Characteristic Of Chloride Ion Transport In Concrete Under Loading Effect

The transport of chloride ions under loading is the main cause of structural deterioration in marine concrete.At present,the research on the transport characteristics of chloride ions mainly regards its transmission coefficient as a constant in concrete.In fact,the transmission coefficient is the change value of many factors influence.In addition,most of the current theoretical and experimental studies focus on the penetration of chloride ions in the one-dimensional direction and less on the two-dimensional direction which is more consistent with the actual situation.Therefore,it is of great significant and practical value for the durability of concrete structure to study the characteristic with variational coefficient of chloride ions penetration on the multi-dimensional direction under loading.The research contents and conclusions have been carried out in this paper.(1)The chloride ion diffusion coefficient varies with the microstructure of the concrete.On mesoscale scale,the concrete is considered as a three-phase composites which are composed of aggregate,mortar matrix and interfacial transition zone.Based on the micromechanical theory and its equivalent homogenization,the equivalent diffusion coefficient of chloride ion transport in saturated state is obtained.And its solving formula is determined.(2)The pebble(ellipsoidal)and gravel(polygonal)aggregates in natural environment are selected as the research objects.The equivalent spherical volume and shape factor characteristic parameters are used to quantitative characterize the two aggregates.Then,on this basic theory,based on the nearest surface distribution function and the spherical particle accumulation model of Lu,the ITZ theoretical calculation model is established around the ellipsoidal and polygonal aggregates.And the theoretical model is verified by tests.The calculation program of ITZ volume fraction is written based on Visual Basic 6.0 language,and realizes its visualization and functionalization.Finally,the influencing factors of ITZ volume fraction morphology,volume fraction,maximum particle size and grading of aggregates are analyzed systematically.The simulation results show that the effects of ITZ volume fraction around ellipsoidal aggregate are the ITZ thickness,the aggregate volume fraction and maximum particle size of aggregate.It indicates that reducing the ITZ thickness is the most effective measure to improve the interfacial performance.(3)Based on Fick's second law,one-dimensional and two-dimensional diffusion model with variable coefficient of chloride ions in concrete were established.The solution model of equivalent diffusion coefficient of chloride ions in concrete is embedded in the above diffusion models.The numerical solution of partial differential equation is derived by using the C-N and ADI format in finite difference method.The numerical solution is performed by using MATLAB.Compared with the experimental results,the prediction accuracy of variable coefficient is significantly improved.In addition,based on the theory of elastic mechanics and material mechanics,they were established that one-dimensional and two-dimensional diffusion models with variable coefficient of chloride ions in concrete under loading effect.The one-dimensional model was tested and verified and it shows that the results are basically consistent.The theoretical simulation results show that the concentration distribution of chloride ions is stepwise decreasing from surface to inside in concrete which is eroded by chloride ions.The concentration of chloride ions varies at different diffusion time and section depth.The compressive stress slows diffusion process of chloride ions and tensile stress accelerates it.For example,when diffusion time is 200 d and the section depth is 15 mm,the concentration of chloride ions under tensile stress is higher than that of compressive stress at 26.7%.In the case of two-dimensional diffusion,the value is 42.1%,which indicates that the two-dimensional diffusion is higher than one-dimension.