Existing Concrete deterioration...

[youngstructural]

Hello All;

Given an older reinforced concrete showing some signs of deterioration, including minor cracking across the members, autogenous healing, pitting and some minor discolouration, do you typically down-grade the concrete capacity? Do you down-grade the resulting member capacities (flexural, shear, axial, etc)?

If yes, what, if any, standard or guideline do you use for this purpose?

This is related to my previous post regarding the capacity of an existing bridge, however I believe it can stand alone.

Thanks,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[msquared48]

With the cracking, I would be concerned with salt intrusion and oxidation of the rebar. I would also be concerned with spalling in freezing weather.

If oxidation of the rebar is present, I would definitely downgrade the structure - no question - subject to necessary repairs to the concrete cover to prevent further degredation of the structure. Oxidation being present, I would consider cutting a core sample over some rebar in a few less critical places showing the same signs of concrete damage, then reducing the capacity considering the area of bar lost. I would also consider the bond strength to see if that was compromised too.

Would it be possible to take a core over one of the cracks to see how far the cracking penetrates?

Mike McCann
MMC Engineering

 

[youngstructural]

Thanks for the reply Mike,

There isn't any particularly obvious rust staining, and with this being on the west coast of New Zealand, there isn't a lot of salt in the air. It also would get regular washes with rain, and there are few, if any, rust stains on the bridge. Freezing weather is also unlikely.

I don't think that the bars are rusting, however given the age, and the slightly orange tinge to the concrete which is otherwise very light gray, I can only presume that the level of carbonation is advanced. However, that does not mean that it is yet of structural concern, and I fully recognise this fact. Concrete gets stronger, but the Ph shift can harm the steel... It just most likely hasn't yet done so.

Coring to review one of the more severe cracks has been suggested, but the client is awaiting the completion of peer review of our report prior to undertaking any work. And I'm trying to explore all possibilities and fully inform myself prior to the next meeting with the external reviewers.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[JAE]

Here's a few thoughts (they correspond to Mike's above a bit too)

1. Oxidation of reinforcement results in the rebar expanding and blowing off the concrete cover. In most cases, if there's not a lot a signficant cracking or spalling, the reinforcing is usually in not too bad of shape.

2. With oxidation comes the loss of bar cross section - but usually at discreet locations (i.e. near a crack or spall). On either side of the concrete distress, the reinforcing bars usually are not as affected. So the loss of section occurs at localized places on the beam or slab and these should be considered when analyzing the members for strength. Usually cleaning off a bar down to gray steel and measuring gets you a "current" diameter and thus a downgraded bar area.

3. Loss of bond/development (per Mike above) is also a concern but with localized damage, sometimes this is a lesser concern if the bars are continuous past the oxidized areas.

4. Beyond reinforcement oxidation, you could also have damage to the concrete itself. With salts in the water, the concrete matrix itself can be weakened. There are numerous ramifications of this but one that you see a lot is where the concrete is calcified a bit and powdery, able to be easily scratched with a hand tool. I'm not an expert in this aspect but I think the chlorides can severely damage the portland cement matrix over time.

5. The fixes are numerous for these things - I'd refer you to the ACI documents here:

ACI 345.1R-06: Guide for Maintenance of Concrete Bridge Members

546.1R-80: Guide for Repair of Concrete Bridge Superstructures

546R-04: Concrete Repair Guide

 

[darthsoilsguy2]

apart from all that was said, i've heard that cracked concrete should be designed with Strength Design not Allowable Stress Design as one of the new changes to code in USA. (don't know how much that matters in kiwiland now)

 

[hokie66]

YS,

From your description, and in the absence of spalling, I would not think the bridge requires downgrading due to the cracking if the cracks are flexural in nature. Shear cracks would be a different thing altogether. Carbonation would be a concern, and I would definitely want to know the pH.

 

[SlideRuleEra]

YS - I have read all of both this and the companion thread. There are a few thing that concern me. All discussion is about "downrating" the allowable loading because of deterioration. This assumes that you know the "safe" load for the bridge if it were in "like new" condition. What is this number based on? Load tests? Detailed analysis of the design? "Grandfathered", because it has worked successfully for a long time?

About the cracks - Are they random? Is there a pattern to indicate that some type of failure is occurring?

An 80+ year old bridge was almost certainly designed using working stress assumptions. Reinforced concrete from that time may not have adequate shear reinforcement (compared to the amount of steel provided to resist moment).

I'm sure your are aware that concrete shear failure can occur suddenly, without prior warning.

As darthsoilsguy2 indicated, working stress assumptions rely on:

the bond between the concrete and the metal reinforcement remains unbroken throughout the range of working stresses

That may not be the case now, considering the cracking.


