Graduate Thesis Or Dissertation
 

Analytical methods for the study of migration of chloride ions in reinforced concrete under cathodic protection

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  • The migration of chloride ions in concrete with steel reinforcement under cathodic protection was investigated. Mortar blocks (15 cm x 15 cm x 17 cm) of various composition (water to cement ratio, chloride ion content) were cast with an iron mesh cathode embedded along one face and a thermally sprayed zinc anode applied to the opposite face. Current densities of 0.033 and 0.066 A/m² were applied to the blocks over a period of one year at constant temperature and humidity. The zinc face was covered with a pond of saturated Ca(OH)₂ to prevent polarization of the zinc-concrete interface. Over the course of polarization, potential versus time curves were recorded, and samples of mortar were extracted for determination of chloride concentration. An ion chromatography method was developed for the analysis of small samples of mortar for chloride. The method allowed us to measure chloride concentration in mortar samples with a long-term overall relative standard deviation of 3.2% in the concentration range of 1 15 mg/L in the water extract of the mortar. Under the conditions of the study, no significant migration of chloride ions could be detected over the one-year test. This result was not inconsistent with that which was expected based on a simple transport model of the system. Random fluctuations in the chloride concentration profiles that were observed were attributed to the inhomogeneous pore structure of the mortar on the scale of the sample size and the associated inhomogeneity in the chloride distribution. Future studies of these phenomena should be designed with: (i) larger blocks and larger samples of mortar for chloride analyses; (ii) an automatic misting device to obviate the need for the Ca(OH)₂ pond required to prevent the polarization of the zinc-mortar interface, and (iii) higher current densities, longer periods of polarization, or both.
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