| Sulfate corrosion is one of the main environmental factors that reduce the durability of concrete.The reaction of diffused sulfate ions(SO42-)with hydration products in concrete will cause cracking and spalling of the structure,while long-term load will lead to micro cracks in concrete,accelerate the diffusion of sulfate ions,and seriously affect the service life of concrete structures.Concrete is a complex heterogeneous material,the diffusion of sulfate ions within it is influenced by many factors.At present,there are few studies on the diffusion characteristics of sulfate ions from the meso level,and establishing a microscopic model for sulfate ion diffusion is of great significance for theoretical research on sulfate attack on concrete and solving practical engineering problems.Therefore,this thesis is based on the meso-structure of concrete with random aggregate model,a meso-model of sulfate ion diffusion reaction in concrete without load and under load was established and the influence of different factors on the diffusion characteristics of sulfate ions was studied.The main research contents were as follows:（1）The generation method of concrete random convex polygon aggregate is improved.Based on Monte Carlo method and aggregate grading theory,triangles or quadrilaterals are generated on a circular matrix,and then the points inside the circle and on the boundary are randomly selected to realize the random generation of hexagonal to nine-sided aggregates,two-dimensional circular and convex polygon coarse aggregate models were established by MATLAB software.Considering the existence of interface transition zone（ITZ）,the interface area of circular and polygonal coarse aggregate is created.（2）The meso-sulfate ion diffusion reaction model of concrete was established based on aggregate model.According to Fick’s second law and the theory of chemical reaction dynamics,considering the different diffusion coefficients of the three phases of concrete,the meso-sulfate ion diffusion reaction model of concrete was established for numerical simulation,and the rationality of the model was verified.The influence of mesh generation method and ITZ phase generation method on suifate ion diffusion were analyzed.It is found that the overall mesh refinement of the geometric model can replace each material to divide the mesh alone,the diffusion effect of sulfate ions obtained by ITZ membrane formation method is equivalent to that obtained by field formation method.（3）The erosion and diffusion characteristics of concrete sulfate ions and their influence laws were obtained.The numerical simulation of sulfate ion diffusion in concrete coarse aggregate,ITZ,initial boundary concentration and water cement ratio was carried out by COMSOL software,and the effects of different factors on sulfate ion erosion diffusion were analyzed.The results show that the shape and distribution position of coarse aggregate has little effect on the diffusion of suifate ions,with the increase of coarse aggregate,the maximum diffusion depth of sulfate ion at the same time,the ion concentration at the same depth and the erosion rate decrease,coarse aggregate content is the largest aggregate factor affecting the diffusion of sulfate ions.The diffusion rate of sulfate ions can be improved by increasing ITZ thickness and diffusion coefficient,but the overall impact of diffusion coefficient is smaller than that of ITZ thickness.（4）The influence mechanism of bending load on sulfate ion erosion and diffusion characteristics of concrete is revealed.The sulfate ion diffusion mesoscopic model under load was established to simulate the sulfate ion diffusion,and the feasibility of the model was verified.The influence of the shape,content and bending load of coarse aggregate on the diffusion of sulfate ions in the tensile zone of pure bending section was analyzed.The results show that the sulfate ion erosion depth of concrete with polygonal coarse aggregate is slightly smaller than that of circular under the same load level,the increase of sulfate ion diffusion depth of low aggregate content concrete decreases with the extension of erosion age,increasing the load level,the concentration and diffusion depth of sulfate ions increase. |
