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03290 EFFECT OF CEMENT ALKALINITY UPON TIME-TO-CORROSION OF REINFORCING STEEL IN CONCRETE UNDERGOING CHLORIDE EXPOSURE

Product Number: 51300-03290-SG
ISBN: 03290 2003 CP
Author: Jingak Nam, William Hartt, Kijoon Kim, Lianfang Li
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A series of 91 concrete G109 type specimens was exposed to cyclic wet-dry ponding with a 15 w/o NaCI solution for approximately four years. Mix design variables included 1) cement alkalinity (equivalent alkalinities of 0.97, 0.52, and 0.36), 2) water cement ratio (0.50, 0.41, and 0.37), 3) presence versus absence of coarse aggregate, and 4) presence versus absence of fly ash. Corrosion potential and macrocell current between bottom and top bars were monitored in order to determine the time at which active corrosion commenced. Subsequent to corrosion initiation, specimens were autopsied and determinations made of 1) the effective chloride diffusion coefficient, 2) pore water pH, and 3) the critical chloride concentration for corrosion initiation. Time-to-corrosion for specimens of the individual mix designs conformed to a distribution that was represented using Weibull analysis. Specimens fabricated using the highest alkalinity cement exhibited times-to-corrosion that exceeded those with the two lower alkalinity cements by as much as a factor of five. Chloride threshold concentration was also variable and for each mix design increased with increasing time-to-corrosion with data for all mixes conforming to a common concentration-time trend. Macro-voids that impinged upon the upper side of the top reinforcing bar facilitated corrosion initiation and may have contributed to the distributed nature of the time-to-corrosion and chloride threshold concentration data. Keywords: concrete, cement concentration, air voids. alkalinity, reinforcing steel, corrosion, chloride threshold
A series of 91 concrete G109 type specimens was exposed to cyclic wet-dry ponding with a 15 w/o NaCI solution for approximately four years. Mix design variables included 1) cement alkalinity (equivalent alkalinities of 0.97, 0.52, and 0.36), 2) water cement ratio (0.50, 0.41, and 0.37), 3) presence versus absence of coarse aggregate, and 4) presence versus absence of fly ash. Corrosion potential and macrocell current between bottom and top bars were monitored in order to determine the time at which active corrosion commenced. Subsequent to corrosion initiation, specimens were autopsied and determinations made of 1) the effective chloride diffusion coefficient, 2) pore water pH, and 3) the critical chloride concentration for corrosion initiation. Time-to-corrosion for specimens of the individual mix designs conformed to a distribution that was represented using Weibull analysis. Specimens fabricated using the highest alkalinity cement exhibited times-to-corrosion that exceeded those with the two lower alkalinity cements by as much as a factor of five. Chloride threshold concentration was also variable and for each mix design increased with increasing time-to-corrosion with data for all mixes conforming to a common concentration-time trend. Macro-voids that impinged upon the upper side of the top reinforcing bar facilitated corrosion initiation and may have contributed to the distributed nature of the time-to-corrosion and chloride threshold concentration data. Keywords: concrete, cement concentration, air voids. alkalinity, reinforcing steel, corrosion, chloride threshold
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