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Maintenance Tips

f there is a single point of agreement in the industrial pro-tective coatings business, it is that concrete is porous. Manyfactors influence the porosity of concrete, including the

capillary and gel pores found within the concrete matrix,entrapped air during placement (otherwise known as bugholes),and even entrained air voids from the deliberate addition of air-entrainment admixtures. All said, concrete is a challenging sub-strate for the monolithic application of high-performance protec-tive coatings. If the porosity of concrete is not properly addressedduring surface preparation, coatings applied over concrete candevelop pinholes, blowholes, or air inclusions (foaming), resultingin a discontinuous film that, at best, provides short-term barrierprotection to the underlying substrate.

Surface preparation is always critical to the coatings process.Many industry standards exist to ensure that concrete, both newand existing, is properly prepared before application of a protec-tive coating. SSPC: The Society for Protective Coatings andNACE International jointly developed a surface preparation stan-dard applicable to this process: SSPC-SP13/NACE No. 6 SurfacePreparation of Concrete.

Within this standard is a requirement to repair gouges, bug-holes, and other surface anomalies before coating application.Otherwise, as indicated in the standard, surface air voids, pin-holes, or excessive porosity may affect the application or perfor-mance of the coating. The maximum substrate void size or surfaceporosity that can be tolerated depends on the coating system andthe intended exposures (i.e., mild, moderate, severe). But if voidsare not filled before the coating is applied, the trapped air vaporexpands and may affect the performance of the coating. Trappedair vapor is of real concern for liquid-rich (neat) coatings, whichhave a tendency to outgass or pinhole because of excess porosityat the concrete surface. Cast-in-place concrete walls generallycontain an abundance of bugholes that form during concreteplacement.

If a suitable material is not pushed into these bugholes beforeapplication of the topcoat, air will become entrapped within thebughole cavities and will then be released into the protective coat-ing. This phenomenon is known as bughole-induced outgassing. Toprevent bughole-induced outgassing, a material must be forcedinto the bughole cavity, to displace the air within. Such materialsinclude epoxy fairing/putty patching compounds, epoxy poly-meric mortars, and epoxy-modified cementitious resurfacers (also

Preventing Air-Induced Coating Failures on Concrete By Vaughn O’Dea, Tnemec Company, Inc.

Typical cast-in-place concrete wall containing exposed bugholes(upper half of photo) and application of a greenish-colored, epoxy-

modified cementitious resurfaur (lower half of photo)

Outgassing caused by bugholes that were not remediated before the coating was applied. Photos courtesy of the author

Typical cast-in-place concrete following SSPC-SP 13/NACE 6 surfacepreparation to expose bugholes

I

Continued

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called epoxy modified cements [EMC]). Of the materials commonly used to

reduce the formation of bughole-induced outgassing within a lining sys-tem, epoxy-modified cementitiousresurfacers, in the author’s experience,has proven to provide the most favor-able results. The epoxy-polymer modifi-cation allows the cementitious productto be applied as a thin-patch material

while maintaining its physical proper-ties. In other words, epoxy-modifiedcementitious materials can be applied asa parge coat (a thin coat of cement mor-tar applied to concrete for refinement ofthe surface) at a thickness of approxi-mately 1/16 inch, or even feather-edged down to 1/32 inch. Another ben-efit of the epoxy modification is that thedensity of the cementitious mortar

increases, thereby eliminating the sus-ceptibility of the mortar to outgassingwhen topcoated.

A successful protective coating appli-cation over concrete achieves a mono-lithic film to provide barrier protection.Concrete, whether new or existing, isundisputedly porous; has a surface thatis often irregular; and likely containsnumerous bugholes or other cavitiessusceptible to outgassing, unless thesubstrate is resurfaced before coatingapplication. The key to preventing bug-hole-induced outgassing and other air-induced coating failures on concrete isto fill the voids, bugholes, and other con-crete cavities with a suitable material,thereby creating a contiguous concretesurface before topcoating.

Bibliography SSPC-SP13/NACE No. 6 (latest revision),

“Surface Preparation of Concrete,”

(Pittsburgh, PA: SSPC, and Houston, TX:

NACE).

O’Dea, Vaughn and Savage, Dan,

“Preparing Concrete for Coating: Make

Sure Your Work Is Up to Industry

Standards,” JPCL (December 2005), 34-

38.

O’Dea, Vaughn, “Lengthening Tank Life:

Rehabilitating Corroded Concrete

Structures,” Public Works Magazine

(April 2006).

Feucht, Albert E., “Failure Analysis of

Coatings on Concrete: A Failure

Avoidance Tool,” JPCL (March 2001),

41–47.

Vaughn O’Deais the TechnicalSales Manager,W a t e r /Wastewater forTnemec Co.,Inc., KansasCity, MO. He isan SSPC-certified Protective CoatingsSpecialist, a NACE-certified CoatingInspector, a NACE-certified CorrosionTechnician, and an ICBO-certifiedReinforced Concrete Special Inspector.

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