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Outgassing: It’s Not Just Hot Air


By Warren Brand

I am intimately familiar with the fine art of “eye-rolling.” As the followers of my blog know, I have three daughters, ages 16, 19 and 22.

As you can imagine, I’ve had more than my share of eye-rolling exhibitions over the years. The following is only the briefest of lists leading to the aforementioned eye-rolling:

  • What time will you be home?
  • Please clean up your room.
  • Why are you watching New Girl when you have homework?
  • Why did you lose your phone … again?
  • Please don’t put me into a nursing home when I get old.

And so forth.

I was not, however, expecting to encounter this facial gesture during a meeting discussing remediation of a catastrophic coating failure on a very large concrete secondary containment area.

Bulging Blisters

A multi-coat polyurea system had been installed and was peeling, bulging and otherwise failing sporadically throughout this containment area. And it appeared that the failure was slowly progressing. One day a coated wall would look perfectly normal, and then a few days later, a two-foot-diameter blister filled with water would appear.

Photos: Warren Brand

This is an example of the blistering that comes with outgassing, although much smaller than what we saw on the project discussed here. Photos in this blog post are not from the project described.

We were initially called in to perform a failure analysis. While quite competent to perform a failure analysis, I am not a big fan of doing so unless there’s a compelling reason to perform one. The owner didn’t intend to sue anyone, and there was no other reason, so instead we recommended they hire us to develop a remedial repair plan.

The mode of failure was complex and varied, but mostly characterized by debondment, both adhesively and cohesively within various coats.

In some cases, there were blisters of 2 feet and 3 feet in diameter, some filled with water and some not. There were sections where you could literally grab a chunk of coating and pull off a section 3 feet wide by 10 feet long.

While we weren’t hired to conduct a failure analysis we presumed that the existing coating fell victim to outgassing and other issues while curing.

While we would expect to see blistering from outgassing, we surmised that the coating (which had less than 40 percent elongation and which was applied at between 40 and 60 mils) simply got hard before having a chance to create small, discrete blisters, and instead forced the coating off in large sheets or very large blisters.

Recommended Repair Plan

In our repair design, we suggested using a completely different material with more than 400 percent elongation.

We would identify those areas within the containment area where the coating was sound and well adhered, and remove those areas that were bad.

Another secondary containment area

Although a view of a different secondary containment area, it is representative of the one we encountered in the failure analysis project.

We would apply the new material to the exposed concrete and roughly 6 inches over the existing, well-adhered material.

We specified wet abrasive blasting on all exposed concrete to an ICRI 6 to 8 and provided the contractor with profile chips as comparators. The existing coating would be lightly abraded.

And here’s where the eye-rolling came in.

Temperature Considerations

Part of our site-specific application specification required that the new material be applied to all properly prepared concrete “…in declining surface temperatures.”

We explained that outgassing was a very serious concern (as we believed that’s what led to the current failure), particularly since there was no vapor barrier installed behind the concrete when built.

The applicator was patient, but was struggling to keep his eyes level. This temperature requirement would be logistically difficult. It was the dead of summer—a particularly hot one in the Chicago area—and the surface temperature could literally change as clouds passed overhead and hid, or exposed, the sun.

Our specification necessitated that the contractor work in the late afternoon, as the sun was setting, and, further, actually use an IR thermometer to document the declining temperatures of the surface. (The surface temp didn’t matter when overcoating the existing polymer.)

Blisters or fisheyes

Smaller, elongated holes penetrate throughout concrete slabs; the interconnected capillaries allow water vapor and air to pass through. As the concrete surface cools, the concrete will “breathe” in, absorbing air and humidity. As concrete warms, it will “exhale.”

Now, there are many other times when I’ve experienced the eye-rolling phenomenon, and, I must admit, when I have participated as well. Two cases in point: the cold wall effect and chloride contamination (SCAT testing). While I believe these are real, I think they are highly overrated and take the blame for far more failures than they actually account for.

But outgassing is a clear and present danger whenever painting, coating or tiling concrete, particularly outside in changing temperatures, and particularly if no waterproof membrane was installed or applied to the backside.

The Ins and Outs of Outgassing

Concrete is porous; picture Swiss cheese with smaller, elongated holes penetrating web-like through the concrete slab.

These interconnected capillaries allow water vapor and air to pass through, often all the way through, the concrete slab depending on the formulation and thickness of the concrete. As the concrete surface cools, the concrete will “breathe” in, absorbing air and humidity. As concrete warms, it will “exhale.”

Just a few weeks earlier, the area had been hit by a substantial rainstorm. My home office was hit hard and the rain dampened the floor of our furnace room, a harmless, yet annoying event not uncommon during torrential Chicago rainstorms.

