How to consider rebar corrosion in capacity calculations? 1

[GalileoG]

I am evaluating an underground structure which is about 50+ years old and which is still in service. This structure will see a temporary increase in surcharge load and I want to check whether or not this structure is capable of withstanding this new load. I have record drawings showing reinforcing, etc..

I have done a visual survey and there is a little spalling here and there and a lot of effluence. In my calculations, I want to take into account the potential that rebar is slightly corroded and that the full cross-sectional area of the rebar cannot be relied upon.

Does anyone know where I can find guidance that can help me quantify reduction of rebar cross-sectional area based on, say, age or visual condition of structure?

I am designing per Canadian standards but also eager to find out what other codes/standards have to say about this.

Much thanks for your help.

 

[Enable]

It's a temporary increase in surcharge? Why are you not shoring the structure? I live in Canada and repair parking structures for a living. 100 out of 100 times if we are adding load we shore for it especially if it's temporary.

There's absolutely no guidance on what you're asking as far as I know. And frankly what you want to do is not a good idea either. You've done a visual inspection...but there's so much else that you did not see. Is the bar placed in the right location? Are wall thicknesses as intended? Were the walls backfilled with the right type of material? Is an underground stream present that nobody accounted for? Is the concrete strength up to snuff? Were some elements made more rigid than intended and hence will take more load than the designer accounted for? What settlements have actually happened? What events have taken place over the history (i.e. someone driving into a garage column causing stresses to redistribute)?

You want to provide a design value only taking into account degradation of rebar but without knowing those other things? I think that's just plane silly. If something goes wrong in any way, due to any of those unknowns, it's on you because you provided a working value taking into account an as-it-sits condition.

If you are hellbent on doing this the only thing I can think of is A) scan all elements for bar placement that you will be checking and B) perform half-cell testing on the same to get a feel for level of corrosion, which leads to C) guess some value.

 

[SlideRuleEra]

I have record drawings showing reinforcing, etc..

Assume all questionable rebar is one size smaller than specified and perform calculations with the temporary surcharge load. This will give you an idea of the resilience of the design.

If the design fails... the end.

If the design passes... barely? ... or with a comfortable margin?

Then you will be in a better position to decide if you want to spend (your client's) money on additional investigation of existing conditions.

http://www.SlideRuleEra.net

 

[Settingsun]

I agree that a rough sensitivity check is the way to go. You don't have enough info to go into more detail.

My experience is more related to salt exposure than buried structures, but the corrosion propagation period is the minor part of the structure life in that case. The majority of the life is the time it takes for the nasties to penetrate the cover concrete. Once the reo starts corroding in earnest, the days are numbered. I recognise that your load is temporary, but any significant loss of strength needs to be addressed promptly regardless.

 

[bugbus]

Only thing I will add is that any significant pitting corrosion will reduce the ductility of the bars significantly, which may be just as important, if not more so, than the strength reduction from the steel loss. There are plenty of papers on this topic.

I would be careful not to rely on any moment redistribution or hinge formation (or anything in that realm) in the corroded regions.

 

[dik]

I've been involved with the design of dozens of parkades, and all the above is on the mark. The only redeeming feature is that the original design may have been designed using Working Stress Design and Limit States design gives you approximately 10% increase for both flexure and shear. The half cell testing is very useful... it gives you a quick idea of the condition of the reinforcing. Have you checked with the city to see if they have drawings on file? They often have archives with drawings. Are there any drawings available? If not, you have to use a covermeter to determine the bar size and spacing at various locations to see if it's compatible. Is the structure clear span (60' approx) or are there intermediate columns. If adding any additional loading add temporary reshoring.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

Thanks SRE... forgot about that old 'rule of thumb'...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik