Quantitative Characterization Of The Connectivity And Tortuosity Of Defects And Their Effects On Permeabilities Of The Cementitious Composites

Cement-based composites are frequently exposed to harsh environments for long periods of time with little or no repair.Thus,the durability of cement-based composite is one of the civil engineering field’s research hotspots.From the nature of degradation,the destructions of the microstructure and durability for cement-based composites are induced by the chemical reaction of the solid matrix with aggressive media(such as carbon dioxide,chloride and sulfate ions).It should be noted that pores,interfacial transition zones(ITZs)and cracks are the most common transmission channels for aggressive media.In general,a complete microstructure-guided laboratory measurement on defect’s percolation behavior and tortuosity is time-consuming.To avoid the unnecessary cost,the cement paste,mortar and concrete can be numerically assumed as granular composites with complex microstructures.However,the majority of previous numerical modelling studies on cement-based composites ignored the shape-and size-polydispersities of defects.To address the above issue,this work aims to quantify the relationship between defect compositions(shape-and size-polydispersities),microstructures(percolation threshold and tortuosity)and macroscopic property(permeability)of cement-based composite.To construct the polyshaped-polysized particle packing system,the superovals and superovoids are employed in this work.Combining the expression formulas of superoval and superovoid with the geometric potential,the contact detection algorithoms of superoval and superovoid are developed.Based on the soft ovoid packing system,this paper proposes a model to quantify how pore shape-and size-polydispersities affect the percolation threshold and tortuosity of porous network,and the above model is verified by comparing with the simulated results.The studies find that the spherical porous network possesses the maximum percolation threshold and tortuosity in the monoshaped-monosized porous network,and that the percolation threshold and tortuosity decrease as the dimensionless excluded volume of pore increases.The shape and number fraction of pores in a polyshaped-monosized porous network are the fundamental parameters that quantify the percolation threshold and tortuosity.Furthermore,the percolation threshold and tortuosity of porous network increases with the decrease of the ratio of pore minimum size to the maximum size.Based on the excluded volume of hard core-soft shell system,this work proposes a theoretical model for predicting ITZ percolation threshold,and this model is verified by comparing with the corresponding values from Monte Carlo simulation.The impacts of ITZ thickness,aggregate shapeand size-polydispersities on ITZ percolation threshold are evaluated.The results show that the ITZ percolation threshold of monoshaped-monosized aggregate packing system increases as aggregate sphericity enlarges.When the sphericity of aggregates is given,the ranking of ITZ percolation threshold for aggregate shape type follows: Oblate ellipsoid ≈ Oblate ovoid > Platonic particle >Prolate ellipsoid ≈ Prolate ovoid.The ITZ percolation threshold of polyshaped-monosized aggregate packing system is influenced by the shape and the number fraction of aggregate.The effect of aggregate size-polydispersity on the specific surface area is the essence of changing ITZ percolation threshold.Moreover,the ITZ percolation threshold decreases with the increase of ITZ thickness.According to the dimensionless excluded volume of crack,this work proposes a framework to characterize the influence of crack shape-and size-polydispersities on the percolation threshold of crack network,and the framework is verified by comparing with the results form simulation.It can be found that the circular crack network has the maximum percolation threshold of the monoshaped-monosized crack network,whereas the sharper crack shape has lower percolation threshold.A linear superposition model including the contribution of percolation threshold from each monoshaped-monosized crack network can be used to measure the effect of crack shape-polydispersity on the percolation threshold of polyshaped-monosized crack network.The percolation threshold of crack network decreases as the increase of the ratio of crack maximum size to minimum size.Furthermore,this work investigates the how aggregate shape-and size-polydispersities affect the tortuosity of crack network.It can be deduced that the tortuosity of crack network diminishes as aggregate sphericity increases.Moreover,the tortuosity of crack network grows with the increase of the volume fraction of aggregate with smaller sphericity.However,the aggregate size distribution has no effect on the tortuosity of crack network.Based on the quantified percolation thresholds and tortuosities of defects,this work establishes a complex theoretical framework for predicting the permeability of cement-based composite,and the above framework is verified by comparing with the experimental and simulated results.The findings indicate that the influence of pore shape on the permeability of cement paste can be divided into percolation control part and surface area control part.In the percolation control part,the permeability of cement paste decreases with the increase of percolation threshold.In the surface area control part,the permeability of cement paste enlarges with increasing surface area.Concrete with polyshaped-polysized aggregates has higher permeability than concrete with monoshaped-polysized spherical aggregates.The permeability of cracked concrete increases as the crack tapering parameter grows from 0.0 to 0.5,and reduces first and then increases when the aspect ratio goes from 0.2 to 5.0.This study proposes a "three-multis" model including multi-phases(pores,ITZ,and cracks),multi-polydispersities(shape-and size-polydispersities),and multi-structures(percolation threshold and tortuosity)to characterize the influence of defect composition on the permeability of cement-based composite.The results reveal a complex relationship between the internal composition,microstructure,and macro-property of composite.To some extent,the research findings can provide theoretical guide for the performance optimization and design of cement-based composites,as well as support the development of mathematics,physics,computer science,and other relevant subjects.