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.

 

[planc]

After the slab was cast, cardboard type ceiling was immediately put below. Any wetness can destroy the ceiling, like the ceiling that got water damaged last week.

Anyway. Have you seen other slabs with many cracks like it too?

How do you check whether a rebar has yielded? Have you tried chipping from below the slab and checked if the rebar has yielded?

Also what is the cheapest strain gauges that can be put under the slab to check for micro strain changes?

 

[planc]

Btw.. hokie66, in addition to the above, is it not drying shrinkage cracking only occurs on top of the slab, and not beneath the slab?

 

[hokie66]

Absolutely wrong. Drying shrinkage cracks are often full depth. You may be thinking of plastic shrinkage cracks.

 

[planc]

I can't find actual photos at google of slabs yield lines cracking. I'd like to see how big are the cracks when they are yield lines. Anyone seen or got some photos?

 

[CELinOttawa]

Planc: You're confusing drying/shrinkage cracks which have their backside (bottom) restrained, ie: slab on grade, with suspended slab behaviours... Hokie's dead on.

 

[hokie66]

CEL,

Even for slabs on grade, drying shrinkage cracks are often full depth. The restraining force is on the bottom face, so that face is in tension. While most folks put slab on grade reinforcement near the top, some studies have shown that it works best near the bottom. (Waiting for the brickbats)

 

[dik]

That's why I generally place rebar 2" from the top... you cannot see the cracks on the bottom so easily...

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

-Dik

 

[planc]

Also what if the drying shrinkage cracks occurred in between the main rebars and distribution rebars meaning pieces of it fall down between the rebars mesh? The main bars were distances 5 inches and the distribution rebars every 12 inches. The drying shrinkage cracks could become small pieces enough to fall down between the rebars.

 

[hokie66]

You worry too much. That can't happen due to intergranular friction.

 

[dik]

@planc... that's why it's critical to have your sawcutting done at the right time, 4 to 6 hours after finishing.

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

-Dik

 

[planc]

when i showed the crack photos to another engineer. His first thought was it was structural cracks and said there might not be enough top and bottom distribution  rebars at L/4  for the one way slab and suggest demolishing the slab and inserting bars (see attached photo with the red lines he drawn for alleged missing rebars). But in the design we have sufficient distribution rebars. Do you put much more distribution reinforcements at negative moments supports at L4 for the one way slab?

 

[planc]

See attached grid model of slab.

We know the l long span can bend more. That's why we reinforce the short span s with negative moment bars as it has 1/4 less moment.

My structural friend reasoning (he was not a veteran anyway) was that since the long side can bend more, just like a long ruler. He has to put more rebars at the l side (or parallel to the long side) so it can strengthen more.

But is it not the short span s negative moment bars and midspan positive moment bars can do the strengthening? Or must you really need to put tension bars at the long side at L/4 of s (in addition to putting the so called distribution reinforcement)?