What seal do you use for slab crack/leak?

[planc]

See attached photo. There are cracks in the slabs. Does the Y shape cracks tell whether it's shear or flexural cracks? the right and left side is the longer side compared to the top and bottom (half as wide).

What compound can totally seal the crack? All I know is injecting with epoxy. What other alternatives besides injecting epoxy? Can cementitious compound really fill the gaps? What did you use? Or you just leave the cracks alone?

If I don't seal the cracks. Client said what if his bottle of juice or basin fell down to the floor. Then the occupant below the floor would have the liquid drip on his head.

 

[CELinOttawa]

Slab patterns can be "read" to tell whether they are flexurally governed or majority shrinkage in nature. There is no such thing as a crack that is wholly one and not at all involving the other.

You should look at the Johansen yield lines; they are very simple and predictable and show the classic flexural patterns.

What are the support conditions? Looks at first glance like flexural cracking.

As to your injection question: NO. Injection is never a solution; you need to stop the water. Membrane or integral waterstop. Injection makes clients feel better, and shifts the leaking to an adjacent microcrack, only to reinitiate the cycle again...

 

[planc]

Attached is the support conditions, the right small column is the stairs column.

It is one way slab.

What you think? Shear or flexural crack? There is sufficient reinforcement. No one demolish cracked slabs and pour new concrete again?

And if it is not a roofdeck. You still put waterproof membrane on every floor??

 

[CELinOttawa]

Well that is surprising... From the plans that would not be a flexural crack, but the pattern really looked like a classic yield line pattern.

Regarding the membrane: No, only areas expected to have moisture. Did my eyes deceive me, or is there water dripping through?

Honestly a very strange crack to have occur. Are you seeing or suspecting any support movement? Is there a significant concentrated load on the top of that suspended slab?

 

[planc]

Please see the attached photo of the exact rebar details taken before the 4" slab was poured. The drawn violet Y lines were the cracked lines now. All rebars used were 0.4" (10mm) grade 60. It is one way slab with the top bar at one way edge distant every 5 inches. The other span has same sized rebars every 1 foot for distribution reinforcement.

3" cement and sand topping were put above the 4" slab because it was a roofdeck and the 3" topping on top of the 4" slab were sloped to run the rainwater to the drain. But then 2 years later a thin metal roof was put above the floor/slab. So we removed the 3" topping these weeks. Then it rained yesterday and the water from the front flowed to the slabs (the photo in first message with water drops were below the slabs).

So what do you think is the solution?

1. Put carbon fiber below it?
2. Demolish the slabs and repour concrete?
3. Seal the crack with epoxy injection?
4. The cracks were not wide so the rebars didn't yield, so maybe leave it as it is? If there are yield lines, does it mean the rebars have yielded and the slab needs to be demolished?

What do you think?

 

[CELinOttawa]

You didn't answer about the source of the water...

The water source must be addressed. Holding the water inside the slab is a really, really bad idea. That eliminates your option 1. Option 2 is expensive and potentially unnecessary. Option 3 is only going to shift the cracks to another location if you don't stop the water.

Option 4 works as long as the water stops.

Follow up question: Any risk of freeze-thaw cycles?

 

[planc]

In the floor. There was a portion in the front which is the balcony. The reason water got inside was because all the topping inside were being removed. So it's lower than the balcony topping. But the balcony will be covered as well with roof next week so there is no more balcony and all toppings in the floor will be removed. And so no rain water can get inside anymore because the small balcony would be totally sealed with sealed window and roof.

No risk of freeze-thaw cycles. But if the rebars have yielded already. The slab segments might collapse. If the slab would be demolished. Where exactly would you put the new pour? Right at the edge of the beam boundaries?

After the slab is demolished. Fresh rebars would be put at middle bottom for flexural strength.

 

[Enable]

I absolutely forbid you to even consider demoing this slab because of a few hairline cracks!

First, concrete and cracking are synonymous. If it's field cast it cracked, even if you cant see it. Learn to love it!

Second, epoxy is for non-moving structural cracks and not used as a waterproofing measure (usually). You are looking for a polyurethane injection. As CFLinOttawa noted, they are hit/miss and sometimes they work other times just push water to adjacent cracks. But it'll cost you $500 to get an injector out there and so is usually the first measure but since you are redoing the balcony you probably dont even need to do that.

