Wastewater Treatment Tank structural concrete issues


A few days ago I posted this dilemma on Eng-Tips; however, I only received a few comments so I thought I would post here to see if I get more response.

Here is my post on Eng-Tips:

I’m a PE in a consulting firm brought in on a structural issue at a wastewater treatment facility. After Googling I determined both this site and www.SimpliEngineering.com were sites that could potentially answer my questions. I noticed there are many common usernames between both sites. Are these the same sites, or is one better than the other?

I’m looking for solid pointers from seasoned structural engineers - preferably ones with concrete experience.

Thanks for your input.

Two issues:

  1. We have a coating on the inside of the structure that appears to be failing. We believe the coating to be asphaltic.
  2. Underneath the coating that has failed, we’ve noticed the concrete is soft and friable.

I see that the two sites have some common users. Hopefully I can get some guidance here that I didn’t get on Eng-Tips.



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This is not my field but welcome to the site.
The other site was sold and the new owners looked to be about to make some unpopular changes.
This site was created so that we would have a home if the other site deteriorated.
We’re the new guys on the block and we are growing.

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Not my area for sure, but I think this may have some useful information for your situation: The Basics of Deteriorating Concrete at Wastewater Plants: Tips on Causes, Repair, and Resources

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Welcome to this site. I’m sorry you didn’t get enough response on Eng-Tips to address your issues. As you noted, the sites have quite a few common members…we think we have the cream of the crop of common members on this site :grinning:

The article posted by SuperSalad is quite a good overview of conditions that can occur. Here are my comments to address your specific issues:

  1. Your coating is failing and you believe it to be an asphaltic coating.

Asphaltic coatings are sometimes used for concrete structures subjected to aggressive environments such as wastewater treatment plants. Asphaltic materials application onto concrete usually requires good surface preparation and a primer. When both are not present or poorly done, the coating will fail. Once failure of the coating occurs, the concrete beneath the coating will deteriorate in the aggressive environment.

Since coating failure has occurred, it will be necessary to remove the tank from service, re-route the influent to other tanks temporarily or to portable tanks as necessary, and then remove the existing coating so that a proper inspection of the exposed concrete can be done.

  1. You mentioned some friability in the exposed concrete surfaces.

This is a good example of deterioration of the surface due to either carbonation or chemical attack. Once you have the concrete exposed, you can determine the extent of the issue and take samples for a proper petrographic examination of the concrete.

You can also look at crack patterns in the concrete to see if those need to be repaired individually or whether only a better surface coating will take care of them. ACI 350 gives some guidelines on crack widths so that you can make that assessment.

You can also do a ground penetrating radar survey of the exposed and cleaned surface to determine the rebar pattern and approximate depth. In areas where the concrete cover over the rebar is shallow, you should physically remove the concrete to observe the rebar condition. If it has started to corrode, it will require specific remedial action including excavating the concrete at least 3/4 of an inch below the rebar, applying a rust inhibitor to the rebar and repairing with a high quality repair mortar.

Once you have surveyed the tank’s interior surface you will be in a better position to provide remediation specifications that include necessary crack repair, rebar repair and surface preparation for a new coating. Coal tar mastic is what I have used in the past for such applications. It is quite good for these applications and should give you years of protection for the concrete. One such product is Bitumastic 50 from Carboline (…for disclosure, I have no connection to Carboline other than having used and specified their coatings in the past)

Good luck with your project and if you need more info, don’t be shy about asking here. We have a good group of experienced engineers, some of whom have likely faced your very issues in the past.


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Evan - After following the inspection / evaluation / repair steps that Ron has covered, there are other coatings available to protect concrete from wastewater. Examples are polyurethane and epoxy based products, some containing glass flakes. Here is one discussion: “New Coating Protects Concrete in the Wastewater Industry”

I don’t have experience with wastewater applications but do with the similar problems resulting from processing coal flue gas desulphurization (scrubber) waste stream at electric generating stations.

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Thanks, SRE…you are correct as usual! There are numerous other coatings, I offered that one because of my experience with it.

@EvanS…SlideRuleEra is one of those experienced engineers that I mentioned. His experience and advice are always on target and helpful.

That’s true Ron. Us older ones definitely know the effects of acid.

Thank you all for the thoughtful replies. Your advice will help me immensely on this project. As engineers, we are always learning, right?

Even though it will be my first engagement at a wastewater facility, the help you’ve given allows me to walk in confidently asking the right questions, and have more familiarity with my surroundings and what to keep an eye out for.

Because so many of you hang out on more than one engineering forum, I’ll post back to the other site so others can see these incredibly helpful replies. I’m sorry if I’ve created some confusion between the two sites. It won’t happen again.

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No confusion. It’s all good.

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