Concrete Tank Construction Errors 2

[slasecki]

I have an interesting situation. A contractor poured a concrete tank that I designed (48' dia x 16' tall). The walls are 12" thick and have two layers of vertical bars and hoops. During the initial water test, there were a lot of leaks. The photos of the leaking areas were near the bottom of the tank, and they appeared to have been patched. I was concerned that the concrete had not been properly vibrated and hired a testing agency to perform GPR testing. They located some void areas; however they obtained more concerning data regarding the cover over the rebar. The cover for the vertical bars varied from 1/2" to 6 1/2" (2" was specified).

As the EOR, do I even allow the option for a coating to provide long term protection for the bars? I checked the impact of the new "d" and this is not a structural problem.

 

[SlideRuleEra]

As the EOR, do I even allow the option for a coating to provide long term protection for the bars?

IMHO.. no way. If fact, if you did, the Owner (whom I presume you represent) should have you removed from the project. Demo the tank and have the Contractor do it right... at no cost to the Owner.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[TehMightyEngineer]

Agreed with SRE, I'd document all the deficiencies and let them know what they need to do to remedy it. Most likely demo and replace will be the most efficient. A coating should be added protection or a repair of an existing tank; not something that should be acceptable on a new tank.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[Ingenuity]

Before the tank is deemed to be demoed, I would conduct a few invasive probes (chip and remove concrete cover to rebar) to verify that the cover is indeed in agreement with the NDT testing results. Depending on the GPR unit, its calibration, the dielectric constant, the age of the concrete and the experience of the operator, determining concrete cover to some accuracy with GPR can be challenging.

 

[TehMightyEngineer]

Agreed with Ingenuity, I've done GPR work and most of the time it's quite accurate but I've also have it miss entire bars or have inaccurate measurements of clear cover. Plus, it will provide a good opportunity to document exactly why the tank needs to be demolished so there is no quibbling about whether the tank was acceptable or not.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[RickyTickyTavi]

GPR, at least with new concrete, is not as good at determining rebar depth as with old concrete. The high water content attenuates the signal quite a bit. We used to truth and verify any NDT testing done if it was critical.

 

[kmart30]

I'd love to be the fly on the wall when you tell the owner and contractor they have to tear it down and start over because of a few inches of cover. I'm on your side and agree with your concerns but I have a feeling the news wont be accepted very well...

 

[dik]

I'd also give the contractor an option to remedy it... just to tidy up any legal 'loose ends'.

Dik

 

[TLHS]

I'm almost as worried about the issue with the excessive cover as I am the limited cover. Your bars are basically in the centre. Are you going to be able to hold crack widths to watertight levels like that?

 

[Ron]

slasecki....you indicate the issue is not a structural deficiency. That makes the demo and replace argument a lot harder to sell. The contractor will hire another structural engineer to say it's fine...just coat it. It is what construction defect litigation is all about. As everyone knows, only the attorneys will win.

As the EOR and for the owner, you have to be strong in your argument to demo and replace. Here are a few items to consider....

1. The structure has been compromised, even if marginally acceptable. Check your calcs and have them peer-reviewed.
2. The contractor has forced the owner to incur inordinate maintenance for the life of the structure because of the construction defects. This can be costly so do life cycle analysis and increased costs due to loss of serviceability, including cost of coating and recoating on a periodic schedule. Include corrosion repairs due to lack of proper cover.
3. The owner did not get what they contracted for.
4. The building code has been violated.
5. Loss of serviceability is more difficult to prove than loss of structural integrity.
6. Stick to your guns.

dik makes a good point....you'll have to give the contractor an opportunity to propose a repair....look at it critically!

 

[JStephen]

If you complain that it leaks and has inadequate concrete cover and they should tear it out and replace,
And if they do so and it still leaks due to your design,
Then you could find yourself paying for the next tear-out/redo.
So be real sure your design is good, or else be real clear that the tear-out is due to concrete cover alone.

 

[dik]

Been there, and still have the bruises...

Dik

 

[dik]

Good list Ron, captured for future use...dik

 

[dik]

JStephen:

It may not be that simple... in the same situation, I would check my calcs, then retain another engineer... then put the contractor on notice that there was a serious problem.

Dik

 

[SlideRuleEra]

salasecki - I'll assume that a 48' diameter x 16' high tank is one part of some process at a plant or facility. If so, talk with the Owner about two things:

1) What are the consequences of having a marginally acceptable tank "fail" while in use and taken out of service for repairs/replacement (unplanned forced outage)? Perhaps the tank is not essential or there could be other tanks that can take over... or maybe not.

2) Is the tank's current construction schedule critical for the overall facility? Making (possibly temporary) repairs and putting the tank in service may be required... or there could be time for additional investigation or replacement.

In an industrial application, there are usually other factors to consider than just the tank's technical "flaws".

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Shu Jiang PEng SE]

Leak occurs due to the existence of cracks in concrete. Crack occurs when concrete take too high tension, which is due to the un-appropriate wall thickness in some cases. Are you sure that the wall will work with proper concrete cover for the rebars? You may double check the calculation with ACI 350 or something similar before jump into any conclusions.

 

[IFRs]

Can you build a 48' diameter x 16' high steel tank inside and be done with it?

 

[Ron]

IFRS....my first job out of college was designing and constructing steel tanks, including ones like this. That's exactly the repair approach we would have taken!