Micro-pore Structure And A Dual Porous Media Permeability Model Of Concrete Materials

With the increase of service duration of existing concrete structures,the durability problem of concrete structures causes huge economic loss and resource waste.Permeability is an important parameter to characterize the durability of concrete materials.Accurate prediction of permeability of concrete materials is of guiding significance to evaluate the service life of concrete.The permeability prediction model establishes the functional relationship between the pore structure characteristics and permeability,and the characterization and selection of pore structure parameters are the basis of permeability predicting model.The commonly used characteristic parameters of pore structure are porosity,characteristic pore diameter and specific surface area.In addition to these macroscopic parameters,the research on the microstructure of pore structure shows that the characteristics of tortuosity and pore shape determine the network connectivity and liquid retention degree.Both the micro characteristics of pore structure that have important contribution to and influence on permeability.The existing permeability model of single porosity medium is suitable for the single-phase medium of mortar matrix,and it is not suitable for concrete materials without considering the contribution of aggregate mortar interface area to permeability.It is an important work for durability characterization of concrete materials to study the micro pore structure characteristics of mortar matrix and interface area of concrete materials,and to establish the permeability model of dual porosity media suitable for concrete materials.In addition,the water vapor in the environment directly or indirectly causes the durability problems such as freeze-thaw damage,ion ingression and steel corrosion.As the adsorbate,water vapor can be used to analyze the pore structure of concrete materials,and at the same time,it is helpful to master the mechanism of environmental humidity on the pore structure of concrete materials.In this thesis,the isotherm adsorption model is established by the water vapor adsorption method,which is used to analyze the pore structure of concrete materials.The characteristic parameters of pore structure of concrete materials are analyzed by comparing the water vapor adsorption method with the mercury injection method and the gas adsorption desorption method.And the characteristic parameters that can characterize the permeability are selected.The desorption hysteresis loop of water vapor is analyzed,and the desorption characteristics of concrete materials are obtained based on the characteristics of the dual phase media model between the mortar matrix and the interface,the permeability prediction model of the dual porosity media model is proposed.The main research contents and conclusions are as follows:(1)The water vapor adsorption characteristics of concrete are studied.Moisture absorption test was carried out.And the thermodynamic parameters of the adsorption process are analyzed.The influence of w/b and fly ash content on the thermodynamic process of water vapor adsorption was analyzed.The effect of concrete material proportioning on thermodynamic parameters was discussed.The results show that the smaller the w/b is,the stronger the moisture absorption capacity of dry concrete material is.When the ambient humidity is 80%,the moisture absorption capacity of 0.3 w/b is 5%-10% higher than that of 0.4 w/b,and the moisture absorption capacity of concrete with 0.4 w/b is 17%-28% higher than that of 0.5 w/b.(2)The water vapor isothermal adsorption model of concrete material is proposed,and the pore structure parameters that characterize the permeability characteristics are analyzed and determined.The GAB adsorption model for cement paste and mortar materials was modified in low relative humidity section of 0-58% to make the GAB model suitable for concrete materials.A new adsorption model was proposed in high relative humidity section of 58%-100%,i.e.a power function model between cumulative adsorption volume and ambient relative humidity called Freundlich adsorption isotherm.Combining with GAB model revised in low relative humidity section,a complete water vapor adsorption model is formed in the whole relative humidity region of 0-100%,which has a good fitting effect to the experimental value of adsorption capacity.The pore size distribution obtained by modified GAB model was compared with that measured by mercury intrusion method and nitrogen adsorption method.And appropriate measurement for different size range of pores was determined.For dry concrete materials in this research,the range of permeability porosity gained by MIP is 2.68%-5.22%.Hydraulic radius could be used as the characteristic pore size,and it ranges from 26.6nm to 43.3 nm.The pore structure tortuosity calculated by CPSM model based on mercury intrusion method ranges from 4 to 9.(3)The ratio of different pore shapes in the pores of concrete materials are determined.Water vapour desorption test is carried out.According to the curves of desorption volume changed with time,the parameters of the commonly used desorption rate model were determined.The hysteretic loop of water vapor adsorption and desorption isotherm was analyzed.And the effects of proportioning of concrete materials on the volume fraction of ink bottle pores and capillary pores were studied.The results show that desorption rate is a power function.And volume fraction of capillary pore is around 22%-43%,volume fraction of small throat and big volume pore is 15%-25%.For concrete with a certain w/b,the more fly ash content,the larger the proportion of capillary pore volume.Increasing of fly ash content reduces the volume fraction of small threat and big volume pores.For concrete materials with the same fly ash content,the decrease of waterbinder ratio increases the proportion of capillary pores and small throat big volume pores.(4)The permeability prediction model of dual porosity media suitable for concrete materials is proposed.Conbining mercury intrusion testing and scanning electron microscopy(SEM),the micro-crack widths in the ITZ of 12 groups of concrete materials with different proportions are studied.By simplifing the calculation model of ITZ,the calculation equation of ITZ porosity was determined.Based on the pore structure of mortar matrix and the microstructure of ITZ,a permeability model of dual porous media for concrete material is proposed,and the model constant for concrete material was determined.Further study shows that permeability calculated by the dual porous media permeability model is in agreement with experimental data which means the proposed permeability model could effectively predict the permeability of concrete materials.The pore shape-related constants in the dual porous media permeability model increase linearly with w/b.