A Random Walk Method for Predicting the Chloride Ions Diffusion Coefficient of Cement Based Materials

Author:Zhang Cong Yan

Supervisor:zheng jian jun

Database:Doctor

Degree Year:2017

Download:65

Pages:132

Size:9150K

Keyword:

Concrete is one of the most widely used building materials,from buildings,roads to hydrolic engineering.Therefore,the durability issue of concrete affects various aspects of national ccomomy and construction security.As one of the most serious problems in the durability of concrete structures,ingress of chloride ions into concrete causes the reinforcement corrosion and the degradation of the properties of reinforced concrete structures,which has become the focus of the researchers all over the world.It is of great significance how to better study and analyze the diffusion performance of chloride ions in concrete for the durability design and assessment of concrete structures.The existing experimental methods have the disadvantages of complex processes and longer time and the theoretical studies are typical of various mathematical derivations,sophisticated,and difficult to be understood.Therefore,based on previous work,the diffusion behavior of chloride ions is related to Brown motion and the chloride diffusion coefficient of cement based materials is obtained.To better understand the diffusion behavior of chloride ions in cementitious materials and to verify the random walk method,a chloride diffusion test of cement paste and concrete with different mixing proportions is designed.An accelerated method is adopted to measure their chloride deffusivities.From the water/cement ratio and the cumulative distribution function for cement particles,cement pasticles of various sizes are generated and distributed in a simulation element.The periodic boundary conditions are introduced to eliminate the wall effect.In view of the interactions between cement particles,the cement hydration process is simulated and an evolution model of the microstructure of cement paste is established.The random walk method is adopted to obtain the chloride diffusion coefficient of cement paste.Through comparison with experimental results,the validity of the method is verified.For concrete with circular aggregates,an equivalent aggregate model is introduced and concrete is reduced to a two-phase composite material,consisting a cement paste and equivalent aggregates.The stereological method and the effective media approximation are used to derive the analytical solutions of the equivalent interfacial transition zone(ITZ)thickness and the chloride diffusion coefficient of the equivalent aggregate.The main factors that affect the equivalent ITZ thickness and the chloride diffusion coefficient of the equivalent aggregate are analyzed.The random walk method is used to obtain the chloride diffusion coefficient of concrete.After the validity of the method is verified with experimental results,the effects of the thickness and chloride diffusion coefficient of ITZ on the chloride diffusion coefficient of concrete is evaluated.For concrete with ellipitical aggregates,the equivalent ITZ thickness and the chloride diffusion coefficient of the equivalent aggregate are derived.Concrete is modeled as a two-phase composite material composed of the cement paste and equivalent aggregates and the chloride diffusion coefficient is obtained with the random walk method.The effect of aggregate shape on the chloride diffusion coefficient of concrete is analyzed emphatically.For concrete with spherical aggregates,the equivalent ITZ thickness and the chloride diffusion coefficient of the equivalent aggregate are derived.Three-phase concrete is reduced to a two-phase one and the random walk method is adopted to compute the chloride diffusion coefficient of concrete.Finally,the validity of the method is verified with experimental results.