Multiple Corrosion Protection Systems for Reinforced Concrete Bridge Components

Abstract

Eleven systems containing epoxy-coated reinforcement (ECR) in combination with another corrosion-protection system are evaluated using the rapid macrocell, southern exposure, cracked beam, linear polarization resistance, and field tests. The systems include bars pretreated with zinc chromate to improve the adhesion between the epoxy and the reinforcing steel, two epoxies with improved adhesion to the reinforcing steel, one inorganic corrosion inhibitor (calcium nitrite), two organic corrosion inhibitors (Rheocrete® 222+ and Hycrete™), an epoxy-coated bar with a primer containing microencapsulated calcium nitrite, three epoxy-coated bars with improved adhesion combined with the corrosion inhibitor calcium nitrite, and multiple-coated (MC) bars with an initial 50-μm (2-mil) coating of 98 percent zinc and 2 percent aluminum followed by a conventional epoxy coating. The systems are compared with conventional uncoated reinforcement and conventional ECR. The coatings on all bars are penetrated to simulate the effects of damage during fabrication and placement in the field. The results presented in this report indicate that the coated bars provide superior corrosion protection to the reinforcing steel and that bars with damaged coatings initiate corrosion at chloride contents within concrete that are several times greater and corrode at rates that are typically two orders of magnitude below those exhibited by conventional reinforcement. Limited additional protection is achieved using bars with the primer coating, MC bars, and concrete containing the corrosion inhibitors calcium nitrite and one of the organic corrosion inhibitors, although the latter resulted in reduced compressive strength and reduced resistance to surface scaling. The differences in costs over a 75-year design life are relatively small for coated bars. Cracks in concrete directly over and parallel to the reinforcement, such as found in bridge decks, result in earlier corrosion initiation and higher corrosion rates than obtained with intact concrete for all systems. Epoxies that provide initially high adhesion to the underlying steel provide no advantage over conventional epoxy coatings. All coated bars that were evaluated exhibited corrosion losses at openings through the coating. A reduction in adhesion between an epoxy coating and the reinforcing steel occurs after a period of exposure to corrosive conditions. This reduction increases with increasing chloride content in the concrete and in the presence of cracks and decreases with the use of corrosion inhibitors, with the use of MC reinforcement, and with electrical isolation of the epoxy-coated bars from each other. Corrosion products form under the coating where adhesion has been reduced. For periods up to five years under exposure conditions representative of those in bridge decks, the reduction in adhesion between an epoxy coating and the reinforcing steel did not affect the rate at which coated bars corrode.