Crack in old floor slab 1

[TPNY]

I ran into an engineer's inspection report written for my company by the only engineering firm in the area that is willing to do reports for condo conversions.

Before I reach my point, I would like to interject the following: the report was written by a very young, inexperienced, green engineer and signed by his boss, the PE, who never visited the site; needless to say, a lot of cracks were identified incorrectly as were the reasons for the cracks developping. The building is an old matress factory that was turned into apartments in the early 70's. Being the only engineer in the company (also the only one in the history of the company) these reports get thrown at me to evaluate when something looks suspicious.

Now the situation... the engineer identified a long crack in the floor slab running about 3-4" from an adjacent beam and going the entire depth of the slab (the slab was poured monolithically with the beams and girders; two-way slab). The crack -obviously -is from shear failure and most likely occurred during the factory-era. His solution (because he doesn't like to see cracks in concrete) is to cut the whole crack out of the floor slab, and re-pour. When I read this, I was floored (no pun intended)! I thought that practicing professional engineers were more competent than this!!! There are about a dozen easier solutions that would actually "fix" the problem (if it's needed- a thorough analysis has not been approved, so I haven't been able to investigate further).

Has anyone else had any problems like this? I hope I'm not the only one... wait a minute... I DO hope I'm the only one!

 

[mubash]

The slabs in shear do not fail normally as claimed by designers.The failure might have been due to excessive loading that caused shear crack may be heavy point load at some time during factory era.Now it should be seen that if slab is relieved from excess load does it make any difference.For this purpose the behavior of slab crack is to be watched by tell tale.It should also be seen if the crack is not due to less provision of negative steel.
Has the crack made slab shear and surface is uneven from top i.e one side up and the other down?If it is crack not going to widen then stiffening by suitable steel strips be provided by welding to the reinforcement.In this way the excessive shear will be taken into account and I hope it will be ok without pulling it down or recasting it again.

 

[AggieYank]

Assuming by "2 way slab" floor slab you mean a slab on grade, this is what I'd do.

1) Cut out the slab to the beam on one side of the crack, and 18" from the crack on the other side, the whole length of the crack.
2) Drill and epoxy (5000 psi, 2 part mix) slab reinforcing matching existing a minimum of 6" into slab on either side of cut out area.
3) Provide a keyway in the slab on either side (2" x 2").
4) Provide 5000 psi, 2 part epoxy on the entire surface before pouring new concrete.

I'd love to see the eleven other ways that you would recommend.

 

[sigma1]

I just have a question for Zo40.

Why did your compnay had to hire this "not so competent engineer" when they have one on their payrol?

I also agree with "AggieYank" and cannot wait to see your solutions to the problem.

 

[pappyirl]

Not sure what you mean by calling a young engineer incompetent. Especially as it is a viable solution seeing as the problem hasn't really been investigated. The PE who signed off the report has to take alot of responsibility for the content. Perhaps he is the one you should be calling incompetent.

By two way spanning, I assume you mean the slab spans in two directions.

For your solution to work AggieYank, you would also need to reinstate the reinforcement in the direction parallel to the crack as well as the reinforcement perpendicular to it. The slab would need to be backpropped while all this remedial work is being carried out.

I agree with mubash, in that more investigation of the crack and the loading history is required to come to an economic solution. It might transpire that the dowel action of the bars and the spanning action of the slab is sufficient to cater for the lighter apartment loading. If not, there are plenty of 3rd party products on the market for fixing these type of problems, e.g bonded CFR plates.

I have, luckily, never encountered this problem and I don't know what the most economic solution would be. We have a new build slab at the moment designed for car park loading. The client wants to change to a retail space. We are looking into strengthening the slab with the 3rd party products mentioned above.

I agree with you however in that there are dozens of solutions to this problem. It would be interesting to hear which one you finally decide on.

 

[TPNY]

The slab is not on-grade, so pappyirl is correct. The inspection report is for a condo conversion (this is the ONLY engineering group in the city that will TOUCH an inspection for a condo conversion- I believe it's due to the high rate of lawsuits) and they check all aspects of the building, not just the structural elements. Also, building inspections that are inclusive of checking for mold, mechanical units, plumbing, etc, are not my forte.
Furthermore, my group is owned by, but separate from the company, so they just wanted my opinion on this particular instance.
I don't think that cutting the slab would be a good idea, because I don't believe you can economically achieve a better bond between the existing concrete and the newly poured concrete. Also the way to reinforce concrete to resist shear forces is through the use of vertical, or slanted ties. Correct me if I'm wrong, but cutting the slab, repouring the crete and tying it all together with some horizontal reinforcing just turned one joint into two joints. Also, the original horizontal rebar has been cut, and it would take more work to determine if the new, shorter development length is adequate.
As far as the young engineer goes... I don't mean to be rude (we were all inexperienced at one point).

 

[AggieYank]

Zo40. I believe with the use of a 5000 psi, 2 part epoxy, you can achieve a true bond between concrete less than 5000 psi, assuming the concrete is properly cleaned prior to coating with epoxy.

My fix (very similar):
1) Cut out the slab to the beam on one side of the crack, and 18" from the crack on the other side, the whole length of the crack.
2) Drill and epoxy (5000 psi, 2 part mix) slab reinforcing matching existing a minimum of 8" into slab on either side of cut out area.
3) Provide a keyway in the slab on either side (2" x 2").
4) Provide 5000 psi, 2 part epoxy on the entire surface before pouring new concrete.

