Chloride Diffusion Into Concrete Considering The Heterogeneity Of Materials And The Blocking Effect Of Reinforement

Chloride-induced corrosion of steel reinforcement was treated as one of the significant factors for the durability of the reinforced concrete structures exposed to marine environments.The ingress process of chloride ions into concrete has been become a hot issue on the durability of reinforced concrete structures.At the meso-scopic level,the reinforced concrete can be generally regarded as a multiphase composite material composed of mortar,coarse aggregate,rebar and interfacial transition zone(ITZ).The heterogeneity of materials for concrete(coarse aggregate content and random distribution)can significantly affect the chloride diffusion characteristics.The rebar embedded in a hardened mortar paste can obviously enhance the chloride concentrations at the apex of steel reinforcement due to its blocking effect and as a result shorten the service life of reinforced concrete structures.Additionally,the two-dimensional diffusion behaviors of chloride ions at the corner of the reinforced concrete components can result in the initial corrosion time for reinforcement was shorter than that of chloride one-dimensional diffusion,which was not conducive to the safety of the structure.Nevertheless,the current researches on the aforementioned problems mainly focus on the effects of either coarse aggregate only or steel reinforcement separately on the chloride one-dimensional diffusion characteristics into concrete.For the chloride two-dimensional diffusion,most scholars generalized the concrete as a kind of homogeneous materials and devoted to develop different numerical algorithms for exploring the chloride two-dimensional diffusion laws in the macro-scopic numerical model of concrete.The common influences of heterogeneity of concrete materials and the blocking effect of reinforcement on chloride diffusion into concrete,especially on the chloride two-dimensional diffusion characteristics in concrete is still a problem that needs to be further studied.For this paper’s study,the physical experiment,the theoretical model and the numerical simulation method were combined to explore the aforementioned issues.The specific works were as follows:(1)During the existing studies,the drying-wetting cycle test method(dry/wet ratio is 1:1),which kept the experimental specimens completely drying for the half of one exposure period and completely wetting for the other half of one exposure period,was generally adopted to approximately simulate the marine tidal cycle.However,the effect of chloride ions ingression into concrete was not the most unfavorable when the ratio of dry to wet was equal to 1:1 in the real marine tidal environment.This paper employed the artificial marine environment automatic simulation device developed independently by our research team to indoors simulate the real-time rising and ebb tide process which is more approximated to the actual marine tidal environment.This device was used to carry out the indoor physical experiment for chloride natural diffusion into various experimental specimens(including the concrete and reinforced concrete specimens with different volume fractions of coarse aggregate and diameters of rebar)exposed to man-made marine tidal zone and to explore the chloride diffusion characteristics,especially,to analyze the two-dimension diffusion behaviors,into experimental specimens under the real-time tidal cycles.(2)The chloride concentrations into different experimental specimens measured by the physical experiment of chloride one-dimensional and two-dimensional diffusion were analyzed,and the impact factor of coarse aggregate volume fraction and the indirect and direct blocking effect coefficients of steel bar were proposed to respectively quantify the influences of heterogeneity of concrete materials and the blocking effect of reinforcement on chloride diffusion.On the basis of the Fick’s second law,the prediction models of chloride one-dimensional and two-dimensional diffusion considering the coupling influences of heterogeneity of concrete materials and the blocking effect of steel bar in marine tidal environment were established.The predicted values by the two models were in good agreement with the experimental measurements,validating the accuracy of the prediction models.(3)In previous studies,the thicknesses of interfacial transition zone(ITZ)were usually taken as 10～100μm,and the corresponding chlorine diffusion coefficients of ITZ were taken as dozens or hundreds of times than those of mortar.This value range was too large,and the results of these important parameters(thickness and chloride diffusion coefficient of ITZ)still required to be determined by experimental method.For this paper’s study,the prediction model of normalized chloride diffusion coefficient for ITZ considering the common effects of thickness of ITZ,volume fraction of coarse aggregate and diameter of rebar was built by deeply analyzing the experimental data.The prediction model can be utilized to directly confirm the chloride diffusion coefficient of ITZ on the precondition of a known chloride diffusion coefficient of mortar,and this can lay a foundation for the numerical simulation method of chloride diffusion into meso-scopic models of concrete and reinforced concrete.(4)Concrete or reinforced concrete was a representative heterogeneous and multi-phases composed materials,thus,using the macro-scopic numerical model considering the homogeneous of materials to estimate the chloride diffusion behaviors into concrete or reinforced concrete need to be further discussed.In this paper,the algorithm of random generation and random placement for three-dimensional spherical coarse aggregate was proposed and employed to establish the three-dimensional meso-scopic numerical models for concrete and reinforced concrete by the consideration of the properties of multi-phases composites for mixtures and the effect of coarse aggregate distributions at a certain region around the rebar due to the steel reinforcement embedded into mortar.The chloride one-dimensional and two-dimensional diffusion characteristics for the meso-scopic models were explored by numerical simulation using the finite element method,and the experimental measurements were adopted to validate the accuracy of the meso-scopic numerical simulation method.(5)The physical experiment used to investigate the long-tern chloride diffusion behaviors into concrete must consume a lot of time,manpower,material and financial resources,thus,there were a great deal of difficulties carrying out the long-tern test.Consequently,most of the existing researches have chosen the relatively short exposure periods.Whether the chloride diffusion model,which was built by using the short-tern experimental measurements,can reasonably predict the long-tern diffusion behaviors of chloride ions,this issues required to be further studied.For this paper’s study,the long-tern chloride diffusion concentrations evaluated by the meso-scopic numerical simulation method proposed by this paper were compared with the tested long-tern chloride concentrations with the maximum exposure time of 20 years,and this paper presented that the meso-scopic numerical simulation method can be adopted to replace a part of physical tests and to explore the long-tern chloride diffusion characterestics for concrete.Subsequently,the long-tern meso-scopic numerical simulation method was employed to demonstrate that the prediction model of chloride diffusion built by indoor short-tern experiment can accurately predict the long-tern chloride concentration profiles into concrete.On the basis of this,the durability service life for reinforced concrete structures were further predicted using the chloride diffusion prediction models proposed by this paper.