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Durability of Exposed Rebar in Elevator Shaft

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KootK

KootK

Structural
Oct 16, 2001
18,590
I'm reviewing another engineer's fix at the behest of the owner.

While pouring the elevator shaft walls, the inside forwork gave a bit. As a result, the walls were cast about 65 mm thicker than planned for about three upper floors. To get the elevator to fit, the contractor ground the extra 65 mm off of the interior of the walls. In some locations, this exposed or nearly exposed the interior mat of rebar.

The specified fix involved two measures:

1) A Sika mix skim coat has been applied to the interior to cover the bars. The manufacturer's min specified thicknesses is 19 mm. In some places, the installed thickness is as little as 6 mm. Essentially, I consider it to be an exposed rebar condition.

2) Some vertical channels have been bolted to the outside of the shaft, presumably to replace interior rebar that is no longer effective.

So my questions are:

1) From a durability perspective, how big of a deal is exposed rebar on the inside of an elevator shaft if that rebar is no longer being relied upon for strength?

2) How well do we like exterior replacement reinforcing? I've seen this done to reinforce beams and slabs often. I've wondered if bolt hole slip didn't render these solutions ineffective because io the strain required to fully engage the externally applied steel.

Thanks for your help.

KootK

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
 
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I am not a fan of feather edging any cementitious product, it eventually fails. Surface prep is key. If the thickness tapers to 6mm, I would consider taking out more concrete to increase the thickness. Have them jackhammer it out using 15lb jackhammers so that the steel rebar does not get damaged and treat it exactly like a concrete delamination patch. The roughened surface will provide a much better bonding surface and combined with the recommended depth will provide a durable repair.

If rebar is exposed on the surface, I would not be too worried as I don't thing the interior of an elevator shaft is an aggressive environment. However, it would be worth it to put a thin coating over these areas to provide some additional protection. Generally cheap and easy to apply an epoxy or elastomeric coating.
 
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Do you even need the rebar?

Is this a high rise core? Or a 4' pit?

How exposed is it to weather really?
 
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The rebar is no longer required as long as the external reinforcing will do its thing.

The wall shouldn't be exposed to weather at all.

It's floors six to nine of eleven total

I'm in the no big deal camp myself. I'm just fishing for issues that I might not have considered.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
 
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Paint. Put painting in the maintenance schedule at 10 year increments and walk away. All you need to stop rust is to deny the bars oxygen or moisture. The right zinc-rich protective paints will do both, in addition to providing sacrificial zinc to corrode first. Yes there are such paints available for protecting concrete. No, I don't remember trade names or have any resources on hand while in Vancouver.
 
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Thanks gentlemen. Do we care if the rebar rusts?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
 
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Rust itself is minor, the progressive failure of the concrete through rust jacking is not.

If the rust is limited to rust spotting and does not extend to delamination, 'No, we don't care'. Once the process of delamination begins, this typically continues much like a cancer and the answer quickly becomes 'Yes' as the stratification of concrete undermines the system's strength and integrity.
 
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KootK:

The missing concrete has compromised the fire protection of the slab. I don't know if that's going to be a problem with your code or jurisdiction. You might want to ask the AHJ if it's of any concern.

Regards,

DB
 
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DBronson,
As I read it, the thickness of the wall has not been reduced from the planned dimension. The issue is the reinforcement on the shaft side.
 
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@CEL. That's just the kind of scary advice I was hoping for. I may persue the paint strategy. It will need to be a concrete painting endeavour. If you're able to suggest something when you get home, that would be great. One tricky bit is that cover is extremely low in some locations 6mm. In such cases the painted concrete may just spall off altogether in the future. Cutting out the rebar where cover is low seems excessive given the exposure conditions.

@Hokie: correct on all counts.

@DB. I had the same concern. Whether it's needed of not, the original fix has all elements externally. Fireproofed.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
 
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The fix for this focussed on restoring tension capacity to what is essentially a shear wall flange. I'm starting to wonder about compressive behaviour. Under flexural compression, the wall will be loaded eccentrically due to the fact that all of the reinforcement will now be on one face (outside). Of course, the unbraced height of the wall is only the floor to floor height.

What's your gut feel guys? Is compression worth worrying about? Note that my scope here is limited to qualitative review. I won't be analyzing the structure to work out loads or anything like that. At least, not yet.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
 
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Nope; Even if there is an unbalanced strength effect, the strength is likely far and away above that required. Return walls also make it quite unlikely that the wall section would buckle as a column.
 
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Was vertical FRP applied to the interior face of the shaft a considered option? Depending on the numbers, it may be possible to delete the proposed exterior channel steelwork.

The FRP would provide both tensile capacity and also durability protection to the inside face. Three layers of carbon in a wet lay up would be only about 2mm thick. Easy to apply over three levels, or more, given it is the inside face of shaft. Guide rails brackets etc for the elevator can still be accommodated with FRP.

But, a coating would be required to cater for smoke/flame and fire issues.

Just a thought.
 
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It's actually smack dab in the middle of the back wall of a three bay elevator bay. Returns are aways off. Probably why the Formwork bulged. The walls would basically have to work as an unreinforced compression member. Still, I'm sure it's fine, as you say CEL.

@Ingenuity: it would have been FRP if it was my affair. Can you elaborate on how it would provide durability to the inside face? The rebar could still rust and rust jack even with the FRP in place right?

Are the epoxy sprays for concrete or rebar sewer? Can you point ms to something.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
 
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KootK said:
Can you elaborate on how it would provide durability to the inside face? The rebar could still rust and rust jack even with the FRP in place right?
Click to expand...

A wet-lay up epoxy resin/carbon matrix will provide a 'barrier' and encapsulate the deficient concrete cover and protect the rebar (from one face only), assuming full coverage over the applicable area/a. Somewhat similar to CELinOttawa idea on 'paint', but the FRP will also provide tensile strength too, and less maintenance.

I recall that a prof from U of T (Mike Thomas, I think) did some research back about 10 years ago on FRP encapsulation to concrete columns for ACI-365. Interestingly, these were columns with preexisting corrosion to rebar and the encapsulation arrested further corrosion. Interesting study - was controversial at the time by repair specialists (engineers and contractors) that have traditionally practiced the removal of corrosion and defective concrete PRIOR to any encapsulation.
 
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Thanks Ingenuity. I'll recommend a paint and an FRP option to the owner, representing different levels of durability.

A question for everyone: if I took a length of bare rebar, placed it on top of the elevator cab, and came back in twenty years to check on it, what would I see? Rust? Good as new? I want to be realistic about the corrosion potential in this environment.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
 
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It would be dirty, maybe a bit rusty, but useable.
 
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I agree; I doubt the bar would even need to be wire brushed before being used. I don't think the corrosion would have progressed enough to have loose rust...
 
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If the same piece of rebar were nearly exposed in the face of a wall rather than sitting on top of an elevator cab, would the corrosion be such that rust jacking would be a plausible concern?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
 
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