Critical Chloride Corrosion Threshold for Galvanized Reinforcing Bars

Abstract

Galvanized reinforcement is evaluated to determine the chloride content required for corrosion initiation. The bars conform to ASTM A 767, except that no chromate treatment was applied. Specimens containing the galvanized bars are subjected to Southern Exposure test conditions that are terminated upon corrosion initiation, after which the chloride content at the level of the reinforcement is determined. These data are compared with chloride surveys performed on bridge decks to obtain an average time to corrosion initiation. The time to corrosion initiation for galvanized reinforcement is compared to conventional reinforcement and MMFX Microcomposite reinforcement. The galvanized reinforcement specimens were also examined after testing for signs of hydrogen formation. The test results show that galvanized reinforcement has an average critical chloride corrosion threshold of 2.57 lb/yd3, which is greater than conventional steel (1.63 lb/yd3) and lower than MMFX steel (6.34 lb/yd3). Galvanized reinforcement exhibits a wider range of values of chloride content at corrosion initiation than conventional reinforcement. Critical corrosion threshold values for galvanized reinforcement range from values comparable to those exhibited by conventional steel to values three to four times that of conventional steel. Autopsy results revealed zinc corrosion products on the bars. Hydrogen gas evolution did not appear to increase the porosity of the concrete in the non-chromate treated bars relative to conventional reinforcement in air-entrained concrete. Some galvanized bars, however, showed signs of corrosion, including loss of the pure zinc layer, which may be due to the lack of chromate treatment or due to loss of metal in presence of high-pH concrete pore solution. Based on chloride surveys of cracked bridge decks in Kansas, galvanized steel can be expected to increase the average time to corrosion initiation at crack locations from 2.3 years for conventional steel to 4.8 years for bars with 3 in. of concrete cover. Corrosion initiation can be expected to occur at an average age of 14.8 years for MMFX steel. All three systems will exhibit significantly longer times to corrosion initiation in uncracked concrete.