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October 7 - October 13, 2013

Zinc metalizing has been applied to concrete surfaces to provide cathodic protection to rebar in the concrete. What are the problems, if any, with applying zinc metalizing to concrete, and what is the best approach to addressing them?

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Selected Answers

From Willie Mandeno of Opus International Consultants Ltd. on October 17, 2013:
Problems include 1) maintaining an electrical connection between the zinc coating on the concrete surface and the rebar; 2) keeping the zinc bonded to the concrete due to acid generated at the anode; and 3) preventing the zinc from being passivated as the surface pH changes. A better approach is to use purpose-designed internal sacrificial anodes to provide cathodic protection or inert external anodes coupled to an impressed current system. Another alternative, if chloride penetration or carbonation have not yet reached corrosion initiation levels at the rebar, is to apply an external barrier coating system such as an elastomeric, high-build acrylic to reduce entry of contaminants into the concrete.

From Simon Hope of Bilfinger Salamis on October 16, 2013:
Main issues are related to the degree of cure of the concrete substrate, retained moisture and surface contamination, all of which cause adhesion problems if the levels do not  meet those specified for the coating method. Zinc is used as a sacrifical coating on the external of concrete strucure with rebar reinforcing that has a known potential to corrode. The zinc acts as an anode and as such needs to be in direct electrolytic contact with the rebar, which means that there needs to be some residual moisture within the concrete to enable this. Surface contaminations such as laitance, oil, grease and old coatings need to be removed by methods such as abrasive blasting, which also provides a mechanical key for the applied zinc system. Zinc comes in a variety of forms for this type of application, either as a thermally sprayed metallic wire or powder, or more easily applied as a liquid coating with an extremely high proportion of entrained metallic zinc powder, normally over 95% by weight. There are no major issues as to applying the coatings, but what must be remembered is that these coatings are not barrier coatings but are sacrificial and as such need to be monitored regularly for degradation and will be required to be built up when depleted; otherwise, protection of the rebar will be impaired.

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Tagged categories: Cathodic protection; Concrete coatings and treatments; Rebar; Zinc

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