| Concrete in service is rarely at fully saturated state due to water evporation.This has a significant influence on its transport properties.It is therefore important to understand the mechanism of water transport in unsaturated cementitious materials,which serves as an indicator for concrete durability assessment and service life prediction.In unsaturated concrete,there exit both water and air in pores and the mechanism of water transport in such condition remains unclear.Although experimental methods can be used to study water transport in cementitious materials,they are unfortunately time-consuming,expensive and sometimes inaccuracy.Moreover,most of them are unable to directly exhibt the water transport process.In contrast to experimental tests,computer simulation is more economic,efficient and can be used for visualization.In recent years,it gradually becomes popular in studying water transport mechanism of unsaturated concrete.Lattice Boltzmann method is suitable for modeling multi-component multi-phase medium transport,solving fluidsolid coupling problem and multi-scale numerical simulation because of its high accuracy and efficiency.Besides,this approach supports parallel calculation and can be exteneded for cosidering complex physical boundary conditions.So far,the Lattice Boltzmann method has been successfully used to predict water permeability of cement paste at a fully saturated state.However,the actual distribution of water and gas in unsaturated cementitious materials and their interactions are not paied enough attention yet.Besides,the contributions of calcium silicate hydrate(C-S-H)gels and interficial transition zone(ITZ)to water transport in concrete are rarely taken into account.Lattice Boltzmann method is utilized in this work to study the water-gas transport in cementitious materials.Main work and findings are presented as follows.(1)Recent progresses in the application of experimental methods and computer simulation techniques in studying water transport in unsaturated cementitious materials are firstly summarized.It is found that different water saturation degree of the specimen can explain the significant difference in the reported water permeabilities.There exists a certain relationship between the permeability and the effective porosity.According to the distribution of water in cementitious materials,it can be divided into saturated region,transitional zone and unsaturated region,respectively.Water permeability is determined by both water penetration under the external pressure and water adsorption due to capillary force.(2)A multi-component multi-phase Lattice Boltzmann model was developed to distribute water and air in cement paste at various water saturation degrees.The influence of wetting characteristic on the exitance of water and air in cement paste was studied.It is observed that capillary force is a key factor for distributing water and air in the pores.When cementitious materials change from hydrophilic into hydrophobic,large pores are filled by water instead of small ones.When the system is of neutral wetting characteristic,the distribution of water and air seems to be insensitive to pore size so that the water is uniformly distributed in all pores.(3)On the basis of actual particle gradation curve,a three-phase mesostructure model of mortar was constructed.A partial-bounceback Lattice Boltzmann model was developed to investigate the effect of aggregate fraction and ITZ structure on the permeability of mortar,such that the competitions between the ITZ effect,aggregate dilution effect and tortuous transport path can be demonstrated.When the thickness of ITZ is small or the porosity is low,the effect of aggregate dilution and tortuous transport path become dominate,water permeability of mortar decreases at an increased aggregate content.As the ITZ thickness or porosity goes up,the enhancement effect of ITZ on water transport becomes significant and the permeability of mortar rises as aggregate content increases. |
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