Study On Microscopic Crack Structure And Macroscopic Permeability Of Concrete

During the service of the hydraulic engineering structure,the concrete is subjected to external loads and environmental factors.Micro-cracks will inevitably occur,affecting the permeability of the concrete significantly and even threatening the safety of the project.The permeability of concrete is closely related to the structural characteristics of the micro-cracks.How to accurately describe the structural characteristics of the concrete crack network and generate numerical model attract people's attention.This paper focuses on the permeability of concrete and tries to establishe the relationship between micro-cracks and macro-permeability.Systematical researches were carried out based on macroscopic physical tests,microstructure observations,and numerical simulations.The main research results obtained in the dissertation include the following aspects:(1)Based on macroscopic experiments,the influence of layer interface on the concrete permeability was evaluated,focusing on the effects of different intervals and treatment methods.The specimens were casted layer by layer.According to the different initial and final settling times,four different types of pouring layers,namely bulk specimen,hot joint,warm joint and cold joint,were subjected to water permeability and electrical penetration tests.The results show that the coefficient of linear correlation between permeability coefficient and the initial current is above 0.90,indicating that the electrical test can be used to evaluate the permeation performance as a rapid detection technology.Treatment methods also have important influence on the permeability of concrete.The application of modified mortar containing PVA fiber can significantly improve the anti-permeability performance of pouring interface.(2)The microsturcture of layer interface was observed through microscopic tests.Scanning electron microscope observations show that the hydration products near the hot joint are tight,while the sturctures near the cold joint are porous.Microhardness test results show that the width of the interfacial transition zone(ITZ)is about 40 ?m,and the width of overlay transition zone(OTZ)of the hot joint,warm joint and cold jonint is about 60 ?m,80 ?m and 100 ?m respectively.Fluorescence microscopy observations show that the micro-cracks around the pouring interface can be divided into three types: matrix cracks,bond cracks and plane cracks.The length of micro-cracks follow the lognormal distribution.The curve of permeability parameter ? can be used to characterize the direction of crack netwrok.As the layer interface grows weaker,the ? curve gradually tends to be flat elliptical from circle,indicating that the direction of crack network changes from isotropic to anisotropic.Meanwhile,the dominant direction of the crack network tends to follow the layer interface.(3)Based on the embedded element technique,the permeability of threedimensional crack network was simulated.The embedded element technique can embed cracks into matrix and allow the cracks and the matrix to be meshed independently,which solves the difficulties of meshing in conventional continuous mesh method.Combining the embedded element technique with elastic analogy method,the seepage field can be obtained through the degradation of the corresponding stress-strain field,realizing the simulation of permeability of three dimensional crack network.Based on different three-dimensional crack networks,such as periodic network,random network,and OTZ network,the applicable range of the effective medium theoretical m odel was verified,and the influence of different structural parameters on the permeability performance was analyzed.