Inadequate Cover Under Mat Foundation 2

[bookowski]

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?

 

[BAretired]

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

 

[MotorCity]

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?

 

[bookowski]

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.

 

[Teguci]

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!

 

[KootK]

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.

 

[WinelandV]

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.

 

[JoshPlumSE]

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.

 

[bookowski]

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

 

[Teguci]

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.

 

[MotorCity]

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

 

[bookowski]

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.

 

[KootK]

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.

 

[WinelandV]

What kind of rational remedy would there be?

An expensive one.

 

[bookowski]

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.

 

[CarlB]

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.

 

[bookowski]

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".

 

[KootK]

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

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.

 

[bookowski]

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.

 

[hokie66]

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.