×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Durability of Exposed Rebar in Elevator Shaft

Durability of Exposed Rebar in Elevator Shaft

Durability of Exposed Rebar in Elevator Shaft

(OP)
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.

RE: Durability of Exposed Rebar in Elevator Shaft

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.

RE: Durability of Exposed Rebar in Elevator Shaft

Do you even need the rebar?

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

How exposed is it to weather really?

RE: Durability of Exposed Rebar in Elevator Shaft

(OP)
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.

RE: Durability of Exposed Rebar in Elevator Shaft

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.

RE: Durability of Exposed Rebar in Elevator Shaft

(OP)
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.

RE: Durability of Exposed Rebar in Elevator Shaft

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.

RE: Durability of Exposed Rebar in Elevator Shaft

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

RE: Durability of Exposed Rebar in Elevator Shaft

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.

RE: Durability of Exposed Rebar in Elevator Shaft

(OP)
@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.

RE: Durability of Exposed Rebar in Elevator Shaft

(OP)
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.

RE: Durability of Exposed Rebar in Elevator Shaft

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.

RE: Durability of Exposed Rebar in Elevator Shaft

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.

RE: Durability of Exposed Rebar in Elevator Shaft

could you use one of the 'epoxy' sprays used in sewer structure applications?

Richard A. Cornelius, P.E.
WWW.amlinereast.com

RE: Durability of Exposed Rebar in Elevator Shaft

(OP)
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.

RE: Durability of Exposed Rebar in Elevator Shaft

Quote (KootK)

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?

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.

RE: Durability of Exposed Rebar in Elevator Shaft

(OP)
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.

RE: Durability of Exposed Rebar in Elevator Shaft

It would be dirty, maybe a bit rusty, but useable.

RE: Durability of Exposed Rebar in Elevator Shaft

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...

RE: Durability of Exposed Rebar in Elevator Shaft

(OP)
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.

RE: Durability of Exposed Rebar in Elevator Shaft

I think only if not a controlled internal climate or in some way it gets wet. Carbonation will cause the loss of the passivating environment of the concrete, but it won't cause corrosion on its own.

RE: Durability of Exposed Rebar in Elevator Shaft

you could call Parsons, Permaform or Sprayroq. These materials are waterproof, stick to anything and I thick they are both a pretty blue color. If you sandblast the concrete and bars, it will seal them from the water that may cause rust. there are other manufacturers. look it trenchlessonline.com

Richard A. Cornelius, P.E.
WWW.amlinereast.com

RE: Durability of Exposed Rebar in Elevator Shaft

I would not be concerned about corrosion in an interior elevator shaft. If you have that kind of potential, the elevator rails and cars will rust first. If you want to coat the area with thin cover, be sure you extend the coating well beyond the exposed bar, else you may promote corrosion by trapping moisture near the steel when water gets in through cracks, etc. It would probably be better to leave the bar exposed/keep thin cover and let the concrete and bar dry between exposures (if the exposure is cyclical.)

BUT, you now have reinforcement which will not be effective, particularly in compression. This will not be fixed by a channel placed on the opposite side of the wall. I doubt the elevator shaft walls would be cast-in-place if they were not needed for lateral force resistance, which is precisely what was compromised by loss of cover. The wall will probably not function properly as a seismic shear wall with this kind of defect. If MIGHT be OK for wind, since the force, displacement, and probability of code-level wind exposure on a short building would be relatively low compared to the strength of concrete shear walls.
--
Also, rust is not like cancer - that is, once you remove the exposure to water and air, the corrosion stops whether you remove the rust or not. Rust only begets more rust when the rust provides a pathway for ingress of water and oxygen. An effective treatment has to seal off the steel from exposure, which is essentially impossible with cementitious materials in weather-exposed concrete.

RE: Durability of Exposed Rebar in Elevator Shaft

(OP)
Thanks so much TX. You're the first to express concern about the channel reinforcing. Can you elaborate on that? In my view:

1) For compression loads, the wall needs to work as unreinforced from floor to floor and the channels are useless.
2) For shear wall tension, the channels on the outside successfully address the rebar lost on the inside. I was initially concerned about effective development length on the channels but that checks out.

