Corrosion Resistance of Duplex Stainless Steels and MMFX Microcomposite Steel for Reinforced Concrete Bridge Decks

Abstract

Chloride-induced corrosion of reinforcing steel in concrete is one of the major durability concerns in reinforced concrete structures. In Northern America, the cost of maintenance and replacement for highway bridges due to corrosion damage is measured in billions of dollars. Of corrosion protection systems, reinforcing steels with inherently good corrosion resistance have received increased attention. In this study, the corrosion performance of duplex stainless steels, including 2101 and 2205 duplex steels in both “as-rolled” and pickled conditions, and MMFX microcomposite steel were compared with the corrosion performance of conventional and epoxy-coated steel using laboratory tests. These tests include rapid macrocell tests, corrosion potential tests, bench-scale tests (the Southern Exposure and cracked beam tests), and two modified versions of the Southern Exposure test to determine the critical chloride threshold. The rapid macrocell tests were modified by replacing the simulated concrete pore solutions at the anode and cathode every five weeks to limit the effects of changes in the pH of the solutions. The corrosion resistance of the steels was evaluated based on the corrosion rates, corrosion potentials, mat-to-mat resistances, and critical chloride thresholds measured in these tests. Based on laboratory results, along with data from bridge deck surveys and field experience, the service lives of the steels for bridges decks were estimated and the cost effectiveness was compared based on a life-cycle cost analysis. Results show that, in all rapid macrocell tests, replacing the test solution helps maintain the pH and reduces the corrosion rate and loss of steel. It is recommended that the test solution be replaced every five weeks. Statistically, effective chloride thresholds for reinforcing steel can be determined based on chloride samples from modified Southern Exposure and beam specimens. Results show that conventional steel has the lowest corrosion resistance, with chloride thresholds ranging from 0.91 to 1.22 kg/m3 (1.53 to 2.05 lb/yd3) on a water-soluble basis. Epoxy-coated steel [with four 3.2-mm (0.125-in.) diameter holes in the coating in each test bar to simulate defects of 0.2 to 1% of the bar area] has good corrosion resistance, with corrosion losses ranging from 0.4 to 6% of the values for conventional steel. MMFX microcomposite steel exhibits higher corrosion resistance than conventional steel, with corrosion losses between 16% and 66% and chloride thresholds, 3.70 to 4.07 kg/m3 (4.72 to 6.86 lb/yd3), equal to three to four times the value of conventional steel. Bridge decks containing MMFX steel will be less cost effective than decks containing epoxy-coated steel. Pickled 2101 steel and nonpickled and pickled 2205 steel exhibit significantly better corrosion resistance than conventional steel, with corrosion losses, respectively, ranging from 0.4% to 2%, 0.4% to 5%, and 0.2% to 0.5% of the value of conventional steel. Conservatively, the chloride thresholds of the steels are more than 10 times the value of conventional steel. Overall, 2205 steel has better corrosion resistance than 2101 steel, and pickled bars are more corrosion resistant than nonpickled bars. Pickled 2205 steel exhibits the best corrosion resistance of all the steels tested, while nonpickled 2101 steel has similar corrosion resistance to MMFX steel. The life cycle cost analyses show that in most cases bridge decks containing duplex stainless steels provide lower total life-cycle costs than bridge decks containing conventional, epoxy-coated, or MMFX steel. Pickled 2101 steel represents the lowest cost option.