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Inadequate Cover Under Mat Foundation
2

Inadequate Cover Under Mat Foundation

Inadequate Cover Under Mat Foundation

(OP)
Just got an inspection report in for a 4'-6" mat that was poured 2 weeeks ago. The inspector notes that only 1" of cover was provided instead of the specified 3". Clearly this is not very useful 2 weeks out. I have notified the owner. Two questions:
- Owner is asking how it can be fixed. I hate to say that it can't, but short of pulling up 400 yards of concrete plus a story of walls I don't see any options?
- Beyond notifying the owner is there any other action to take?

RE: Inadequate Cover Under Mat Foundation

That is a tough one. There is no easy way to correct it. Was the contractor advised of the error prior to the pour?

BA

RE: Inadequate Cover Under Mat Foundation

Couple of questions....

If the inspector observed that it was not correct, why wasn't it fixed before the pour?

Any chance it was shrinkage steel that he measured to?

RE: Inadequate Cover Under Mat Foundation

(OP)
I don't know but my guess is that at this point the contractor would say that he was not advised, why would he say otherwise when there's nothing in writing. It's likely that the inspector did tell them but there's no way to know. Going forward I've asked for an official go/no go from the inspector at the end of each day.

Definitely not temp steel, multiple rows of #9s.

RE: Inadequate Cover Under Mat Foundation

1" cover is enough structurally, but not enough to protect the reinforcement from low pH soils. Maybe propose cathodic protection with some kind of high pH soil treatment? Yikes!

RE: Inadequate Cover Under Mat Foundation

Any chance you've at least got a mud slab under it?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Inadequate Cover Under Mat Foundation

Perhaps the way forward is for the contractor to apply a lengthy warranty to that concrete? 5, 10, 20 year? (not sure what's reasonable). That way the project keeps moving, but the contractor isn't off the hook.

RE: Inadequate Cover Under Mat Foundation

So, the issue here is mostly corrosion protection, right? The capacity of the slab is not immediately adversely affected by this change in cover.

This is way outside my area of expertise. However, I would think that something along the lines of the following could be done (hopefully by someone more familiar with the topic of corrosion than myself):

1) Establish some estimates about the degree of cracking and the rate of corrosion that will occur in reinforcement.
a) This may require some chemical evaluation of the soil / site to determine how corrosive the water leaking into the cracks will be.
2) Determine at what point the slab becomes truly damaged or ineffective for its estimated use.
3) Explain to the client the expected reduction in design life of the structure / slab. Let the client (and lawyers) determine where to take it from there.

RE: Inadequate Cover Under Mat Foundation

(OP)
No mud slab - I wish. That alone was probably a sign of little quality control. Architect specified waterproofing as crystalline admix.

Cathodic protection on contractors dime - that is interesting. That would really make them feel the pain.

Same job/contractor as the honeycomb wall I posted a while back (that was all torn out by the way) http://www.eng-tips.com/viewthread.cfm?qid=401760

RE: Inadequate Cover Under Mat Foundation

Feel the pain - nothing. This is an end of company mistake. ACI allows a protection system to replace concrete cover if the building official agrees that the protection is equivalent (commentary next to ACI 318-7.7). Coring 54" (53"?) to get to the rebar is going to hurt. But it won't hurt nearly as much as using a concrete eraser on 400 yds of concrete. Propose it and then try to get yourself to the sidelines on this one as soon as possible. You might want to inform your insurance provider as well in this stupid litigious society.

I believe you can monitor a cathodic system, so this might actually be an acceptable long term solution.

RE: Inadequate Cover Under Mat Foundation

Maybe pressure grout below the slab (i.e. a post installed mud mat)? Might be tough to do without lifting the mat

RE: Inadequate Cover Under Mat Foundation

(OP)
I've never done cathodic and don't know much about it - but, the top and bottom bars are essentially continuous in this thing. There are u-bars around all the ends, the chairs are #8 bent bars etc., I wonder if they could use the top bars and somehow verify continuity through the bottom. Although it seems like to do that you still need to get to the bottom to verify.

RE: Inadequate Cover Under Mat Foundation

Could one get cathodic protection to the bottom middle of something this large? Or would you have to core holes into the mat at intervals? At large mats, I usually specify 6" of cover. One is pretty terrifying.

I hate to say it but I think that the concrete has to come out. I don't think that anyone other than the structural engineer should be blessing this as acceptable. And for you to assume the liability of doing that seems ridiculous. I have a hard time imagining an educated owner wanting to do this any other way either. Say you monitored it and discovered problems in ten years' time. What kind of rational remedy would there be?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Inadequate Cover Under Mat Foundation

Quote (KootK)

What kind of rational remedy would there be?

An expensive one. :)

RE: Inadequate Cover Under Mat Foundation

(OP)
Rational remedy in 10 years = fill the subcellar with concrete + lots of dowels into the walls and it becomes the new mat.

I really can't imagine removing it.

RE: Inadequate Cover Under Mat Foundation

In how many places did the inspector measure the cover? What cover does the contractor claim was provided? I expect on a typical project the cover varies a lot more than we think, so maybe it's actually 1"-4" with an average of 2". Perhaps the contractor can propose a program to measure actual cover provided, at least around the perimeter.