Several years ago I worked on the evaluation of a very "troubled" reinforced concrete structure. It had both inadequate shear reinforcement and poor construction. The bottom line, was that we did a load test on the structure based on then current ACI procedures. The structure passed, but we assumed that the test load was the "ultimate" load and back figured downward from this known value to arrive an "acceptable" load.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[youngstructural]

Thank you all for your replies thus far...

Hokie: There is some spalling, but it is only just initiating, and is occuring in absence of rust staining. Given that rust is 8 to 9 times the volume of the base metal, it would not indicate any appreciable steel are loss from the bar. Please correct me if this is not the case in your experience. I've attached the relevant photo.

Slideruleera: I have complete drawings of the bridge and have reviewed the shear capacity in detail. I have many times in my experience found existing structures without sufficient shear stirrups/capacity, however not as of yet in New Zealand. I do not know why, but even 80 and 90 year old structures here have very modern, tightly spaced, stirrups, though they are typically not spaced tightly in zones to sustain plastic hinging (unsurprising) and are often open-top style, and therefore often need work due to seismic detailing concerns irrespective of ULS capacity. I really don't know why the Kiwi engineers were are "over doing" the shear regarding the codes of the day, but it seems to be a consistent and useful theme.

The cracks are typically across the sections, very small, and often are bleeding calcium/lime staining from autogenous healing. They are nearly always free from rust staining, with the exception of the top of piers which are heavily stained from autogenous healing across their breaths and showing signs of rust staining. I'll post a couple more for you to get a feel...

Also, regarding the load testing, I am confident that this bridge would pass a load test. That's not really my issue; The fact is that, irrespective of my confidence, I must ensure a certain level of safety in fulfilment of my duty of care to the public. I don't think this bridge would fail under the coal trucks, but I DO believe that it would be operating at less than the code/standard prescribed level of safety. And that is the core of my problem.

Cheers,
And thanks again for all of your help and posts!

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[hokie66]

YS,

I would want to knock out that loose concrete so you can actually see the reinforcement condition. I think some chemical testing of the concrete is warranted. If the pH is getting down below about 10.5, carbonation will have advanced to the extent that the steel will become unprotected by the alkalinity of the concrete, especially in porous areas. Cleaning and coating may be indicated if that is the case.

There are a couple of straight lines on the beam soffit which suggest that consolidation of the concrete under the bars was not very good.

 

[apsix]

YS
I would assume it's corrosion causing the spalling in the photo, until shown otherwise. I wouldn't expect rust staining as it is not in an area with free water to cause the staining.
As Hokie said; check extent of carbonation.
If it is over salt water there would be chloride ingress, but you've ruled that out from being significant.

Hokie
Aren't those lines due to fines loss thru' the gaps between formwork boards?

 

[hokie66]

apsix,

Yes, now that I look at the photo again, I agree with you. The lines are nice and straight and evenly spaced from the right side. The voids would not be so regular if it is lack of compaction. But poor consolidation is still likely a factor in the deterioration of the structure.

 

[youngstructural]

Hokie: I would have definately tried to knock off the covering concrete, however this was an inherited photo from a previous (the month prior) inspection. I was a two-man crew with no access equipment, so was not able to access the underside of the beams up close.

Apsix: I definately think it is corrosion, and there is some minor rust staining, so there's little doubt. You're quite right about the lack of free water to feed the stains, of course.

Just to play devil's advocate: We won't have lost much steel area even with corrosion initiated; Rust is 8 to 9 times thicker than base metal. I still have concerns regarding deterioration and applied the deterioration 0.9 factor. Remember that it is an external peer reviewer who is arguing that the 0.9 is not necessary.

The problem keeps dancing around in my head... I'm being reviewed by an engineer with 40+ years bridge experience, so I'm happy to learn from him if possible, but not to simply defer to his opinion when it flies in the face of what I have been taught and learnt. He keeps insisting that several code clauses should not apply, and that I have been overly conservative. I'm just not convinced. And to boot, his firm has previously reviewed this bridge (without bothering to search out the drawings) and declared it safe, so I'm not completely convinced that his motivations are purely technical.

Thanks again all,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[hokie66]

I see your problem. You could probably dispense with the deterioration factor for today, but what about tomorrow? It looks like the bridge, even though it is old and has performed well and may continue to do so, is entering a period of decline if nothing is done to rehabilitate it. Evaluation of existing structures is not precise science, there is a lot of engineering judgment involved. Stand your ground.

 

[msquared48]

As a corollary to hokie - don't stand you ground on the bridge...

Mike McCann
MMC Engineering

 

[youngstructural]

Thanks Hokie; The coal mine will be using the bridge for ten to twenty years. I think I'm well prepared to stand my ground, and very glad for all of the input.

Thanks all; I'll definately let you know how it turns out. I'm guessing I've already put a dent in their arguments because we met Wednesday and they were supposed to get back to us early this week. Tomorrow's nine days and counting. We'll see how it turns out.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...