Corrosion assessment of epoxy coated reinforcement in concrete structures
Final Report Abstract
The above findings show that in ECR with coating breaks there is a substantial increase in cathodic strength in preconditioned ECR compared with non-preconditioned ECR. The increase can be attributed to the disbonded region created by the cathodic disbondment preconditioning around the preexisting coating breaks. The results suggest that there is intrusion of pore water in the disbonded region sufficient to create electrolytic contact between the underlying steel in the disbonded region and the rest of the concrete, and maintain an appreciable electrochemical reaction rate. The thickness of the disbonded region is not expected to be very large, given that the steel appears to be still in the passive condition and hence metal loss would be insignificant, and also because concrete embedment would have prevented any marked blistering. Nevertheless, given the extent of the effects observed the conductance of the electrochemical path appears to be appreciable. The results thus suggest that the disbonded condition could become a significant added contributor to the cathodic component of a possible macro-cell corrosion process acting on ECR. Since the cathodic reaction can be dominant in determining the overall corrosion rate of ECR examination of this issue merits continuing attention via additional experiments and formulation of appropriate models for the corrosion rate of steel in concrete when corrosion macro-cell is activated. The results also showed that cathodic contributions could be significant even without high levels of moisture, so consideration of enhanced macro-cell action due to the presence of disbonded regions and it possible effects should not be limited to marine structures. Other atmospheric exposure regimes such as deicing salt service should merit attention as well. The corrosion detectability in concrete by half-cell potential measurements is significantly impaired due to coating electrical insulation. Even though, defects in the coating in the size of the permitted defect sizes reacts as an anode, no pronounced potential gradient at the concrete surface is found independently of the used half-cell measurement procedure. The resulting potential field in epoxy-coated reinforcement is dominated mainly by the resting potentials of each rebar and the moisture distribution. In field measurements potential gradient in a range up to 200 mV were measured in a concrete sea structure made with epoxy-coated reinforcement. It could be concluded that corrosion in epoxy-coated coated reinforcement can be detected with a certain defect size, anode size respectively. The detectable anode size Is probably much greater than the permitted defect size and much greater than the detectable anode area in conventional uncoated reinforcement. As a consequence early corrosion starting in a small defect cannot be detected with half-cell potential measurement. Only, if the anode size increases due to anodic delamination or other adhesion problems reinforcement becomes observable. But then It is expected that the structural damage is much severe and the needed repair actions is more cost-intensive. The question arises If the benefit of epoxy-coated reinforcement with prolonging the service life of concrete structures is eliminated by the poor corrosion inspect-ability. Further electrochemical inspection tool such as polarization resistance and/or electrochemical Impedance spectroscopy should be included in the investigation of corrosion detectability. Lau found out that the cross-sectional area of coating pore defects can be estimated as being inversely proportional to the ohmic resistance of the electrolyte assumed to be filling the defect. As the consequence with known resistance the defect size can be estimated. These results are a first step toward applying EIS as a diagnostic method to assess the performance of ECR in quality evaluation and field applications. In the next step EIS should be tested on-site and not only on bore holes. Furthermore, it should be investigated if the combination of several inspection tools enhances the corrosion detectability. In line with current knowledge concrete structures with epoxy-coated reinforcement are nor inspectable and consequently it is not suitable to propose an inspection guideline as It was one target of the research project.
Publications
- Extent of Cathodic Reaction on Epoxy Coated Rebar with Partially Disbonded Coating, In: Corrosion Conference and Expo 2017 : New Orleans, Louisiana, USA, 26-30 March 2017, New Orleans. S. 2210-2221. ISBN 978-1-5108-4034-8
Kessler, S., Sagüés, A.A., Lau, K.