Third, if you believe it's moving / potentially a bigger issue you put a strain gauge on it and wait. Then think about doing other things after you've reviewed.

Fourth, walk the structure. It'll talk to you and tell you if this is completely unusual or is in line with what is going on elsewhere. This can aid your decision of what to do.

 

[EcoGen]

What @Enable said. Nothing of what you have presented so far warrants demolition.

I'll also add that you shouldn't be sealing the crack from below since it can trap any moisture in the slab if it does eventually get in there. What does the top surface of the slab look like? If you can see the same cracks from above you should rout and seal the crack from above. Or if you're really worried about the smallest drop of water getting through that crack, apply a multi layer membrane coating system on the entire slabs top surface. If you're still expecting some water to come in periodically (like periodic rainwater, not a random bottle of juice which I doubt will do anything) then hydrophobic injections from above may be the way to go.

EcoGen Consultants LLC
Structural Engineers
ecogenconsultants.com

 

[dik]

Not even that... cracking is unusual. It doesn't make sense. Maybe an early overload from construction and the pattern is different than that, too. If it needs to be sealed there are products like Vandex by Euclid that will form a crystalline seal within the crack. Careful, because remedy may be more obtrusive than the little cracks. Spill a bottle of water on the surface to see if it leaks through; I doubt it will.

http://www.vandexit.com/

https://www.xypex.com/about/crystalline-technology

Are those little things on the soffit drops of water from above? I'm surprised. If so, and the cracks are small, then Vandex would do the trick.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[planc]

I learnt from client there was glass partition added before on top of the crack. So it could be from overload. Since cracks formed the Y shape yield lines. That means the rebar at bottom bars have yielded alreary? Why, is it possible to have Y yield lines without the rebars yielding?

The slab top still has waterproof membrame to be scrapped off (after rest of topping of the floor removed). The floor will be made water free so no worry about flooding occuring again.

 

[dik]

The Y shaped yield lines are going to the wrong locations... that's the puzzle.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[planc]

We removed the ceiling covers and noticed there are 3 Ys interconnected. See attached pictures.

Have you seen anything like it? Does it mean the slab form many yield lines and these are already slab segments?

How do you tell normal concrete cracks from the rebars yielding? They are hairline cracks but the even with rebars yielding, the cracks can still be hairline. See next message for the actual locations of the cracks superimposed on the actual rebars photo.

 

[planc]

see attachment and see above message for the description of multiple Ys. All the violet lines are same hairline cracks.

 

[IRstuff]

how is there this much water dripping from the cracks? Are there embedded water pipes in the concrete? Are they possibly broken?

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[hokie66]

You report that the slab is only 4" think. Very thin for 4 layers of bars. But I think the cracking is just restraint shrinkage cracking. The cracks start a bit off the beams on both side, but don't always line up.

 

[planc]

IRstuff, see message on March 14, 2:37pm about the balcony with topping/screed not yet removed and it rained last week, but the balcony will be totally covered next week so no more problem about flooding or any rainwater.

hokie66. The 4" slab has only 2 layers of bars, top and bottom. NOT 4 layers. And it's not restrain shrinkage cracking because after the slab was placed, no waterproof was put for about 5 months and client said it rained many times and absolutely no leak in the slab. So it means the cracking occurred years later and so they were not restrain shrinkage crack.

Let's say (as one of the possibilities) the columns move differently causing cracks in the slabs or just movement from traffic as the front is major traffic. Would the slab still be useable or must they be demolished and replaced?

What is the behavior of slabs with many segments cracked but the rebars didn't yield yet. And how do you tell if the rebars yielded or not? How big is the crack when rebar has yielded?

 

[dik]

and do you have shrinkage steel at right angles? maybe 4 layers...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[planc]

Yes, it's a one way slab, the left and right beam with top rebars are the negative moment bars in the slabs. The vertical rebars in the photo are all shrinkage and temperature rebars. See attached photo again.

 

[hokie66]

plank,

By 4 layers, I mean top and bottom in each direction. That creates a total of 4 layers. 4" is a thin structural slab, regardless.

You cannot rely on anecdotal statements from a client to determine when or how the slab cracked. Drying shrinkage cracking can initiate some months after a slab is cast, and can continue for years. So my opinion stands that these are (probably harmless) drying shrinkage cracks due to restraint of the beams.