The project is in a low seismic zone. For the curious, the attached PDF shows the repair detail that I'm reviewing.

Regarding the rusting of elevator rails etc, are those not often brass or other corrosion resistant metals?

The project is situated in a location where flooding is a relatively common concern. The client fears that, during a flood, there will be stagnant water and thus high humidity within the shaft. Pretty sharp on their part really.

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

RE: Durability of Exposed Rebar in Elevator Shaft

I can't see the channels helping. They will just be along for the ride. I just don't understand the philosophy.

All the elevator rails I have seen have been steel.

RE: Durability of Exposed Rebar in Elevator Shaft

TX: Rust IS like Cancer... I've heard others object to the analogy, but it holds quite well. Cancer does not continue to grown when denied ATP and oxygen any more than rust does when denied water and oxygen. The point of the analogy is not that it is exact, but that it is a simple way to accurately think of the behaviour. Once present, rust is much harder to control and arrest than simply preventing it in the first place. It will also spread form an initial source. Concrete which is "healthy", ie: non-carbonated or salinated, is far more resistant. Mix designs matter as well, just as ethnicity. When you think about it, particularly for someone from a family affected by cancer like myself, the parallels are deeply disturbing.

It isn't perfect, but it is helpful to think of when dealing with rust early on. I assure you that from what I've seen, Kootk will be able to handle the analogy without running off to read about chemo.

RE: Durability of Exposed Rebar in Elevator Shaft

(OP)
Rust is like grapevine trunk disease. I understand it poorly and it makes me want to imbibe faster.

Thanks for your help guys. This has been very educational for me. As I mentioned above, my client is worried about periodic flooding of the elevator pit leading to high humidity and corrosion. Is that a valid concern? I would expect that it would take prolonged exposure to moisture to induce serious corrosion. Getting wet once every five to ten years wouldn't do anything appreciable, would it?

@Hokie: I believe that the philosophy is simply to use the channels as replacement vertical steel for rebar deemed to be compromised by a lack of cover. As I mentioned at the top, I've always had concerns about bolt slip with that kind of reinforcement. But then, I have concerns about a lot of things.

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

RE: Durability of Exposed Rebar in Elevator Shaft

Getting wet occasionally isn't going to do any shaft any good, but the staying wet or much more frequent cycles are what would present a problem.

Note that the moisture level does not have to be particularly high to continue existing rust, but it will slow. There are good charts available relating temperature and humidity to rates of corrosion for various steels.

Err on the side of caution where in doubt. The client will be looking for blood, or at least a token corpuscle, should it all go wrong after they've seeked repair. I know you tend not to want to intervene, but I'd counsel specifying protection with these unknowns.

RE: Durability of Exposed Rebar in Elevator Shaft

If you really feel a need to coat the bar and have very little clearance, you could go with a product like Sika Armatec 110. This provides corrosion protection in thin coatings. Alternatively, a coating like ZRC zinc rich coating will protect the exposed bar sufficiently for intermittent moisture exposure in an otherwise dry environment.

You mention flooding, and I would think that "once every 5-10 years" is a non-issue UNLESS it is salt water/seawater. If it is salty, or has some other source of chlorides or another corrosion promoter, then definitely coat the bar and area of thin cover.

The ability of a channel on the opposite face to do the job of the inadequately bonded rebar seems problematic. I have done a fair amount of repair work and I agree with Hokie on the channels. But if you have done the engineering and computing, then it must be a good solution.

RE: Durability of Exposed Rebar in Elevator Shaft

(OP)
Thanks TX. Sika was already applied before I got to the party. Sika 212.

Quote (TXStructural)

But if you have done the engineering and computing, then it must be a good solution.

Ha! Flattery will get you everywhere. My scope doesn't even include the non-durability engineering side of things. All that I've looked at was how quickly I could "develop" the channels as I was worried that it might take a floor height or something ridiculous like that. As I've mentioned, I worry about bolt slip and strain compatibility. However, similar repairs are regularly done on the underside of slabs by better engineers than me.

@Hokie/TX: you've both mentioned concerns about the channel reinforcing without elaborating on what those concerns are. I've attached a sketch showing how I think things work. If you would review it and articulate your concerns, I would be grateful.

