Numerical Simulation On Chemo-mechanical Damage Evolution Of Concrete Exposed To Sulfate Environment

Exposed permanently to sulfate environments,such as marine,saline lake and groundwater,concrete structures not only bear the dynamic and static load actions,but also suffer from the external sulfate attack（ESA）from environment medium.Sulfate ions in environment diffuse into concrete and have chemical reactions with the cement-hydrated products.Its induced chemical damage will result in the decrease of mechanical performance of concrete.However,the stress effect induced by load action will cause the microcrack propagation and lead to the mechanical damage,which can accelerate the progress of sulfate attack on concrete.Thus,the combined action of ESA-induced chemical damage and load-induced mechanical damage is a primary reason for the decrease of mechanical property of concrete material and service life of structures.In this paper,the evolution of chemo-mechanical damage of concrete in sulfate environment has been studied by using the experimental investigation,theoretical modeling and numerical simulation.This work is helpful for predicting the service life of concrete structures in sulfate environment.The main contents of this paper are as follows:（1）A corrosion experiment of cement-paste/mortar specimen immersed into Na₂SO₄ solution was performed to study the ESA-induced failure of concrete.The time-varying macro-and microscopic performances of the specimen were obtained.The failure of concrte is revealed to be the result of macro-micro interaction of diffusion-chemical-mechanical behavior.It involves a cycle of the ionic diffusion,chemical reaction,microstructural damage of cement matrix,macroscopic cracking of concrete and its induced accelaration of ion diffusion.Additionally,the results show that,the whole process of concrete failure under ESA can be divided into two stages,namely the dormant period and detrimental duration.The two stages can be explained by the combination of crystallization theory and volume increasing theory.（2）For the diffusion and reaction of sulfate ion in concrete,an unsteady diffusion-reaction model of sulfate ion and a calculation method of sulfate products were respectively established by using the Fick’s law and kinetics of chemical reaction.In the model,the boundary movement caused by concrete cracking is considered.Based on the finite difference method,the numerical solutions of one-dimensional and two-dimensional diffusion-reaction models are obtained.Meanwhile,an experiment of cement-mortar specimen immersed into Na₂SO₄ solution is performed,and the concentrations of sulfate ion and sulfate products in the specimen are measured.The diffusion-reaction model was validated by comparing the experimental results with those of the model.（3）Considering the influence of chemical damage of concrete caused by sulfate diffusion and reaction on its mechanical property,an elasto-plastic damage constitutive model of concrete coupled with chemical damage is established.Based on the backward Euler implicit algorithm,the plastic part of constitutive model is numerically solved.Meanwhile,a simplified testing device was designed to measure the stress-strain curves of cement mortars immersed in exposed to Na₂SO₄ solution.The constitutive model was validated by comparing the experimental results with those of the model.Its establishment lays a foundation for analyzing the chemo-mechanical response of concrete under sulfate attack.（4）Aiming at the microstural damage of cement matrix caused by sulfate chemical reaction,a representative volume element（RVE）was chosen as microscopic research object.Busing the solid increasing theory and crystallization theory,two microscale models were respectively established to analyze the chemo-mechanical response of RVE.Meanwhile,on the basis of the microscale model of unconstrained RVE,the macroscopic eginstrains of concrete in spherical,cylindrical and rectangular coordinates were obtained by using the coordinate transformation and homogenization method.The acquisition of macroscopic ceginstrain is the premise of analyzing the macroscopic chemo-mechanical response of concrete under ESA.（5）Considering that the failure of concrete under sulfate attack is the result of macro-micro interaction,a criterion of concrete boundary movement is proposed,which relates to the microscopic damage of RVE and macroscopic deterioration of concrete.On the basis of microscale model for the chemo-mechanical response of RVE,a macro-microscale model is established to analyze the chemo-mechanical response of concrete in the dormant period and detrimental duration of ESA.By the numerical analysis of macro-and microscopic chemo-mechanical response,the evolution of chemical damage of concrete caused by ESA was quantitatively characterized.（6）In order to study the chemo-mechanical response of concrete under the combined action of sulfate attack and external load,a finite element governing equation of this problem is established by using the Galerkin method and variational principle.Base on the Meshless method and Newton-Raphson iterative method,the finite element governing equation is numerically solved.On the basis of a series of diffusion-chemical-mechanical models and their numerical solution methods,a computer analysis program is composed by using MATLAB.It was used to evaluate the performance degradation and service lifes of concrete box structure and sulfate-resistant concrete pile in sulfate environment.