CHARACTERIZING CORROSION BEHAVIOR OF DISBONDED EPOXY COATED REINFORCING STEEL - UPDATED RESULTS

摘要

An in-house research project employing epoxy coated reinforcing steel bars (ECRs) was launched in March 2006 at Turner-Fairbank Highway Research Center of Federal Highway Administration (FHWA). The main objective of the study was to investigate effectiveness of disbonded ECRs and it was based on a concern whether ECR could be effectively corrosion resistant, even if it lost its original coating adhesion property. The present lab study is being carried out with ECRs having various conditions including coating disbondment. Poor quality concrete with w/cm = 0.55 and a high performance concrete mix containing blast furnace slag were used to cast seven 6 in. × 6 in. × 9.5 in. (15 cm × 15 cm × 24 cm) long prism specimens, which, after 28-day curing period, were exposed to an accelerated corrosion test called a modified Southern Exposure test. Test variables include artificial defect size, defect density, coating disbondment, clear cover, concrete quality, and bar type of bottom mat. For control purpose, prisms with black steel and defect-free virgin ECRs were also made. The first preliminary test results were presented in Corrosion/2007 conference. This paper addresses updated laboratory test results pertaining to the corrosion behaviors of individual ECRs and uncoated control bars. They have been monitored for about 360 days using open circuit potential, macrocell current, and electrochemical impedance spectroscopy. It was found that corrosion performance of the ECRs, in terms of macrocell current, was mainly influenced by the number of defects and the bar type (ECR vs. uncoated) in the top and bottom mats. This observation was still true for some ECRs exhibiting active corrosion indicated by open circuit potentials (values more negative than -350 mV vs. CSE). On the other hand, coating disbondment produced in conjunction with hot water immersion test and 4-step cathodic polarization did not create noticeable negative impact on corrosion performance of defective ECRs. In addition, electrochemical impedance spectroscopy data analysis using an equivalent circuit modeling technique confirmed that coating disbondment did not mean losing the epoxy coating's barrier function against steel corrosion inside concrete.