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

RE: Durability of Exposed Rebar in Elevator Shaft

Well I for one don't think 212 is the right product at all... It is not a repair mortar or epoxy coat at all, but a base plate and finishing non-shrink, nonmetallic grout.

I would definitely talk to a Sika rep; they are most helpful. I would have expected Sika 622 or 623.

RE: Durability of Exposed Rebar in Elevator Shaft

(OP)
I'll do that CEL. Thanks for the recommendation.

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

RE: Durability of Exposed Rebar in Elevator Shaft

KootK,
Gut feeling mostly. External reinforcement for composite action is tough to achieve in practice.

RE: Durability of Exposed Rebar in Elevator Shaft

KootK,
The detail shows 4 anchors per channel above and below the questionable stories. Has the capacity of the anchors been checked? (Also, I tend to agree with Hokie66 concerning the fix)

RE: Durability of Exposed Rebar in Elevator Shaft

(OP)
That's actually the only thing that I checked wannabeSE. Great minds think alike.

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

RE: Durability of Exposed Rebar in Elevator Shaft

If the channels are only applied at the floor level with the exposed bars I would lean towards agreeing with Hokie in that they are "just along for the ride". If they are being used to replace tension steel for a shear wall I would think they should extend to the floor above and below and develop the tension into the adjacent floor levels thru bolted connections, similar in action to a hold down in a wood building. (4) bolts also sounds a bit light if we are talking about developing the tension capacity of channels.

RE: Durability of Exposed Rebar in Elevator Shaft

Disregard the first part of my post I replied before looking at the detail.

RE: Durability of Exposed Rebar in Elevator Shaft

(OP)
No sweat Celt83. Someday I'll figure out how to include pretty pictures within my posts so that people can't miss 'em. Your hold down analogy is great; I wish that I'd thought of that. I'd be a lot happier with the channel business if it had something akin to a hold down take up mechanism.

My scope with this is mostly limited to durability. I don't have -- and won't have -- estimated loads to work with. For that matter, I don't even know the depth or extent of the compromised rebar cover. When this first came up, development length on the channels caught my eye even though it's technically not within my purview here. I did a quick check based on the bolts plowing through the channels. To my surprise, that worked so I didn't take it any further.

Considering the concerns expressed in this thread, I decided to dig a little deeper, if just for sport. And I think that there's a problem. The limiting failure mode appears to be shearing of the 1/2" bolts. According to my Hilti catalogue, that comes in at 14.5 kN. The factored yield strength of the channels, at 300 MPa, is about 430 kN per channel. Like you, I've assumed that the designer of the fix intended to use mobilize the entire yield strength of the channels. By my math, it would take 30 bolts to do so.

There are only 23 bolts installed along the entire length of each channel. That means that, at best, only 40% of the channel cross sections can be mobilized (23 x 0.5 / 30). And that would only be the case if the rebar being "spliced" were right behind the channels and nearly in contact at the mid-length of the channels. Things only get worse when one considers the length of the vertical gap being bridged and the likely lateral distance between the "spliced" rebar and the reinforcing channels.

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

RE: Durability of Exposed Rebar in Elevator Shaft

(OP)
So I did a little Sika research and spoke to a couple of their representatives. I thought that the group here might enjoy a summary of that:

1) Per CEL's comment, SikaGrout 212 is exactly the wrong product. Not only is it non-shrink, it's actually slightly expansive and generally requires opposing surfaces to constrain it. Apparently the big issue is successful installation. Somehow, in this case, the installation went off without a hitch. I suspect that the expansive nature of 212 will make it more prone to post-install failure over the long haul however.

2) According to the rep that I spoke with SikaTop 122 would have been the way to go. It works for vertical surfaces, can be applied in ridiculously thin lifts, and has built-in corrosion inhibition. The rep claims that 2 mm of the stuff is equivalent to 6" of concrete from a durability standpoint. I couldn't find anything in print to confirm that however so take it with a grain of salt. Looking at the data sheets for 622/623 (CEL's pick), those appear to be as good, if not better than 122. There are a lot of different products available at Sika. It's entirely possible that there's more than one that's fit for purpose here.

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources