Cracks in the concrete intermediate floor, grid-shaped, cast a few weeks ago

[Jack.k]

Hello,
Does anyone have experience with concrete floor slabs? We poured the plate a few weeks ago and I see that cracks have formed in a grid pattern on part of the slab, the part on each side of the stair opening. The remaining slab is okay, what could it be? I am attaching pictures and the reinforcement plan.

 

[Snickarkirre]

Hello! If you measure the squares they are 150x150mm. It looks like it is the carpet that has caused the cracks since there is a grid with straight fine lines. Maybe the carpets have too little täckskickt?

 

[Et1975]

Cracks!

 

[Jack.k]

Hi! If you measure the squares, they are 150x150mm. It looks like the mat has caused the cracks since it is a grid with straight fine lines. Maybe the mats have too little cover layer?

the squares are larger than 150 mm, about 400 to 500 mm, there may be a little concrete on top, about 2.5 - 3 cm. Do these cracks affect the beam?

 

[Jack.k]

Splits in masonry!

Unlikely, then it's good with posts below

 

[Snickarkirre]

the boxes are larger than 150 mm about 400 to 500 mm, there might be a bit of concrete on top about 2.5 - 3 cm.
Do these cracks affect the joist?

Okay strange. They are so perfectly straight and nice, so it feels like they should follow rebar or something. It's probably not settlement cracks but rather some form of shrinkage cracks they should be. No, they shouldn't be a danger because the slab is probably shrinkage cracks as mentioned, so they are likely just on the surface. The question is why they look like they do. There's not a lot of cover but it's approved anyway.

 

[Et1975]

Shows very clear signs of being shrinkage cracks..!

They probably have nothing to do with how much you've shored...

Fresh concrete settles because the fine material in the concrete can't retain all the water. The water is then pressed upwards and heavier particles sink. Where the concrete has more difficulty settling, for example around the reinforcement, cracks can appear.

The cracks are recognized by following the reinforcement pattern. And they could have easily been avoided by revibrating and rocking the concrete - if you had done it in time...

 

[Jack.k]

Shows at least very clear signs that they are settlement cracks..!

They probably have nothing to do with how much you've supported...

Fresh concrete settles because the fine material in the concrete can't hold all the water. The water is then pressed upwards and heavier particles sink. Where the concrete has more difficulty settling, e.g., around reinforcement, cracks can occur.

The cracks are recognized by following the pattern of the reinforcement. And they could have been easily avoided by revibrating and settling the concrete if you had done it in time...

Don't know much about this, but it could be what you mentioned, thanks for the explanation.

 

[Snickarkirre]

Shows very clear signs that they are settlement cracks..!

They probably have nothing to do with how much you've propped...

Fresh concrete settles because the fine material in the concrete cannot retain all the water. The water is then pressed upwards, and heavier particles sink. Where the concrete has a harder time settling, such as around the reinforcement, cracks can then occur.

The cracks can be recognized by following the pattern of the reinforcement. And could have been easily avoided by re-vibrating and settling the concrete if you had done it on time...

I thought you meant that the entire casting with the form and everything had started to settle. That's probably why jack.k wrote that it was propped from underneath. But it was very well explained anyway, so then the problem is solved

 

[Jack.k]

Shows very clear signs of being settlement cracks..!

They have nothing to do with how much you've shored...

Fresh concrete settles because the fine material in the concrete can't retain all the water. The water is then pressed upwards, and heavier particles sink. Where the concrete has more difficulty settling, such as at the reinforcement, cracks can then occur.

The cracks are recognizable by following the pattern of the reinforcement. And they could have been easily avoided by revibrating and mixing the concrete if you had done it in time...

Do you think the slab's strength is affected by what you described, is it dangerous according to you?
I appreciate your comments.

 

[Et1975]

Do you think the strength of the slab is affected by what you described, is it something dangerous according to you? Appreciate your comments

Difficult to say anything in general just based on pictures. Outdoors and other exposed environments, shrinkage cracks can be directly devastating for durability because they lie right above the reinforcement and therefore lead moisture, oxygen, and possibly salt directly to the reinforcement, which can then quickly start to rust.

Indoors in a protected environment, you don't have those problems. But when you have shrinkage cracks, you often have air pockets under the reinforcement, which can have some impact.