Top Cracks Parallel to Metal Deck Slab 3

[gramy13]

I have 2.5" LW concrete on 3" x 16 gage composite steel deck (5.5" total) with 6x6-W1.4xW1.4 WWF draped over the supports. The typical deck span is about 15' and the deck was placed in 3-span conditions. The supports below are grouted CMU walls. As expected the top face of the slab has cracked above the supports, due to negative moment. But there are also small hairline cracks that connect between the cracks above the supports (parallel to the deck span) at about a 6' spacing? Does anybody know what may be the cause of these? Temperature and Shrinkage? Thanks.

 

[a2mfk]

pictures?

 

[gramy13]

Picture attached. Deck was also unshored.

 

[a2mfk]

Do the cracks perp to your CMU wall support extend the full span?

How far apart are they? I was guessing 6-8' based on the stud wall in the background.

My initial thought is the same as you, shrinkage, and if the cracks are more concentrated around the CMU wall than probably because it is a restraint against shrinkage parallel to the wall.

Of course there are the usual questions about the concrete mix also...

 

[gramy13]

Thanks for the response.
Yes the cracks perpendicular to the CMU wall extend the full span. They are approximately 6' apart.
What to you mean by "and if the cracks are more concentrated around the CMU wall than probably because it is a restraint against shrinkage parallel to the wall"

 

[Ron]

The wall is not restaining the shrinkage in that direction. The cracks you are seeing that are parallel to the deck span are shrinkage cracks. Restraint is from the deck corrugations.

As A2 notes, check the mix design and delivery tickets as well. If water was added, and/or your water-cement ratio is over about 0.55, or if the concrete has small aggregate and was pumped, then those will be causative factors for additional shrinkage.

 

[Lion06]

I've never had the pleasure of using it yet, but I've always heard that LW concrete wants to shrink more than NW concrete and care needs to be taken to avoid this type of problem. Probably not a big deal if it's getting covered by carpet. Definitely a big deal if they want to retro-plate it.

 

[a2mfk]

Ron- I don't understand, if the deck is attached to the wall, how is that not acting as a restraint compared to the rest of the deck and slab?

But if the cracks extend full span, and spaced as shown, I think your idea that the deck has acted as a restraint makes sense. If they were say hairline cracks and only a couple of feet long, and maybe more closely spaced, then perhaps that would be caused by restraint of the wall?

 

[gramy13]

I agree they are temperature and shrinkage cracks. Thanks for your input. Most of the area is covered by carpet, but the kitchen areas are to receive vinyl tile. I'm going to recommend they flash patch the hairline cracks at those areas, to avoid "ant trails" from showing up. Do you think the flash patching is enough or would you recommend providing some sort of Sika sealant as well? Thanks.

 

[Ron]

A2...if the primary restraint were the wall, some of the cracks would be diagonal, then pick up parallel to the span. Once the negative moment crack occurs, the restraint of the wall is gone for the concrete (but not the deck), since drying shrinkage starts from the top.

 

[a2mfk]

Ron- gotchya.

I'd wait until the last possible moment to let the slab shrink as much as possible, then fill with an epoxy. This will prevent minor movements at cracks from telegraphing up through your flooring, whatever it may be. But there are lots of different methods of sealing cracks. I am not familiar with "flash patching"..

 

[msquared48]

I take it that the metal deck was not welded or button-punched together?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[gramy13]

yes, the side laps were screwed together.

 

[msquared48]

Yea... Shrinkage...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[vaquers]

I tend to think that if you don't have fixed points the contraction shouldn't be a problem.

I would say that because of the lack of top steel upon the wall (perpendicular to the CMU), the slab is working as a simply supported slab. This situation increased the moment at the midspan (it's higher than assuming the slab as a continuous slab). Taking into account the poisson effect, at the top face you are having tension perpendicular to the cracks. I guess that there is no top steel parallel to the wall. Regards.

 

[msquared48]

vaquers:

Yes, but in that the "top slab" over the deck ribbing is only 2.5", there is no real top and bottom steel as there could be with a thicker slab, as the WWF, or whatever, is centrally located for cover within the concrete.

I guess in this case, any WWF could be considered top steel, and, if the deck is composite with the concrete, the deck would be the bottom steel. Without the composite action, any steel in the slab would be both top and bottom steel.



Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask