Difficult Concrete Soffit Repair

[KootK]

I'm seeking some advice regarding a difficult concrete repair that I'm undertaking. While I'm interested to discuss the possible causes of the problem, we're mostly just in problem solving mode at this time: how best to responsibly investigate and fix this. For context, everything that follows was copied from an email that I sent to our local Sika rep.

My cliff notes:

1) This is predominantly at column locations.
2) We suspect that uncleared snow on the form deck was likely the culprit here, perhaps in combination with vibration issues.
3) All of the stud rails are too low for cover in their current state. We’re contemplating additional concrete cast below the existing soffit level.
4) We have some concern for the snow having melted into the concrete and messed with the water content locally.
5) We have reason to believe that we have this same issue not just beside our columns but above them. This creates a soffit repair access problem.
6) As you can see, these areas are quite congested with two layers of bottom steel, integrity steel in two directions, and stud rails. It’s a lot of stuff to climb over top of with a repair.
7) My instinct with this kind of thing is to chip concrete away at least 1” above the uppermost rebar before coming back with repair material. That, both for repair longevity and for getting a convincing bond between the bars, the repair material, and the existing substrate. As you mentioned, Hilti has confidence in it’s partial depth repair solutions. You’ll need to dust your salesman hat off and see if you can convince me of the same as I’ve no doubt that our contractor would prefer that.

Here’s a rough outline of what the engineer in me has been contemplating as a hard line repair strategy:

1) Sandblast the soffit to clean things and remove any material that might have been weakened by a higher than normal water content.
2) Hydrodemolish the slab through its entire thickness at areas where we will be doing significant repair. This is not how we make friends.
3) Form new, drop panel-ish looking things below the existing concrete soffit to get us right with cover requirements. Vertical dowels etc.
4) Install our repair material from the top. Low/no shrinkage stuff that flows well.

 

[Ingenuity]

Assuming NOT post-tensioned slab. Difficult to tell the depth of the damage. I have chipped denser rebar congestion with success.

Shore slab.
Concrete chip from soffit. Standard ICRI details, like chip behind the bars, remove defective concrete/voids/honeycombs etc.
Place forms, designed for pumping pressure. Include a thickening/drop if needed to make cover work.
Pump SCC from below, and pressurize forms with gate valve.
Strip forms.

Use a contactor that has done this before.

 

[AskTooMuch]

We've used five star structural concrete v/o for vertical application like that. Not that "difficult" to place with trowel.

 

[jayrod12]

Koot,

I'm with you regarding what you'd prefer to do. However, our office does so many soffit style repairs that I'd be amiss to not say it's 100% possible.

Regarding causes, your presumption is as good as any. If it is in fact snow, then I agree with the concern about strength, especially in an area like this where the bottom of the slab is most certainly needing it's compressive strength, I may be able to be convinced if it was on the tension side of the member, but it's not. Maybe a few cores at each location are required to determine actual strength?

I assume you've got a concrete restoration contractor on board already, what does he think about thru-slab repair versus pressure grouting from the underside?

 

[KootK]

Thank you for your help thus far gentlemen.

Assuming NOT post-tensioned slab.

Correct, non-PT.

Difficult to tell the depth of the damage.

Maximum would be about 4" into a 16" transfer slab.

...like chip behind the bars...

How do you feel about sika style solutions that would not chip behind the bars?

Maybe a few cores at each location are required to determine actual strength?

My expectation is that, if you took a core, it would have variable strength over the height of the core / slab thickness. Are there actually test methods that could pick up strength variation at that resolution? I don't know that an averaged core strength is what I need here.

I assume you've got a concrete restoration contractor on board already, what does he think about thru-slab repair versus pressure grouting from the underside

.

No. The location is quite remote and, if we're to bring in a specialist contractor, that may take some doing. Maybe Ingenuity can dust off is passport.

 

[KootK]

With regard to the uncleared snow possibility, I've had several colleagues say something to the tune of "concrete's warm, wouldn't the snow just have melted?". How do we feel about that? This issue is present on a number of columns and, in all cases, the rebar has almost no evidence of cement paste on the underside. Is that common? Does it indicate that there never was wet concrete up against the underside of the bars?

 

[Tomfh]

Chipping behind the bars is better.

Sika products are good and they promise to hang on, but it’s better and more robust if the product is hanging onto the bars. I’ve seen many repairs fall off over the years. The more it’s grabbing onto, the longer it will hold on.

 

[Enable]

What's above the deck we are looking at? Soffit repair is annoying, time consuming, and produces an okay(ish) product and that's only if it is done well. Generally we only default to soffit repairs if the top surface is not available to us (for example under a roof deck with a bunch of expensive overburden) or the patches are minor, otherwise we through-slab.

But your proposal is reasonable for the areas you soffit. In this case you might consider a bonding agent applied to the parent concrete (Sikatop Epocem 110 is our go-to in Ontario), but you need to take care in staying within the open time (about 8 hours in the summer); so the contractor would need to pre-build their forms. But even if you do not go the bonding route you'll have a pretty narrow window on the wetting of the concrete, since you have maybe a day to complete the repair before the SSD condition effectively dissipates (assuming they wet pre-closure though there ways to do that after the forms are up). If you fill with SCC from above make sure you instruct the contractor to core / drill air relief holes lest air pockets be trapped all over the place. Those need to be done at the high spots and before the forms are on / epocem applied. This is a reason pressure grouting from below is sometimes preferred for shear critical areas as Ingenuity indicated.

Let me know if you need the name of another restoration contractor. I know one who does a lot of work in Ontario who has an office in Calgary.

Outside of the lack of concrete those bars look like they are in fantastic shape! Which I unfortunately take to mean we are somewhere inside with expensive shit above.

 

[KootK]

Chipping behind the bars is better.

Thanks for your help Tom.

A clever colleague challenged me on this earlier today. Why is it better to chip above? You know, in addition to just vague notions of better "grabbiness". I invented this explanation on the fly:

1) With a bar at the repair interface, bar bond splitting stresses will tend to pry apart the repair interface.

2) With a bar wholly within the repair, splitting stresses can equilibrate across the repair material and may not tend to pry apart the repair interface. Rather, splitting stresses would tend to pry apart the repair material itself which should be more of a monolithic, concrete in tension condition. In the latter case, most of what you would need from the repair interface would just be horizontal shear transfer.

What do you think?

 

[Enable]

KootK: it's not just about the stresses at the repair boundary but also that the chipping / removal procedures introduce microfractures in the parent concrete at the bar location. So the safe approach is to chip around them, which also aids with the repair staying in-place.

How do you feel about sika style solutions that would not chip behind the bars?

Are they trying to sell you on Sikatop123 or something similar? I would never consider it for a repair size that you are contemplating.

If they have a different product that you can pressure-grout and apply without introducing chipping hammers into the mix (for the reason stated above) I might be into that provided the parent concrete is sound. I also might tapcon and mesh to hold it in place....just because.

 

[KootK]

What's above the deck we are looking at?

It's accessible at the moment. This is the first of two suspended concrete slabs before it transitions to residential wood.

Let me know if you need the name of another restoration contractor. I know one who does a lot of work in Ontario who has an office in Calgary.

Thanks for that and for your help with this in general. Think much, much farther North. Sled dog north.

Are they trying to sell you on Sikatop123 or something similar?

That's where the conversation started with a colleague. I suspect they'll change tack a bit having seen the photos.

I also might tapcon and mesh to hold it in place....just because.

I like that, especially if we go lower than the existing soffit for cover / fire. As you probably know, sika also has some products capable of reducing the cover requirement in some situation.

 

[Tomfh]

Yes having the bars at the interface means it’s easier for the new layer to split off.

