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May 29 - June 1, 2018

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?

Selected Answers

From matthew duncan of Florida Dept of Transportation on June 1, 2018:
I would say that one of the primary technical challenges to metalizing concrete is achieving a suitable bond strength. Concrete does not share the benefits that its metallic counterparts have. Varying amounts of cement, aggregates, and different types of aggregates can result in different adhesion values. In fact, the same mix used to construct different elements (footers, columns, struts) can produce different adhesion values. It is recommended to conduct adhesion testing on each different element to obtain corresponding adhesion values. Influencing these adhesion values is surface prep. It is crucial and can be influenced by many factors: blasting angle, pressure (too high, too low), type of media, and how much time is spent blasting in an area. This is especially important in Florida where “over-blasting” exposes the soft Florida lime rock aggregate, thereby reducing adhesion values. That is why it is key to specify the person(s) conducting the surface prep and metalizing to have verifiable experience of metalizing on concrete. Experienced personnel know how to maintain and trouble shoot their equipment, such as emptying water separators frequently (another frequent Florida problem), and not only knowing how to metalize when things are going well but how to make adjustments and correct issues when things go wrong. Another key issue is preservation of the system negative connection. Depending on the type of connection, we have found that 2 coats of 100% solids epoxy does well to prevent degradation of the connection. This is especially important if the system is in a marine environment where high moisture can accelerate the oxidation of the anode material. This oxidation can be slowed down by the application of a suitable topcoat. Initially, I viewed this as strictly a technical question, but feel that it is prudent to at least briefly touch on the administrative challenges of metalizing on concrete. It is the responsibility of the owner of the structure to have clear plans and specifications so that the contractor knows what is (and is not) required. Minimum adhesion bond strengths (we specify 100 psi), minimum and maximum thicknesses (we have found that excessive thicknesses on concrete have a tendency to reduce bond strength), periodicity of QC and QA testing, and very importantly, specific time frames between surface prep and metalizing and between metalizing and top coating (if a topcoat is specified).

From Bryant Chandler of Greenman-Pedersen, Inc. on May 30, 2018:
For cathodic protection to be effective the zinc metallizing must be connected to the rebar, especially rebar closest to the surfaces. How many connection points are necessary? Are original drawings available showing rebar location and QC/QA records or photos available that verify proper placement? Need to verify with NDE the physical location of rebar and that there is good correlation of the original drawings with existing location. Where concrete spalling exists, perform appropriate repairs. Do the concrete surfaces have sufficient integrity to withstand abrasive blasting? If not, remove questionable concrete and apply troweled repair or spray-applied shotcrete, depending on amount of area needing repair. Verify after surface prep that surfaces are sufficiently clean and dust-free prior to zinc metallizing. Depending on zinc thickness required, two or three applications of metallizing may be required. Verify thickness using UT electronic gages. Apply more metallizing if necessary.

From Grant Blohm of Metalizing Pro Inc on May 30, 2018:
There are a couple of challenges when applying zinc metalizing on concrete. The first is availability to measure the coating thickness at random locations. Currently, there isn’t a gage that will measure a metallic coating on a non-ferrous substrate. To overcome this, steel tabs can be attached to the prepared substrate in random locations and a conventional electronic mil gage can be used to check the coating thickness at that spot. Alternatively, small tape tabs can be attached to surface and the zinc metallizing separated from the tape and measured with a micrometer. The obvious drawback is that locations are limited and the applicators know that these areas must be coated to minimum thickness. The other way to back up the coating thickness is to calculate the spread rate for the zinc wire over a defined area and log the wire usage and area completed on a regular basis. Another area that can be problematic is consistently attaining the required  adhesion strength. This is where the lack of proper surface prep can contribute to the adhesion failures. It is advisable to conduct some testing on the concrete structure prior to prepping a large area, or else one may discover that the area is either over- or under-blasted. The other contributing factor to adhesion failures is not using proper spray technique. During the actual application of the zinc metalizing on concrete, it is imperative that the proper stand-off distance and travel speed be maintained or adhesion failures could be frequent. The metalizing application is best accomplished by using trained, experienced thermal spray applicators who are not only familiar with the proper spray technique but are also versed in the operation and maintenance of the equipment.

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Tagged categories: Coating Materials; Galvanized/thermal spray coatings; Zinc

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