concrete column retrofit 3

[NGW]

A client needs to fix a failing concrete column. The problem is corrosion. It appears that moisture from a very humid indoor environment has started corrosion from the bottom and is working its way up the column. The bottom 18" of reinf. is severely corroded, allowing the column to crush and spawl. I was thinking of removing the damaged material and encasing the lower half with a reinforced concrete jacket. It's 14" diam., about 8' tall, supports a concrete beam and is on a 48" sq. pad foundation. It's in a utility room. Using the pad dimension to assume the column load, the column design seems to be more that adequate. It's not part of the lateral force resisting system. The plans provided aren't enough to determine the applied loads.

'Anyone have advise?

 

[hokie66]

You are probably on the right track. Is it possible to post a photo?

 

[msquared48]

Install a dehumidifier too.

Mike McCann
MMC Engineering

 

[NGW]

Hokie,

No, I don't have a photo. The damaged area has vertical cracking, bulging & spawling outside the rebar. The ties are rusted through and broken. The verticle bars,(6)#8's anf (6) #6 ftg. dowels, are really bad. It's like peeling bark off a tree. It's pretty hammered.

I think the extent of the damage is limited to the bottom.
The entire column seems to be protected with Thoroseal or something.

I'm wondering if it makes better sense to cut the entire bottom third off & add an 24" or so pedestal W/ epoxied dowels. There would be a pinned connection I guess. Cleaner, & the rust will be gone.

If this is a common problem, someone should have a better fix.

 

[JedClampett]

I'd like to know a little more about this corrosion. Concrete columns are used in seawater and fresh water all the time. And corrosion is not an issue. Humidity shouldn't bother a concrete column at all. Unless the moisture is entering through a crack or the reinforcing is in contact with the ground. Is there a chemical environment?
A concrete jacket might not stop the cause of the corrosion.

 

[msquared48]

Are you thinking electroysis?

Mike McCann
MMC Engineering

 

[JAE]

NGW,
I think you are on the right track. Typically if a concrete column is not too severely corroded, the vanilla method is to remove the loose and deteriorated concrete, clean the rebar, add rebar or other reinforcing if necessary, and replace the removed concrete.

In your case, it sounds like the corrosion is excessive so removing and replacing a significant portion of the column might be the way to go.

The other response to this is to verify what caused the moisture/corrosion in the first place (per JedC above). If there is moisture coming in from above, then some remediation of the waterproofing is in order. If it is simply a wet environment, and will continue to be wet, then perhaps other measures could also be included such as sacrificial anodes, epoxy rebar, concrete mix corrosion inhibitors (DCI for example), etc.

 

[apsix]

Also condition of the pad footing should be checked. If it's reinforced it is also likely to have problems.

 

[csd72]

As others have stated, it is important to verify the root cause of the problem so that you can come up with a suitable solution and not just a temporary patch.

It may be worth getting a concrete repair company on board, their reps will often come to site and give you advice. They may be able to tell you straight off what the cause of the problem is.

Once the required chemical considerations are taken care of then it is simply a matter of restoring the load path.

 

[NGW]

Thanks to all of you for your input.
I'll go back and take a closer look. I'll find a concrete repair company.
I'll call the original engineer if possible to confirm the loads.
It's definitely going to take epoxy rebar & some kind of seal at he ground level. I don't think the soil is naturally highly corrosive. Could calcium chloride additive have this kind of effect?

Assuming the footing rebar is corroded, what do you think about just making the pedestal large enough to spread the load instead of R&R of the footing?

 

[InDepth]

Here are a few suggestions:
1) If bulging is occurring, I would highly recommend some shoring of the beams be placed immediately. You will also need shoring when chipping out the concrete...especially if you have to chip to sound concrete...be careful how much you chip out.

2) You need to identify the source of the probably...if you dig deeper you will always find other areas/elements of concern. Is it moisture and/or concrete chemical properties. I would request a material testing and investigation program be performed in the area of concern as well as another part of the structure. The question: Is this a localized issue or a global issue? Older concrete sometimes have chlorides from admixtures...and if their is a waterproofing issue...then it gets activated. Need to define extent of problem, and stop cause.

3) Would recommend spraying all concrete in the area with a migrating corrosion inhibitor.

4) Jacketing the column with new concrete is probably preferred in this case. I would also add anodes (pucks) inside the mix. Make the mix expansive with low water cement ratio. Make sure to prep surfaces properly.

5) Call Structural Group (Structural preservation Systems) for help.

 

[NGW]

InDepth,

By preping the surfaces, do you mean acid etch & bonder?

I've alerted the original engineer via e-mail. If this were my project, I'd want to know about it. I'll wait for his response.

Either way, this has been a learning experience. Migrating corrosion inhibitor and anode pucks? I'm impressed.

Thanks

 

[hokie66]

If calcium chloride was used in the concrete, that is your likely problem. If that is the case, I would replace the entire column.

 

[NGW]

Replacing the column would be tough because of all the pipes hanging from the ceiling.

'Turns out there are no plans or calcs. to be found. The project engineer said there was a water intrusion problem about 20 years ago. There are no visible signs like stains on the block walls. I think the room houses pool equipment, pool acid, chlorine, etc. That's probably the cause.

I'll just recommend further investigation, handle the column problem & move on. I'm thinking 24" sq. x 24" to 36" pedestal with epoxied epoxy coated rebar, 4,500 psi type V concrete (maybe with anodes (pucks)). If further retrofit is required elsewhere, this still needs to be done, even if temporary.

agreed?

 

[hokie66]

Agreed. But get rid of all the loose stuff first.

 

[NGW]

Got it.

Thanks

 

[JAE]

NGW - some sources suggest that using epoxy coated reinforcement is not a good idea in repairs where there is still active corrosion due to chloride content in the surrounding concrete.

What apparently is the concern is that if you use the epoxy coating for replacement bars, the oxidation of the bars is actually magnified due to concentration of the electrical current (from oxidation) at the point where the epoxy ends and the original non-coated bars begin.

There are conflicting opinions on this and you should check them out. ACI has a very good repair document out there that discusses this...not sure of what it is but do a search on the ACI website.

 

[NGW]

Thanks JAE. I'll look into it. Do you mean the already rusted bars will continue to rust at greater rate? Or do you mean the bars above the column will rust quicker? I'm sure they have some rust on them even if not connected to the problem rust below.

I think my epoxy coated bars will be some distance from the Rusted bars in the footing. The rusted bars in the column will be gone.

 

[LittleWheels]

Incipient anode effect means that the reinforcement around the edge of the patch corrodes much faster.

 

[JAE]

Yes, what DaveMinter said - right at the end of the epoxy paint on the original bars is where you get the attack.