RE: Inadequate Cover Under Mat Foundation

(OP)
I don't know - I have not addressed any of it with the contractor yet, just notified the owner. My guess is that the way this will go is "oh yeah, he told us and we lifted the bars up after he left - all good".

RE: Inadequate Cover Under Mat Foundation

How large of an area are we talking about in plan here?

Quote (Bookowski)

Rational remedy in 10 years

It's a shame that there's no way to sawcut some PT down into the mid-depth of the raft without interrupting the perpendicular topside rebar. If only one layer of several would be compromised, you could probably make up for that fairly easily.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Inadequate Cover Under Mat Foundation

(OP)
Photo attached, decent sized area.

Owners already alerted the contractor, as expected they say the inspector is wrong and they will meet with him (I'm sure to hang him over a balcony until he changes his mind) and produce pictures showing the cover. We'll see.

RE: Inadequate Cover Under Mat Foundation

No wonder this sort of thing happened if "Architect specified waterproofing as crystalline admix". There's your first sign of a project out of control. I would just reject the concrete based on the inspection report. Another sign of a project out of control is that the EOR is not in direct control of the inspection. What is this thick mat supporting, anyway? If it is a high rise building, I don't think a 20 year warranty by a shoddy contractor does any good.

RE: Inadequate Cover Under Mat Foundation

If the inspector can't certify that his initial inspection report is in error, one thought would be to have the contractor core the mat in a couple location (selected by SEOR with inspector input) and measure the cover. If they balk at coring that depth, let the contractor propose another method to expose the reinforcing.

RE: Inadequate Cover Under Mat Foundation

I don't know that this would be a compelling argument but... corrosion would not occur if there is not sufficient oxygen and I wonder if there would be a way to determine the oxygen levels under that mat as a way to substantiate the argument.
I don't know about low Ph but maybe that too could be determined.

Is 3" cover required for corrosion protection or is it required (for concrete placed directly against soil) as a way to be more certain that there is sufficient encasement to get bar development? (maybe both).

Seems like coring and evaluation the in place condition would be a good step before abandoning the sea in place.

What is the slab supporting? Big building or is this just for buoyancy resistance? (I've seen that before).

RE: Inadequate Cover Under Mat Foundation

What did the contractor use to support the bottom mat of steel? Did the rebar fabricator provide 3" chairs with sand plates or was some other support provided? I am not saying the inspector was mistaken but I wonder how could this could happen if 3" chairs were provided? Wouldn't this be called out on rebar shop drawings?

RE: Inadequate Cover Under Mat Foundation

Did anyone take a photograph of the steel prior to pouring concrete?

BA

RE: Inadequate Cover Under Mat Foundation

(OP)
Back to this one. In the meantime: I advised the owner & owner's rep about the issue which they took very seriously. They notified the CM that they needed to propose a fix or show that this report was incorrect. The contractor chose to proceed in the meantime. Now we have walls and a couple of elevated slabs and the special inspector has just produced photos clearly showing deficient cover. These are photos without a tape or any reference but it is clear that we are looking at 1/2" to 1" areas, better in other areas. We're back at square one and the owner is demanding that the contractor prepare a proposed fix.

At the end of the day even if they submit signed and sealed repairs I'm on the hook. A tear out at this point is a huge deal so I'm not keen on rejecting a good idea to prove a point - but I'm having trouble imagining what their good idea could be. Is pressure grouting below the mat going to help/feasible/reasonable? Seems like the coring would need to be very frequent to ensure reasonable coverage. Combined with cathodic protection?

RE: Inadequate Cover Under Mat Foundation

In the interest of preserving your sanity, could you:

1) Ask that a corrosion specialist be roped into the project at the contractor's expense.
2) Get the corrosion guy, contractor, and geotech at the same table.
3) Have the corrosion guy sign off on the fix with you nominally approving whatever they come up with?

This might ease your liability concerns and reduce the perception of your firm being the "bad guy". Basically, you delegate the bad guy job to someone more qualified.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Inadequate Cover Under Mat Foundation

Elevator pit is recessed into the footing, right? Any chance that you're confident that at least the rebar below/around the pit has adequate cover?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Inadequate Cover Under Mat Foundation

(OP)
Basically yes to elevator. That is my sleep at night card. The issue (at least as noted) is only in the area around the pit. If the rebar were to completely disappear in this area and the mat become ineffective it would mean that the surrounding soil becomes overstressed in bearing, i.e. I discount this portion of the mat. Since the bearing was controlled by lateral combos this means that in service we're still in ok shape.

I don't plan on giving them this out though since this is a major screw up and don't want to take it on my shoulders.

The corrosion/repair specialist is the route I am suggesting.

RE: Inadequate Cover Under Mat Foundation

2
(OP)
Update in case anyone ever finds themselves in a similar boat. We're doing cathodic protection, compared to any type of tear out or partial tear out it is substantially cheaper, minimally disruptive to the ongoing work, and comes with a service life statement from the engineering company designing it.

RE: Inadequate Cover Under Mat Foundation

Sounds like a decent outcome all things considered. Thanks for the update.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

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