| For steel fiber reinforced cement-based composites,the orientation of steel fiber has a significant effect on the mechanical properties of cement-based composites.When the fiber orientation is consistent with the tensile stress direction,the reinforcement,toughening and crack resistance of the fiber are fully exerted,and the reinforcement effect on the matrix is the most obvious.Due to the filament shape of steel fiber,the orientation of fiber affects the free water migration of cement-based composite mixture system at the initial stage of hydration,and further affects the microstructure of the interface between steel fiber and matrix.The microstructure of the interface between steel fiber and matrix directly determines the bonding property between steel fiber and matrix,thus affecting the mechanical properties of steel fiber reinforced cement-based composite.Free water migration is one of the important characteristics of cement-based composites at the initial stage of hydration,at the beginning of hydration,because of the density difference between aggregate(fiber)and free water,the aggregate(fiber)goes downward and the free water flows upward to the matrix,the bleeding of cementitious composites is the results of water migration.Free water migration can cause local water redistribution in the system,which is one of the important reasons why the interfacial transition zone is different from the matrix.Studying the influence of fiber orientation on the hydration process and microstructure evolution of cementbased composites is of great importance to fully grasp the properties and mechanism of fiber-reinforced cement-based composites.At present,cement hydration is mainly studied through experiments and numerical simulation.With the development of computer numerical simulation technology,many hydration models have been proposed and applied.Traditional cement hydration models mainly focus on the hydration,precipitation and nucleation of cement particles at the micro scale,but do not consider the influence of free water migration and other behaviors on the micro hydration process.In view of the above problems,this thesis mainly studies the migration characteristics of free water in the initial cement hydration system,the hydration process of cement and the evolution of microstructure of cement stone and other factors,and reveals the influence mechanism of fiber orientation distribution on the microstructure,microstructure and macro-mechanical properties of fiber-reinforced cement-based composites.Therefore,this thesis first analyzes the flow characteristics of free water around steel fibers in different directions and studies the bleeding law of steel fiber reinforced cement-based composites at the initial stage of hydration hardening.Based on the free water migration characteristics of cement-based composite system at the initial stage of hydration,a dynamic hydration model is established,through which the influence of free water migration on hydration process and microstructure evolution law is analyzed.The model is used to analyze the pore distribution characteristics of the interface transition zone around steel fiber(aggregate),and the influence of aggregate size and fiber direction on the microstructure of the interface transition zone around steel fiber(aggregate).In terms of macro-mechanical properties research,this thesis simulates the tensile and shear properties of steel fiber concrete and fiber fabric reinforced cement-based composite(concrete canvas)respectively by Lattice model,and analyzes the influence of different fiber types and fiber orientation distribution on the tensile and shear properties of fiber reinforced cement-based composite.The effect of fiber orientation distribution on the properties of fiber reinforced cement matrix composites was studied from the microstructure and macro-mechanical properties of the matrix.The research shows that(1)the distribution direction of steel fibers has a significant effect on the movement of free water.The rising speed of free water around steel fibers in the vertical direction is much faster than that around steel fibers in the horizontal direction,resulting in an increase in bleeding volume of the material.The free horizontal average flow velocity around steel fibers with an inclination of 90°(vertical)is 1.2 times that of steel fibers with an inclination of 0°(horizontal).The bleeding volume of cement-based composite materials reinforced with horizontally oriented steel fibers is less than that of randomly random directions.(2)The free water movement around steel fibers in different distribution directions is different,which leads to the difference of porosity in the interface transition region around the fibers after hydration.The closer the fiber angle is to vertical,the larger the porosity of the interface around the fibers is.(3)the ITZ around coarse aggregate was formed under the effect of packing effect and micro-bleeding effect,at the beginning of hydration,a high water cement ratio zone are formed under the effect of packing,water-cement ratio of the upper and lower surfaces of the aggregate are the same,as hydration progresses,free water migrates,and a micro-bleeding area is formed at the bottom of the aggregate,while the difference in water-cement ratio between the upper and lower surfaces of the aggregate becomes greater.(4)In terms of mechanical properties,with the increase of fiber orientation coefficient,the tensile properties and shear properties of steel fiber reinforced cement-based composites are significantly enhanced.The strength and toughness are significantly improved,the brittleness of materials is significantly improved,and the shear fracture toughness is also significantly improved.(5)In fiber fabric reinforced cement-based composites,due to the good overall properties of fibers,the reinforcement effect of fibers in the direction of the main tensile stress of the matrix is fully exerted,and the ductility of materials is significantly improved.Through the research in this thesis,it is shown that the free water flow characteristics in the cement-based composite system at the initial stage of hydration have an important influence on the microstructure of the hardened matrix.The directional distribution of steel fibers not only affects the mechanical properties of cement-based composites,but also has an important influence on the movement of free water around the fibers and the microstructure evolution of the fiber-matrix interface area after hardening.This feature is of great significance to the study of the macroproperties and durability of steel fiber concrete. |
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