous medium with fine pores filled with two immiscible pore fluids, i.e. the gas mixture and the liquid mixture. Concrete is first modeled as a partially saturated deforming porous continuum of mixtures, i.e. a medium composed of three immiscible phases: the porous solid filled with both the. liquid mixture (dissolved salts) and the gas mixture (dry air and water vapor). The secondary modeling aims to the description of each of three immiscible phases, i.e. the solid, the liquid and the gas mixtures. The gas mixture is composed of dry air and vapor, the liquid mixture contains pore water and the matrix components dissolved from the solid phase to it, the solid skeleton consists of concrete paste and aggregate, both of which are porous, with bound water. The main character of the secondary modeling is to assume that secondary components within each of the three phases are homogeneously miscible between each other.The following three phase change processes caused by high temperature condition are taken into account in the mass assembly of concrete as a mixture medium. They are: (1) the dehydration-chemically bound water in the solid phase is released to be the free water as part of the liquid phase; (2) the evaporation-the free water changes to become water vapor as part of the gas mixture; (3) the desalination-matrix components of the solid phase of concrete are dissolved into the liquid phasesThe mathematical model consists of a set of coupled partial differential equations governing the mass balance of the dry air, the mass balance of the water species, the mass balance of the matrix components dissolved in the liquid phases, the enthalpy (energy) balance and momentum balance of the whole medium .mixture. A mixed weak form for the finite element solution, procedure is formulated for the numerical simulation of chemo-thermo-hygro-mechanical behaviors. Special considerations are given to spatial discretization of hyperbolic equation with non self-adjoint operator nature.To account for the complexity of coupled chemo-thermo-hydro-mechanical behavior in concrete subjected to fire and the fact that occurrence and evolution of the micro-crack or micro-void growth are accompanied with plastic flow process observed in concrete material, a coupled elastoplastic-damage constitutive model with consideration of chemo-induced material elastoplastic-damage effects is proposed in the present work to model the realistic failure phenomena, i.e. loss of both the strength and stiffness, characterized by thermal spalling. The effects of both dehydration and desalination in concrete members exposed to high temperature on the material strength and stiffness are considered in the proposed coupled model. The model is developed on the basis of the damage model by Mazars and the Willam-Warnke elastoplastic yield criterion for concrete at room temperature. The chemical softening and chemical damage, in addition to plastic strain hardening/softening, suction hardening and mechanical damage, are take into account in the model.Based on the previous work for constitutive modeling of coupled elastoplastic- damage and previously defined remm mapping schemes for this type of coupled problems, a three-step operator split algorithm for the proposed coupled chemo-elastoplastic-damage model is developed. Consistent tangent modulus matrices with consideration of the fully coupled effects are derived to preserve the quadratic rate of convergence of the global Newton iterative procedure.The capability and performance of the mathematical model, the coupled chemo-elastoplastic-damage constitutive model and their numerical methods presented in this thesis are demonstrated by the numerical results for a number of example problems, particularly in reproducing coupled chemo-thermo-hygro-mechanical behavior in concretes subjected to fire and thermal radiation and simulating the complex failure processes in concretes at high temperature.