But perhaps most importantly, having the new concrete go behind the bars means the new concrete is dowelled to the old concrete around the perimeter of the repair.

 

[dold]

2) We suspect that uncleared snow on the form deck was likely the culprit here, perhaps in combination with vibration issues.

4) We have some concern for the snow having melted into the concrete and messed with the water content locally.

2&4) Yeah, snow on the ground/deck during a pour can have some rather gnarly consequences (in an extreme example):

I'm not sure what the bottom of the slab looked like as it was SOG, but I suspect as you/your colleague noted, the snow melted pretty much immediately and just totally fouled up the w/c of the slab. I'd suspect the voids are more of a consolidation/vibration issue. Note in that last picture - that's bleed water, no precipitation after the slab was poured. That said, how far can the excess water 'rise' up through the mix without any sort of agitation? The slab pictured above was 5" thick, so at least 5", but 16" seems unlikely? I'd definitely want to get a handle on the in place strength of the slab if you suspect there was water/snow on the deck during placement...

3) All of the stud rails are too low for cover in their current state. We’re contemplating additional concrete cast below the existing soffit level.

3) I've seen a similar problem and fix, less serious than yours as this was just out in the midspan of a parking slab. The fix was as you describe: form a soffit, dowel hooked bars into the soffit and form a "patch", if you will. Quasi-drop cap, maybe 6" thick? I think they took Ingenuity's approach in terms of placement of the repair material, sans the gate valves. I can't recall if they cored from the top and filled up the form or if they went in from the beamside. I wasn't the engineer on this project so I'm not sure what considerations the engineers made when approving the fix - just a bystander. My dad did the formwork (only), and the electricians and rodbusters took care of cramming tendons, conduit, and steel into a remarkably thin slab - you can see how that turned out.

Not much advice here, but some pretty pictures

 

[Celt83]

My expectation is that, if you took a core, it would have variable strength over the height of the core / slab thickness. Are there actually test methods that could pick up strength variation at that resolution? I don't know that an averaged core strength is what I need here.

Send some cores off for petrographic testing. ACI 437 may have some other info on in situ evaluation.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[haynewp]

I just got petrographic done on a project. It can give you w/c ratio, ASCM C856 has guidance on it.

 

[KootK]

Thanks again for the help all.

The petrographic analysis sounds good. Can it be expected to provide data particular to various locations within the slab depth as opposed to just an average of the slab thickness as a whole?

How do we feel about the potential for their to be crappy concrete right over the columns? Can that be assessed in some way? When the chipping is done for the repair, will it be feasible to try to chip laterally into that space?

FWIW: our sika rep does not recommend 123 after seeing what we're dealing with. They like the fix that comes from the top and recommend Sikacrete 08 SCC for that. Based on the discussion here so far, however, I'm starting to tilt towards and undersize fix.

 

[KootK]

Also, what's the best strategy for identifying the extent of the loose/weak soffit concrete. Sandblasting?

 

[Brad805]

Sounds like you need to get in a plane and go to site. You might not have great luck with all the testing options if as remote as it sounds.

 

[Ingenuity]

Also, what's the best strategy for identifying the extent of the loose/weak soffit concrete. Sandblasting?

From a NDT perspective, ultrasonic or impact echo. Sandblasting will not work. Hydro would be excellent, but requires 'calibration' etc. so that not excessive concrete is removed.

Practically - and given the remote location - an experienced person (not necessarily an engineer - experienced construction laborers/masons are often talented in this regard) at the end of a 15lb or 30lb chipping hammer will get a good sense of 'defective' concrete.

 

[KootK]

Sounds like you need to get in a plane and go to site. You might not have great luck with all the testing options if as remote as it sounds.

Perhaps. I'm a little gun shy on the travel presently though. I visited the site three weeks ago and came back sicker than I've been in recent memory. I didn't work for two weeks straight after that and now have a wee bit of a cash flow problem. I don't relish a potential repeat.