Research On The Chloride Diffusivity Of Near-surface Concrete And Its Testing Method

Currently the durability issues of concrete structures have become increasinglyprominent, causing universal attention of both scientific researchers and industrialcircles worldwide. Transport properties of near-surface concrete are the main factorsthat affect the durability of concrete structures, and chloride diffusivity is the mostwidely studied and used transport properties. The studies of the diffusivity ofnear-surface concrete and its testing method not only have important theoreticalsignificance, but also have obvious practical value. Deep explorations of this subject areprovided in this paper. Research contents and achievements are listed as follows.The influence of the microstructure of near-surface concrete on chloride diffusioncoefficient is studied. The near-surface concrete is divided into several layers, and byconsidering the diffusivity of hardened paste and the influence of dilution effect,tortuous effect and interfacial transition zone, caused by the introduction of aggregates,on the diffusion coefficient, a framework for calculating the surface diffusivity isestablished.The in-situ method, i.e., the PERMIT ion migration methods for near-surfaceconcrete chloride diffusion coefficient determination, is studied and improved. Bymeans of testing concrete specimens with different cementitious materials systems, theconversion relation of conductivity and chloride ion concentration in anode chambersolution in steady-state phase during PERMIT test is established. The influence of initialsaturation degree on testing result is examined, and findings show that initial saturationdegree has significant influence on the lag time of steady-state, but has little effect onthe electro-migration rate of chloride ion. The linear conversion relationship betweenthe PERMIT ion migration test and rapid chloride migration method (RCM) isestablished.A laboratory testing methods based on AC impedance spectroscopy for chloridediffusivity determination is proposed. By introducing an equivalent circuit model withobvious physical meanings, the electrical parameter characterizing diffusion property isfound. The formula for calculating the diffusion coefficient is derived via theEinstein-Smoluchowski equation. By selecting the concentration of saturating solution and analyzing the simulated pore solution, the diffusion coefficient formula is corrected,which laid a theoretical foundation for field-testing methods.A new approach suitable for field chloride diffusion coefficient testing is proposedbased on the above-mentioned laboratory method. The testing method can syntheticallyreflect the diffusion coefficient of near-surface concrete and avoid the interference ofrebar through well-designed electrodes arrangement.An EIS testing method involving a reversible electrochemical reaction for chloridediffusion coefficient determination is established. The chloride ion diffusion conditionsare created by the introduction of porous silver electrode and chloride solution. By theselection and design of electrodes, the application range is extended to field use. Theinfluence of rebar is also examined.Types of chloride diffusion coefficients and their applications in servicelife prediction for concrete structures are reviewed. Comments on the mainmodels for all stage of reinforcement corrosion and its consequences are made.The advantages and disadvantages of these models are discussed. Besides,future research directions are suggested.