Crack in old floor slab
Crack in old floor slab
(OP)
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!
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!






RE: Crack in old floor slab
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.
RE: Crack in old floor slab
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.
RE: Crack in old floor slab
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.
RE: Crack in old floor slab
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.
RE: Crack in old floor slab
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).
RE: Crack in old floor slab
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.
RE: Crack in old floor slab
RE: Crack in old floor slab
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.
RE: Crack in old floor slab
This way you don't have to destroy the existing concrete slab. A structural concrete adhesive can still be injected into the crack.
www.SlideRuleEra.net
RE: Crack in old floor slab
RE: Crack in old floor slab
RE: Crack in old floor slab
RE: Crack in old floor slab
RE: Crack in old floor slab
www.SlideRuleEra.net
RE: Crack in old floor slab
where are you?
RE: Crack in old floor slab
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.
RE: Crack in old floor slab