Mesoscopic Numerical Study On Chloride Diffusion In Concrete Considering Aggregate As Stochastic Convex Polygon

In marine environment,chloride erosion in concrete has become a hot topic in durability assessment of concrete structures.Establishing the meso-model of concrete multiphase composites and starting numerical simulation on chloride diffusion in concrete can not only save a lot of physical experiments costs,but also more conveniently explore the influence of concrete meso-structure on the diffusion characteristics of chloride ions.It is an effective research method for a certainty.At the mesoscopic level,it is generally considered that concrete is a multiphase composite consisting of cement,aggregates and ITZ.Among them,cement mortar and interfacial transition zone are medium that allow chloride ion to diffuse,while the coarse aggregate is opposite due to its hardness and compactness.Therefore,the shape,size and distribution of coarse aggregate can have an extremely important impact on chloride diffusion in concrete.However,most scholars assume that aggregate in concrete is circular,spherical,regular polygon and other geometric regular shapes,which is different from the actual situation.In order to make the aggregate shape closer to reality and consider the influence of the properties of multiphase composites on chloride diffusion,in this paper,a random convex aggregate generation with its ITZ is proposed,which is used to establish a numerical model of concrete,based on the finite element numerical model.In this dissertation,a meso-model of random convex polygon aggregate in concrete is established,and the distribution and diffusion characteristics of chloride ions in concrete are discussed by numerical simulation.The specific research contents are as follows:1.In order to simulate the shape and distribution of coarse aggregate in concrete more accurately,a new algorithm for generating and launching random convex polygon aggregate and its interface transition zone is developed.The three-phase mesoscopic model of concrete is established in COMSOL,including cement mortar phase,coarse aggregate phase and ITZ phase,and is used to conduct numerical simulation of chloride diffusion in concrete.The aggregate particle size distribution conforms to fuller gradation,and satisfies the completely random distribution of size and location.Meanwhile,considering the non-overlapping effect of aggregate in real concrete,the concrete model with different coarse aggregate volume fraction and ITZ can be generated.Compared with the convex algorithm adopted by other scholars,aggregate model developed in this method has more real randomness and higher computational efficiency.2.Modifying the open source function library DXFLib locally to realize the interaction between numerical model and finite element calculation software.The aggregate and interface transition zone information generated by MATLAB can be transformed,and saved as DXF format file to import into finite element calculation software.Based on the APP method module of COMSOL Multiphysics,the COMSOL model-method is developed,which can run the program customized according to different requirements.This realize the automation of finite element calculation process,and greatly improve the efficiency of numerical simulation and post-processing.3.Based on the random convex polygon aggregate model of concrete established in this paper,the numerical simulation is carried out with the same condition in physical experient,and the results are compared with the physical experient results and other meso-model calculation results of aggregate shape.The results show that the model established in this paper is closer to the experimental data and has smaller dispersion.In order to further reduce the influence of discreteness of stochastic mesoscopic model,a large number of numerical simulation results are analyzed by probability method,which verifies that the data calculated by the model are in normal distribution as a whole,and the way to select the mean of random sample combination can effectively reduce the impact of discreteness.4.Discussed the influence of coarse aggregate,ITZ and two-dimensional diffusion on chloride distribution in concrete.In addition,the relationship between volume fraction of ITZ and diffusion coefficient of ITZ is obtained by numerical model calculation,and a model of diffusion coefficient multiple of ITZ is established by combining with experimental results.