When I went to clean up the puddle, I noticed bubbling from the concrete. Being the material and technical nerd that I am, I immediately grabbed my phone and videotaped this outgassing phenomenon.

I couldn't resist the opportunity to catch footage of outgassing in action after heavy rains inundated my own furnace room.

It was fascinating to watch the outgassing from various points in the concrete floor change in frequency and location as time went on. All the eye-rolling in the world couldn’t stop it.

Proving the Planned Solution

Meanwhile, back at the ranch (aka, the secondary containment area), we were hired to assist in developing a proof of concept (POC), and did so on two separate occasions.

We were going to evaluate surface preparation and the use of a moisture-tolerant primer in conjunction with the elastomeric topcoat. We were also going to perform destructive (adhesion) tests and non-destructive tests as well.

The first POC went well and as anticipated. We were not on site for the second one, however.

When we came back to inspect the second POC, the area was completely and utterly blistered—like the top of an overcooked cheese pizza.

outgassing blister after abrasive blasting

In our repair plan, we specified wet abrasive blasting on all exposed concrete to an ICRI 6 to 8 and provided the contractor with profile chips as comparators. Shown here is an example of an outgassing blister after abrasive blasting.

I couldn’t figure out what had gone wrong until one of the technicians sheepishly told me that they applied the materials in direct sunlight during rising temperatures. They simply weren’t convinced that outgassing was a thing—and the blisters were the proof.

And all the eye-rolling in the world couldn’t change it.

We examined (cut open) a number of these blisters and found that the overall coating system remained intact as a continuous waterproof membrane. The bubble had formed within the top layer, cohesively as it were, but didn’t go through to the substrate.

Although we determined both POCs were intact, both were removed prior to project completion.


Warren Brand

Warren Brand’s coatings career has ranged from entry-level field painting to the presidency of two successful companies. Over nearly three decades, he has project-managed thousands of coating installations and developed specs for thousands of paint and coating applications. NACE Level 3 and SSPC PCS certified, Brand, an MBA and martial-arts instructor, now heads Chicago Corrosion Group, a leading coatings consultancy. Contact Warren.



Tagged categories: Blistering; Coating failure; Disbondment; Failure analysis

Comment from Jim Johnson, (9/1/2016, 10:57 AM)

A couple of decades ago, when we were a coating applicator company, we learned about the out-gassing of concrete. Any contractor who sees those tiny volcano like pinholes learns real fast. What they often do not do fast is learn how to cope with the problem in a successful manner. We developed a standard procedure we used for all warm weather concrete coating projects. These were mostly secondary containment and immersion service sumps and tanks. We started by abrading the surface, which could be done anytime. Then we would return to the jobsite about 10 pm and start by pressure washing the concrete to remove particulate matter. The evaporation from wetting of the concrete would start dropping the concrete temperature. As soon as it was dry, which often was a matter of minutes, we would apply a 100% solids epoxy primer by roller and we would roll it in as well as possible, applying some pressure to push the primer down into the concrete pores. This would be dry to the touch in about 90 minutes and it was noticeable that though over 90% of the pinholes no longer existed, there were still a few pinholes remaining. We then applied a second coat of primer in the same manner and by then it was turning daylight so we quit for the night. When we returned the next night we would inspect that surface on our hands and knees and would still find a few pinholes, which we would fill with a little more primer. Our goal was a 100% pinhole free surface. I'm sure a pinhole or two occasionally slipped by, but we did our best. After the primer dried, which took about 2 hours, we would apply the first coat of the coating system and head home about daylight again. All work was done after the sun went down and before it rose again. After each coat we performed an intimate inspection for any pinholes and any found were filled immediately. This became standard operating procedure and was figured into every bid price. Our projects were very successful and that success bred excellent customer relations. Once we explained the extra cost was necessary for success customers understood and had no problem with it.

Comment from Mark Teakle, (9/1/2016, 11:29 AM)

I struggle a little bit to follow the theory that out-gassing from concrete can cause interlayer debondment. If the cause of the pinholing is totally due to outgassing then the pinhole would have to go all the way through to the substrate. If you witnessed cohesive failure between coats I believe you need to be looking at a range of other factors including the product being applied (some poor formulations have very narrow over coat windows, the application equipment set up and operation and other causes of possible surface contaminantion between coats.

