Permeability Of Microcracked Porous Solids With Random Crack Networks

The presence of cracks, intrinsic or induced by external actions, can greatly facilitate the flow of fluids thus alter the permeability of porous solids, deteriorating the longterm performance of materials. Mastering the impact of cracks on the permeability of porous solids is fundamental for various engineering applications. This thesis studies on the permeability of microcracked solids with random crack networks through theoretical analysis, numerical simulation and experimental research.Firstly, a new connectivity factor is defined considering the spanning cluster of cracks, fractal dimension of networks and the size of finite domain. The interaction direct derivative(IDD) method in effective medium theory is extended to evaluate the permeability of cracked solids considering the crack opening aperture and finite connectivity before total percolation of cracks. The contribution of connectivity to the effective permeability is equal to arrange parallel cracks on transport direction. By introducing the definition of the hypothetically parallel crack density, the hypothetically parallel cracks are embedded in a host matrix whose permeability are those of the effective medium.Then the permeability of microcracked porous solids incorporating random crack networks is investigated through continuum percolation theory. The crack density is used as the basic percolation variable, and the percolation threshold changes with crack pattern of networks. On the basis of connectivity and tortuosity of crack networks, the scaling law for permeability is established, considering the geometry characteristics, the permeability of porous matrix and the crack opening aperture. Then the permeability of solids incorporating random crack networks is solved through finite element methods.The influence of crack network geometry on the transport properties of cracked cement-based materials is investigated. Cracks are artificially manufactured in mortar and concrete slice specimens to obtain a 2D crack network. The water vapor sorption isotherms(WVSI) are measured for ordinary concretes and mortars. On the basis of mass measurements during drying test, the mean relative mass variation is used to obtain intrinsic permeability of specimens with different levels of cracking extent, also by using the WVSI and water transport model. Then, several geometry parameters including crack density, orientation and crack opening aperture are evaluated for 2D cracked surface. Finally, the relationship between the geometry characteristics of crack networks and the transport properties are investigated.The results show:(1) The geometry analysis reveals that local clustering will not necessarily contribute to the global connectivity of networks. It is also found that too strong local clustering of cracks decreases the probability of the global percolation. The new connectivity factor is proved pertinent to network percolation, related to both crack density and crack clustering degree;(2) The extended IDD model for solids with connected microcracks is obtained and then the validity and accuracy of IDD solutions for non-connected and part-connected crack networks are numerically confirmed by several case analysis;(3) The permeability depends on the crack opening aperture through a power law near the percolation threshold and this dependence disappears as the ratio between the local permeability of crack and matrix permeability exceeds 106;(4) In experimental study the crack density is found to have an important impact on the permeability and electrical conductivity with a rapid increase at ρ ≈0.3, which might relate to the percolation characteristics of crack networks. Both mortar and concrete specimens have strong correlation between the relative permeability and crack opening, with exponents rather near to 2.0; the electrical conductivity is almost linearly related with crack opening. Due to the 2D observation of crack networks, the accuracy of the numerical and theoretical models is examined.