Measurement Of Chloride Migration In Cement-based Materials Using AgNO₃ Colorimetric Method

Chloride corrosion of steel in concrete is a main cause responsible for durability problem of reinforced concrete structures exposed to chloride environment. In that case, migration of chloride ions in cement-based materials is one of the main factors considered for the design of durability of concrete structures. Therefore, investigation on measurement of chloride migration in cement-based materials, especially using quick methods, is rather significant. This study investigated factors affecting AgNO3 colorimetric method and its use for rapid measurement of chloride migration in cement-based materials.Based on phase equilibrium in four-component system of AgNO3-NaOH-NaCl-H2O, equations for calculation of chloride concentration at the color change boundary (Cd) during AgNO3 colorimetric measurement was derived. Results indicated that Cd depended on sprayed concentration and volume of AgNO3 solution. When AgNO3 could react with all Cl- and OH- at the color change boundary, Cd increases linearly with OH- concentration. When AgNO3 react only with partial Cl- and OH- at the color change boundary, OH- has little effect on Cd and Cd increases with molar mass. Calculated range of Cd explained why Cd measured by many researchers change in a broad range.Based on calculation of Cd and investigation on factors influencing Cd, a reasonable AgNO3 colorimetric procedure was proposed and Cd values of cement-based materials were measured according to this procedure. Results indicated that the measured Cd decreased with water/binder ratio. Representative value of Cd was chosen for Cd range of each type of specimen with certain water/binder ratio.Based on diffusion electrical double layer of chloride absorption, chloride ion "condensation" phenomena was explained through measurement and analysis of related parameters. Results showed that there existed real chloride ion "condensation" in cement-based material, so-called chloride ion "condensation" is that chloride ion concentration in expressed pore solution of cement-based materials is higher than that in bulk pore solution owing to expression measurement. Measurement of free chloride in cement-based materials was investigated using water extraction method. Based on good repeatability and good linear or power correlation between water soluble chloride concentrations and free chloride concentration in expressed pore solution, water extraction method could be regarded as an effective procedure for the measurement of free chloride concentration in cement-based materials.Error of electrically accelerated non-steady state chloride ion migration coefficient (Dnssm) using AgNO3 colorimetric method was evaluated. Results indicated that when chloride color change depth was relatively small and sprayed volume of AgNO3 solution was not controlled, error of Dnssm caused by Cd could even reach 200%. When the AgNO3 colorimetric procedure established in this paper was used, error of Dnssm caused by Cd could be less than 5%.Some critical requirements for accurate measurement of chloride apparent coefficient (Dapp) using AgNO3 colorimetric method were proposed and experimentally investigated. The sensitivity and error of Dapp measured by AgNO3 colorimetric method were evaluated. Results indicated that sensitivity and error of Dapp measured by AgNO3 colorimetric method decreased with the increase of surface chloride concentration (Cs) and decrease of Cd. Based on the AgNO3 colorimetric procedure established in this paper, when Cs is equal to 0.53 mol/L (similar chloride ions in seawater) and 2.8 mol/L (chloride ions concentration of immersion solution used in NT build 443), respectively, errors of Dapp measured using AgNO3 colorimetric method caused by Cd changed within 17.1% and 2%, respectively. When average chloride ion chloride concentration in the first 2mm layer of specimens is similar to that in the immersion solution used in NT build 443, Dapp values measured by AgNO3 colorimetric method is very similar with those measured by NT build 443. Therefore, Dapp in cement-based materials can be rapidly and accurately measured by AgNO3 colorimetric method.