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KTA
Challenge #7
Answer - Acid Etching
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Photograph 1 - Challenge microphotograph
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Photograph 2 - Acid etching solution on concrete
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Photograph 3 - Etched surface surrounded by smooth
concrete
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Challenge 7 (Photograph 1) is a microscopic view of the bubbles that form when an acid etching solution is applied to a concrete floor. Photograph 2 shows the same reaction when viewed without magnification. Acid etching is a method of preparing concrete that efficiently removes laitance and efflorescence, and roughens the surface to enhance coating adhesion. Photograph 3 shows the contrast between the roughened acid etched surface and smooth concrete.
When the acid etching solution is applied to bare concrete, it reacts immediately upon contact. After a few minutes of dwell time, the solution is flushed from the surface until the pH from the acid is neutralized. If bubbling does not occur when the acid is applied to the surface, it indicates that the etching solution is not in intimate contact with the concrete. This could be due to the presence of a curing compound, grease, oil, or concrete additives. As a result, when conducting surface preparation, it is not enough to simply apply the solution to the surface, let it stand for a few minutes, and remove it by flushing. It is critical to make certain that bubbling occurs across the entire surface, and that uniform roughening has been achieved.
ASTM D4260, Standard Practice for Acid Etching Concrete, describes the acid etching procedure. SSPC-SP13/NACE 6, Surface Preparation of Concrete, addresses a number of methods for preparing concrete, including acid etching.
KTA can help you specify acid etching or other means of preparing concrete, and can verify the quality of the preparation in the field.
This was a difficult one because there were no correct answers. The most common response was solvent entrapment, although fish eyes, blisters, pinholes, and outgassing received multiple hits. Moisture contamination and debris in the film were also submitted, but the most creative response was a soap solution used when conducting the wet sponge test. One respondent even suggested that it was the sponge itself. Since no one guessed it correctly, everyone was eligible for the prize. The winner, who provided a very good, detailed explanation of outgassing when topcoating a floor coating, was Jeff Trench of Valspar Flooring.
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