Research On The Transport Derformance Of Cement-based Materials Based On A Multi-scale Model

The interface transition zone(ITZ)in concrete has a looser structure and a larger porosity comparing with the cement paste,which provides a fast transport channel for harmful ions invasion into concrete.The research on the microstructure and transport properties of ITZ is significant for the enhancement of concrete durability and service life prediction.However,the current research on ITZ is not sufficient,and the numerical simulation results do not agree well with the experimental results.In this paper,an improved experimental method and a multi-scale model were used to systematically study the microstructure,transport performance and behavior of the ITZ.This article is divided into the following sections:(1)A test method based on the natural immersion method was established to measure the chloride ion diffusion coefficient of the ITZ.The method can intuitively reflect the influence of ITZ on chloride ion diffusion,and be used to quantitatively calculate the chloride ion diffusion coefficient of ITZ.Compared with other methods,this method is the closest to the behavior and mechanism of chloride ion diffusion in real concrete.The test results indicate that the water-cement ratio significantly affects the chloride ion diffusion coefficient of ITZ,but has a limited effect on the relative diffusion coefficient.(2)Based on basic material parameters,formation mechanism and cement hydration principle,the microstructure model of the ITZ was established,and its transport behavior was simulated using finite element method.And the accuracy and effectiveness of the model was verified by comparing the simulation results with test results.The effects of water-cement ratio,particle size distribution,mature age on the thickness,porosity and chloride ion diffusion coefficient of ITZ were quantitatively analyzed by the numerical simulation,and the influential mechanisms of these factors was revealed.The relationship between pore structure and transport coefficient was studied,and the difference between the transport behavior of ITZ and cement pastes was explained by the analysis of three-dimensional pore structure.(3)Based on basic material parameters,a multi-scale model system of cement-based materials was established at the microscopic,sub-microscopic and microscopic scales.Through a large number of numerical simulations,the influence of aggregate random distribution on the randomness of overall transport performance was studied.It was found that the randomness of materials was closely related to research scales,and the relationship between the two was quantified.The causes of randomness were analyzed,and two main sources of randomness were identified and quantified:the fluctuations in aggregate volume fraction and the random distribution of aggregate locations.Based on the error theory,the relationship between the calculated relative error and research scales was studied,which provided a basis for the determination of representative volume element.And the analysis showed that periodic boundary conditions can significantly reduce representative volume element,thereby reducing calculation requirement.(4)A three-phase composite material model consisting of dispersed particles,continuous matrix and ITZ was established to predict the transport properties of cement-based materials.In this model,the amplification factor was introduced to quantify the influence of the ITZ on the composite properties,and the equivalent volume fraction was used to uniformly describe the influence of thickness,relative diffusion coefficient and specific surface area.Compared with other models,the model comprehensively considers various parameters of ITZ and can accurately predict the transport performance of cement-based materials.(5)The differences and similarities of the transport behavior of various substances were studied,and the main transport media of different substances were identified:the capillary solution for chloride ions and charges,the gas phase for oxygen.The relationship between the volume fraction of transport media and transport coefficient was studied,and the penetration threshold effect is revealed from the microstructure.The multi-scale model of cement-based materials was used to predicted transport performance including chloride ion diffusion coefficient,electrical resistivity and oxygen diffusion coefficient,and the accuracy of the model was verified by experimental data.Considering material microstructure and the ITZ,this model can predict the transport coefficients of various substances under unsaturated conditions.