Comment from Warren Brand, (9/2/2016, 9:05 AM)

Jim - we did exactly the same thing here - except we used only one, thick coat of primer, rolled well into the concrete. We knew that there would be pinholes in the primer, but didn't care. We then applied the elastomeric topcoat at 60 to 120 mils. Since both applications were done in declining temps, we didn't really care if the elastomer was adhering to the primer or the concrete. The topcoat could be applied directly to the concrete, but we were using the primer as a belt-and-suspenders approach. We were also concerned about moisture in the concrete. We figured that if there was any outgassing or other issues, it would be visually apparent when the topocat cured, which was less than 24 hours. The system worked like a charm, with only a handful of odd blisters easily identified and repaired. And, further, the blisters did not connect to the concrete - that is, they were cohesive. So even if we left them alone, we would have had a waterproof membrane in place. Mark - excellent observation - and I should have clarified. There were likely multiple modes of failure in this area - it was very large and done at different times applied to concrete with no vapor barrier. What I believed happened that lead to debondment was that a thin (8 to 15 mil) epoxy primer was applied to the concrete (which was poorly abraded and possibly damp in some instances) The epoxy primer was certainly not monolithic and certainly had dozens or hundreds of pinholes per square foot. I believe the primer cured and then when the "elastomeric" topcoat was applied, and cured (I think in less than a few minutes), and then, there was outgassing, the pressure from the outgassing, instead of forming, small, discrete blisters, "pushed" on the entirety of the cured film, causing it to debond into massive blisters, due to poor adhesion of the topcoat to the primer. I also made a mistake and called it cohesive failure, when, in fact, it would be more accurate to characterize it as intercoat delamination, although there were some areas that we also exhibiting cohesive failures.

Comment from HECTOR MEDINA, (9/7/2016, 7:11 AM)

Warren would mind explaining the difference between cohesive failure and intercoat delamination. I didn't get that part of the comment. thanks.

Comment from Warren Brand, (9/7/2016, 8:35 AM)

Hi Hector, my pleasure. Intercoat delamination takes place when one coat of paint does not adhere to another coat of paint. So, if you had a black primer and blue topcoat, the two coats of paint would not stick to each other. A cohesive failure takes place within a single coat of paint, typically thicker materials. So, by analogy, if you had a grilled chees sandwich, and peeled it apart, and ended up having cheese on both sides of the bread, we would say that there was cohesive failure of the cheese. If cheese ended up on only one piece of bread, you would have intercoat delamination between the dry side of the bread and the cheese. Hope this makes sense and hasn't confused the issue.

Comment from Stephen Dobrosielski, (9/9/2016, 8:14 AM)

The grilled cheese sandwich paradigm ... Warren, you made my morning!

Comment from Mark Teakle, (9/12/2016, 5:15 AM)

Hi Warren I believe the root cause of the problem lies in this statement that you made "What I believed happened that lead to debondment was that a thin (8 to 15 mil) epoxy primer was applied to the concrete (which was poorly abraded and possibly damp in some instances) The epoxy primer was certainly not monolithic and certainly had dozens or hundreds of pinholes per square foot." In my 20 years experience as an applicator, formulator and plural component machine manufacturer i can`t say I don`t have problems with pinholing but my experience is that the following factors are critical. 1 - The residual substrate moisture. The % of residual substrate moisture in the concrete will become more of a factor on warming concrete than on cooling concrete however the root issue is residual moisture content. 2- The use of a high quality (read expensive) 100% moisture tolerant epoxy primer. The primer must have a low visocity for good concrete penetration. Always roll the primer into the concrete do not spray apply the primer on concrete. This helps to seal the concrete. Naturally I presume the concrete has been correctly prepared and should be free of blow holes and honeycomb holes from form work. Carrying out the above steps greatly enhances you chances of having a pin hole free elastomeric coating. Substrate prep and repair, residual substrate moisture checks, the correct primer and a high quality top coat product that has not been formulated to a price will in more cases than not provide good pin hole free results. However if you have concrete with high levels of residual moisture (say above 10 - 20%) you can solve this issue by pre-treating the concrete with a lithium based sealer or similar to lock in the residual moisture. It is an extra step but a true "insurance policy".

Comment from Warren Brand, (9/16/2016, 12:42 PM)

Hi Stephen, lol. I got a chuckle writing it.

Comment from Warren Brand, (9/16/2016, 12:47 PM)

Hi Mark, Thanks for chiming in. I'm not sure what the actual "root" cause might be. For example, if it's moisture in the concrete, then it could be because of a failure to install a vapor barrier on the exterior. That also would have reduced or stopped outgassing. Frankly, we didn't bother checking the moisture content of the concrete (I can hear the gasps from here). It simply didn't mater and we assumed it was problematic. So, instead of wasting our client's time and money quantifying how damp is damp, we designed for a worse-case scenario and specified a very high quality, 100% surface tolerant (can be applied to dripping wet concrete) and worked it deeply into the concrete before topcoat application. Project was completed a few weeks ago. So far so good. Only 10 more years to go!

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