-This was my fix for the original thought that slab on grade was in place. Looking back though, if this was slab on grade, I wouldn't require #4.

I'd assume that because the PE stamped a fix for it, he has checked the developement length for the horizontal rebar. You could also assume he checked the shear forces in the new slab. Chances are they don't require any type of ties, but it depends on the thickness of slab, concrete type, and loading. I would ask him all this stuff to be safe.

I'd be interested to hear better / more economical solutions.

 

[TPNY]

The PE hasn't checked a thing in detail; he stamped and signed the report, though. He wants more money, and he gave the general cut-and-replace solution without ever visiting the site. I think that the capacity needs to be checked AS-IS, before anyone starts tearing up the slab. I also believe that the load required to create this shear crack in a 4.5" slab most likely occurred during the warehouse phase.

 

[AggieYank]

I agree, without knowing everything, that the warehouse phase probably caused the crack. From what you've said, the crack goes all the way through the slab. I don't know how you would check the capacity of a slab unsupported on one side, or part of one side.

What options do you have other than cut and replace?

The only thing I could think of is pumping some sort of epoxy into the crack to try to "connect" the two sides and close the crack. The problem with this is that the steel has been cut as well, and must be drilled and epoxied. Assuming this is a small crack, you'll have to cut some out just to be able to replace the slab reinforcing.

 

[SlideRuleEra]

Zo40 - If the existing concrete beam that the crack parallels is "heavy" enough, could you fasten a large, full length angle, maybe and 8x8, to the existing concrete beam? The angle should project far enough from the concrete beam to span the crack and support the slab. Concrete anchors or thru bolts may serve as fasteners to the existing concrete beam. An identical angle on the opposite side of the existing concrete beam may be necessary to offset potential eccentric loading.

This way you don't have to destroy the existing concrete slab. A structural concrete adhesive can still be injected into the crack.

http://www.SlideRuleEra.net

 

[TPNY]

SlideRuleEra, you were one step ahead of me... I was just about to respond to aggieyank with the exact same repair. (The concrete beams are 16" deep) The crack can then be filled, as you mentioned. There are some other variations of this type of fix floating around in me head as well.

 

[AggieYank]

thats a good idea SlideRuleEra, but its not a typical 2-way slab if you do that, since its simply supported at the angle, and I'm guessing the existing slab won't work. I guess you'll have to hire another PE to do the proper analysis.

 

[TPNY]

I wish that I could have more pull concerning the decisions my company makes, but our department is not allowed to deal with that particular project (other than opinions, when asked). I will have to ask around to find out what is happening with it. I wish it were all about engineering, but unfortunately it's all about business, money and politics. Maybe nothing is going to be done about it, but I prefer nothing over the fix the original engineer suggested. I think he was just looking for more money for a "band-aid" solution.

 

[andieO]

Here's a situation.....residential project where the owners are now trying to prove blatant breech and fraud against the builder. The slab was poured 7 months ago (yes...I said 7 months) and the cable were not stressed nor cut. The slab has been sitting idle and unprotected. Do I even get involved or what should I tell the owners to do? Is there anything that can be done to guarantee the itegrity of the slab? Since the house is only only framed, they can sue and tear down to the dirt andstart over. Any recommendations?

 

[SlideRuleEra]

andieO - My recommendation: For the Owners, this is a construction management issue, not engineering. If the Builder did not comply with the specification, he (the Builder) has the responsibility of proving (by hiring his own independent authorities) that the slab is either acceptable "as-is" or what must be done to "make it so". Perhaps the Owner's lawyers can make this happen.

http://www.SlideRuleEra.net

 

[smwpe]

Zo40

where are you?

 

[casseopeia]

Zo40, I used to write condo conversion reports when I lived in Chicago. Typically, one or two engineers (or architects) look over an entire building, including structural, roofing, ADA, HVAC, electrical and environmental (mold, asbetos, etc). When I was bidding the work, you could expect a fee of $1,200 to $2,200 for the report. Take your regular billing rate (mine is $175/hr) and divide into that and see what you come up with for hours. BTW, don't forget to take out expenses such as gas and parking. It's not much to work with in the end.

If I were writing this report and did not have the fee to support a full-on structural investigation by an SE, I would included a contingency for a comprehensive investigation and repair design fee in the report. That's it. It would be very difficult to estimate a repair without a proper evaluation. The choices for the report writer are demand more fee, or refuse to provide a guess for the repair, or don't do this stuff anymore. I am also going to guess that neither the 'green engineer' nor the PE that signed the report are well versed in structural issues. Generally they are mechanical or civil engineers, or sometimes an environmental tech. They may be competent in their own field (a long-shot possibility), but not structural repair.

There is a great deal of pressure exerted by developers or their lenders (usually the ones getting the condo conversion report) to have the engineer provide far more report than the fee will allow. My old company eventually had to walk away from this type of work. No one wanted to pay what it takes and the competition from environmental firms was fierce, hence the low fees. Also, many of these reports are done by 'stringers', who are not actually employed by the compny that produced the report. They are individual subcontractors who work out of their homes and may not have a good network to review conditions for which they may not be familiar.

I could go on about the general sleazy state of that industry, but I think I've answered the question.

 

[TPNY]

I am in the Baltimore/DC area