Damage Analysis And Theoretical Model For Carbonation Depth Prediction Of Concrete Under The Combined Action Of Load And Carbonation

Carbonation can cause neutralisation of the concrete cover,leading to corro-sion of internal reinforcement,which seriously affects service life and causes huge economic losses and resources waste.Existing studies usually use experimental approaches to analyse the changing of carbonation depth qualitatively,while in practical engineering the structure is in a combined environment of load and car-bonation.However,the current research community lacks sufficient attention to this,and there is a clear absence of research and application results in theoretical modelling and meso-scale simulation of carbonation of loaded concrete.Therefore,conducting relevant research to accurately predict the carbonation depth is im-portant to reduce engineering repairs and improve the buildings’service life.This thesis addresses the following issues:insufficient study of non-destruc-tive and quantitative characterising the damage evolution of loaded concrete,the imperfect research on the mechanism of the load effect on the substance diffusion in concrete,and the serious deficiencies in the theoretical model of concrete car-bonation considering load and the meso-scale modelling of loaded concrete car-bonation.The main contents and innovative results are shown as the followings.1.The damage evolution of loaded concrete was revealed based on nu-merical simulation and XCT test.Based on the concrete loading tests and trial calculations with the recommendedσ-εcurves,the parameters of uniaxial loading simulation and each concrete phase’s CDPM were obtained to accurately describe the damage state,load-displacement curve,and the crack width–diffusivity rela-tionship.Subsequently,the distribution evolution of damage factors and defects was determined by simulation and XCT analysis,which makes a breakthrough of large-scale and integral quantitative characterisation of concrete damage under load.It is founded for the first time that the load caused both microstructure de-formation and microcrack propagation,and the two had mutual influence,which theoretically supports the subsequent study of the load effect on diffusivity.2.A method was proposed to determine the substance diffusivity con-sidering load,environment and structure factors.Based on the results of con-crete carbonation tests under load and the research about the diffusivity of con-crete with cracks,a formula that based on the load level and the damage factor was proposed to evaluate the load effect on the diffusivity.Combined with the influence of environmental factors（humidity and temperature）and material factors（deter-mined by the hydration and carbonation processes）,the calculation of the diffusiv-ity of moisture and CO₂ within concrete that comprehensively considers the influ-ence of microstructural deformation and microcrack expansion,was thusly deter-mined,which can effectively cover the whole loading process.3.A theoretical model（LTH-CM）,verified by various approaches,was es-tablished to predict the carbonation depth of loaded concrete.The diffusive transport equation was modified based on the diffusivity considering the load ef-fect.A coupled multi-physical field carbonation model considering the load effect was then established.The values of the diffusivity factors（D_h,D_c,n,α₀,ζ,χ,andα₁）and the reaction factors（v_c,E_r,and m）in the LTH-CM model were determined.The influence of the above parameters and simulation factors（boundary conditions,calculation tolerances and mesh size）on the carbonation depth and carbonation product distribution were investigated,followed by the finalisation of a parameter sensitivity study of the model.The accuracy and applicability of the LTH-CM were verified by comparing the numerical simulation results with the results of con-crete carbonation tests（450 days dry-air chamber carbonation,3 years atmos-pheric carbonation,2%and 20%CO₂ accelerated carbonation,RILEM TC 281-CCC international comparison test for accelerated carbonation of loaded concrete）and with the predicted values from Han models.4.A computational platform for the joint multi-software simulation was established to implement the LTH-CM model at meso-scale for carbonation depth.A number of application studies were carried out to demonstrate the broad applicability of the proposed model.The transfer of mesh and result（principal stress and damage）files between different finite element software has been carried out through JAVA and Python programming.The problem of decreas-ing accuracy in file transfer was also solved by adding nodal data,ensuring efficient and accurate prediction of the carbonation depth of loaded concrete.The compu-tational platform was used to investigate the effects of the fine structure（the dis-tribution,shape and content of coarse aggregate,the interface transition zone and its width,surface crack dimension）,carbonation dimension（unidirectional and bi-directional carbonation）and loading state（uniaxial tensile and compressive pro-cesses）on the carbonation depth,the width of the semi-carbonation zone and the uneven degree of concrete carbonation,demonstrating the wide range of applica-tions of the LTH-CM model.