Research And Application Of Unsaturated Permeability Model In Multi-scale Cement-based Materials

Water,as the main transmission and reaction medium,is one of the dominatant factors that influence the durability of cement-based materials.It is of great importance to effectively predict the water content and distribution in unsaturated cement-based materials in order to evaluating the durability of structures.In this dissertation,kinds of water absorption tests were conducted with different expremental parameters to study the permeability performance of unsaturated cement-based materials.Based on the experiment,the transport mechanism was furthered analyzed to propose the unsaturated pressureless permeability model considering the hygro-gradient effect with the variables of water content and distribution,and the model was used in the analysi of the chloride ingress in the splash zone of concrete specimens.The model was further extended for the unsaturated premeability model under pressure by considering the effect of the interfacial transition zone(ITZ)on the connectivity of cement-based materials and for the evaluation of permeation pressure of specimens.This dissertation includes the following four aspects of works and research results:1.The water absorption tests of mortars were conducted by varing the water to cement ratio,curing humidity and the volume fraction of fine aggregate to study the influence of porosity distributin,initial water content and aggregate fraction on absorption process.The initial water content and the total porosity were considered as the main factors controlling the absorption rate,which provided the basement for the development of numerical model.2.The hydro-gradient permeability model under pressureless situation was proposed.Based on the analysis of stress mechanism and the slip assumption of water in capillary pores,the effect of water content and capillary force was expressed by the slip length formula representing the influence on permeability of water state in unit element volume.The deviation coefficient was also given under non-ideal wetting conditions,and the effectiveness of the hygro-gradient model was validated by the analysis of different water absorption tests and the calculation of sorptivity.3.The existing chloride transport constitutive model of splash zone was improved on the basis of unsaturated pressureless permeability,through the correction of calculation method of liquid water flow rate in the convective term and the equavalent boundary condition of chloride and humidity describing the convection of splash zone.The applicability of the calculation method was verified by the simulation of artifical splash experiment of specimens with different water to cemetn ratios under different situations of splash cycle and time.4.The analytical and numerical models were established for the pressure-induced penetration incorporating the effect of ITZ.The analytical model based on the porostructural parameter taken into account the overall characteristics of connectivity,constrictivity and tortuosity by introducing the characterization coeffieient representing the ITZ effect.Based on the microscopic model of ITZ,the pore connectivity features in cement-based materials containing aggregate was chracterized through the concepts of paste connectivity,the percolation degree and the contribution to the paste connecvivity of ITZ.The models can be used to effectivly predict the penetration pressure of cement mortar and to meet the requirements of material design and performance evaluation.