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Abstract:The chloride-induced corrosion is one of the main forms of deterioration in the transportation system of Tennessee. The main goal of this research is to determine the critical chloride level that causes de-passivation of both reinforcing and prestressing steel in typical TDOT concrete mixes under various exposure conditions. It was found that steel de-passivation (rust initiation) typically occurred at the total chloride ion contents between 0.42% and0.67% by weight of cementitious materials for unstressed prestressing strand, depending on the concrete mix and the exposure condition. When stressed, the prestressing strand seemed to de-passivate at a lower chloride level (e.g.0.24% by the weight of cementitious materials). Similarly, rust first appeared on rebar at a total chloride ion content of 0.43% to 0.56% by weight of cementitious materials, depending on the concrete mix and the exposure condition. The exposure condition greatly influenced the corrosion initiation and propagation. For the indoor exposure without moisture, active corrosion occurred, but stopped after 5 to 6 months exposure due to the lack of moisture. For the indoor exposure with moisture supply or the field exposure with wet/dry cycles, corrosion was sustained when the chloride exceeded the limit. For the salt-water immersion, all corrosion activities were negligible due to the lack of oxygen. The background chloride in typical TDOT mixes with local materials was between 0.05% and 0.1% by the weight of cementitious materials. It was safe for typical reinforced concrete structures. However, cautions should be taken for prestressing concrete structures. The use of supplementary cementitious materials such as class F fly ash reduced or did not significantly affect the acceptable chloride ion limit in concrete; however, it did noticeably reduce the extent of rust. The acid-soluble method (ASTM C1152) provided results that were relatively more constant. The total chloride ion content by the weight of cementitious materials determined by acid-soluble method was preferred as the representation of chloride content in concrete.
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