Investigations On Gas Transport Characteristics And Mechanism Of Unsaturated Cementitious Materials

Concrete is known as an industrial commodity with strategic significance in the history of human civilization,which is used widely in the infrastructure construction of each country in the world.Modern concrete in the long service process is exposed to severe environmental ordeals,e.g.,oceans,salinized soi,chilliness,extreme heat,and drought.The coupled environmental effects on concrete cause performance degeneration thus resulting in economic losses.The durability issues of concrete have attracted much attention in the economic construction of each country,which is also a major problem urgently solved.Penetrability is a vital index of concrete durability assessment,which describes the erosion medium movement in the pore structure within concrete and reflects its compactness as well as its performance.Based on the osmotic medium property,penetrability can be considered as three types:water,ion and gas permeabilities.Among them,concrete gas permeability is related to the durability of buildings in contact with the atmosphere,e.g.,carbonization and alkalinity-reduction of concrete caused by CO2 transport,corrosion of steel bars in concrete caused by O2 transport.Cement concrete building in contact with the atmosphere is a large amount and wide area,which is closely related to human life.Therefore,it is of great significance to investigate the gas permeability characteristics and mechanism of modern concrete.Based on fluid mechanics principle and porous media theory,this paper investigates gas transport characteristics and mechanisms of modern concrete.The main work are as follows:(1)a new technology named gas intrusion porosimetry(GIP)is developed for characterizing the liquid/gas phase pores within unsaturated cementitious materials;(2)combining experimental observations and model calculations,the standard testing method is established for gas permeability of cementitious materials;(3)the permeability of unsaturated cementitious materials is systematically tested and capillary parallel bundle model with graded pores is created for simulating the permeability of cementitious materials;(4)according to the adsorption-desorption theory,the diffusion and permeation of water-vapor in unsaturated cementitious materials are investigated;(5)based on the above characteristics and mechanisms of gas transport,an engineering testing technique is developed to realize the long-term detection of concrete gas permeability.The main achievements are as follows:1.Investigation of liquid/gas phase pores within unsaturated cementitious materialsThe liquid/gas phase porosity and pore size distribution of unsaturated cementitious materials are systematically investigated.(1)Porosity:based on the ideal gas law,a new technology named gas intrusion porosimetry(GIP)is independently developed and the porosity of cementitous materials with different saturation conditions is accurately measured.The effect of water saturation on liquid/gas phase porosity of cementitious materials is systematically studied using GIP technique:with increasing water saturation,the gas-phase pore decreases and liquid-phase increases;the critical water saturation(θc)and threshold water saturation(θth)of concrete,mortar and cement paste are 22.39%～32.38%,26.44%～34.83%,27.73%～36.74%and 91.09%～72.46%,53.53%～70.68%,48.23%～62.38%,respectively.(2)Pore size distribution:based on the pore size distribution of dried cementitious materials,a calculation model is proposed for liquid-gas distribution within unsaturated cementitious materials,the distribution obtained indicates that the liquid-phase distribution in the small pores is higher than the gas phase and that it is inverse in the large pores;with the increasing water saturation,the liquidphase pores increase and the gas phase pores decrease.The critical saturated pore sizes of cementitious materials with various water saturations of θ25,θ50,θ75 are 2.19 nm～5.04 nm,3.04 nm～6.88 nm,and 3.52 nm～11.48 nm,respectively.The critical pore size increases with the increasing water saturation and the water saturation easily affects the critical pore sizes in cementitious materials with lower water-to-cement(w/c)ratios and the influence decreases gradually with increasing w/c ratio.2.Investigation of the testing standard of gas permeabilityThe effects on the apparent permeability of concrete are investigated:various confining pressures(30.0 bar～90.0 bar),various inlet pressures(1.0 bar～40.0 bar),various sample heights(25 mm,50 mm,75 mm and 100 mm)and various permeable gases(O2,N2 and CO2).Based on the microscopic mechanism and gas flow mode,the standard testing method of gas permeability is established using the combined theory and experiment.The optimized parameters are 50.0 bar confining pressure,15.0 bar～30.0 bar inlet pressure and 100 mm sample height.O2,N2 and CO2 can be used as osmotic medium for measuring the gas apparent permeability of concrete and their permeabilities are in the decreasing order,which is determined by the viscosity and molar mass of the gas medium.3.Gas permeability experiment and model calculation of unsaturated cementitious materialsBased on the quasi-steady flow and a capillary parallel bundle model with graded pores,the gas permeability of unsaturated cementitious materials is measured and simulated,respectively.Based on pore distribution,the model is established according to its pore length ratios.(1)Gas permeability is measured for dried cementitious materials:with various w/c ratios,concrete<mortar<cement paste;various fly ash dosages,concrete<cement paste<mortar;various slag dosages,cement paste<concrete<mortar.(2)Gas permeability is measured for unsaturated cementitious materials with various w/c ratios:with increasing water saturation,gas permeability and relative permeability both decrease while the tortuous factor of gas-phase pores elevates;the threshold water saturation(θth)of cementitious materials ranges from 60%to 80%.(3)The gas permeability of unsaturated cementitious materials is calculated using the established model.The modeled value of gas permeability under dry conditions is close to the experimental value.The predicted values of relative gas permeability agree with the experimental values and VGM model values,which indicats the applicability of the model.4.Investigation of the water vapor transport characteristics of unsaturated cementitious materialsAdsorption-desorption and penetrability experiments are carried out for water-vapor transport using the concentration difference and pressure difference,respectively.(1)Water vapor diffusion:desorption of water vapor lags behind its adsorption,and their adsorptiondesorption hysteresis loop can effectively characterize the characteristics of the pore structure.There is a critical humidity of 70%:lower than 70%,gel pores are saturated and higher than 70%,the water saturation increases rapidly with increasing humidity.With increasing water saturation,the gas phase permeability decreases gradually,and the liquid phase permeability is inverse,and their approximate intersections are located at 50%water saturation.Low water saturation(<50%)is dominated by water vapor diffusion,while high water saturation(>80%)is dominated by water vapor diffusion.(2)Water vapor permeability:the permeabilities of concrete,mortar and cement paste under the same conditions are in increasing order;the effective diffusivity of cementitious materials is 1～2 orders of magnitude lower than the diffusivity of free water vapor in the air.5.Investigation of the engineering application of gas penetrability for cementitious materialsEngineering application characterizations are implemented for the gas permeability of concrete under four working conditions:full exposure experiments in normal environments,exposure experiments in harsh environments,demonstration projects and practical engineering applications.By means of embedded sensor,portable testing equipment,recording the process of air pressure drop on site,combining with laboratory tested calibration,the variation law of concrete gas permeability under these four working conditions is obtained,which is similar to the laboratory test results,so as to realize the application of gas permeability in outdoor cement concrete engineering.