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
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
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
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
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
RE: Durability of Exposed Rebar in Elevator Shaft
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 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
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
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
@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
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
RE: Durability of Exposed Rebar in Elevator Shaft
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
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Durability of Exposed Rebar in Elevator Shaft
@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
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
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
RE: Durability of Exposed Rebar in Elevator Shaft
RE: Durability of Exposed Rebar in Elevator Shaft
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Durability of Exposed Rebar in Elevator Shaft
RE: Durability of Exposed Rebar in Elevator Shaft
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Durability of Exposed Rebar in Elevator Shaft
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
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
All the elevator rails I have seen have been steel.
RE: Durability of Exposed Rebar in Elevator Shaft
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
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
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
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
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
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
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Durability of Exposed Rebar in Elevator Shaft
Gut feeling mostly. External reinforcement for composite action is tough to achieve in practice.
RE: Durability of Exposed Rebar in Elevator Shaft
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
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Durability of Exposed Rebar in Elevator Shaft
RE: Durability of Exposed Rebar in Elevator Shaft
RE: Durability of Exposed Rebar in Elevator Shaft
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
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.