Unknown cracks

[Matt_Struc]

Hi all,

I have designed a suspended slab over a basement (so one level). After formwork was removed the slab showed hairline cracks in numerous places particularly around small plumbing openings. The cracks do not follow a pattern that I know, they show near supports, in mid-spans, on the face of beams, radiating out of walls.
Now the slab is a mix of many systems (flat plate, two way slabs on beams, one way slab on walls and combination). All slab types exhibited same cracks. I have ruled out the possibility of these cracks being structural due to flexure or shrinkage and what seem to me as dodgy workmanship.
It has been 6 months now and the slab is fully loaded with two storeys of brick and timber and the cracks seem as they are. Only obvious down effect is that the cracks drip water during heavy rain which to me it is a longterm problem that needs to be addressed now. The slab was poured during heavy rain and it rained continuously for a week after
What do you guys think these cracks are, interested to hear your opinions.

 

[rscassar]

Do you have cylinder breaks and know the timing on when the formwork was stripped and was the slab re-shored for a sufficient time allowing the slab to attain sufficient strength. Was it subjected to construction loading higher than the design loads? How was the contractors supervision of concrete works, did they monitor slump testing and reject concrete? My first check is strength, make sure strength design is sound. Given they seem to be diagonal and random and not in straight lines is good cause it lends itself to restraint cracks. And check the weather conditions on the days following the concrete pour. Was it hot and windy?

 

[jimstructures]

Matt Struc,

Would you go back and edit those photos by turning them so the surface which is the ceiling in the pictures is oriented so the top of the picture is located correctly at the top of the page. Or at least put an arrow which indicates which way is up.

I think its rude to ask the observers of this forum to have to figure all of this out without obvious references in the pictures. If others feel it is obvious which way is up, then I will bow to the majority. Some sort of scale in the pictures would also be helpful.

Jim

 

[jayrod12]

Why have you ruled out shrinkage cracking?

 

[Matt_Struc]

Jumstructures,
You are right, it is wrong to post it that way, sorry I did not check the preview and I was previously checking the photos on my phone. I have updated the post.
Rscassar,
The cracks appeared when the formwork was stripped at 28 days. Since then in 6 month the slab was fully loaded and the cracks have not progressed in any way. The structure on top is a brick veneer two storey timber structure so there is no shoring schedule. The weather conditions were horrible, very heavy rain for a whole week when they poured and during pouring. Any concrete test was not done during the pouring, however generally the factory has good quality control, but yes site sampling should have been done, but contractor is new in the game and completely missed it.
Jayrod12,
personally I have ruled out shrinkage cracking as I have done many slabs bigger than this before without any cracks. Normally I go a bit over moderate crack control when I reinforce my slabs as well.
These cracks are literally every where and in all sort of random spots. Near columns, mid spans, next to walls, you name it and in all directions and length. some are really, others propagate specially near penetrations.

Thanks,
Matt

 

[Enable]

Definitely neat. My first step would be to core the slab to check the compressive strength and to see if the specimens exhibit any layering or other phenomenon of interest.

Do you know if the forms were oild / what condition the face was? It's not uncommon for concrete to adhere to old forms, which can provide unintended restraint during hydration that can cause cracking.

 

[dik]

Cracks look OK to me... what was the slump, and is there a waterproof membrane over... if not there should be, else it has failed. Is the dripping water in the vicinity of the light? Cracking appears to be minor flexural and/or shrinkage cracking. What was the slump of the concrete... you have some fairly rigid elements in proximity.

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

-Dik

 

[Matt_Struc]

Enable,
Yes I have asked the contractor to take three core samples and test for compressive strength. To my amaze the contractor is unwilling, saying the problem will be fixed once they waterproof the top which they will. But I will not sign final certificate without the cores, as you mentioned they are a big factor in knowing the state of the slab. But on the plus side the slab deflection after full load has been applied is within the design predictions.
dik,
Yes in the exposed areas on top it will be waterproofed, and in habitable floors, it has got a layer of cement screed and tiles over it.
The formwork was not oiled, and apparently a condition was to use brand you plywood. My senior said the same thing, about the difficulty to remove the deck under. But would it really cause that much damage?
As you have mentioned yes there are lots of rigid elements. The cracks appears in spans of 7m on columns and even down to spans of 3.5m between two long rigid walls which made me rule out flexure cracks.
If they are shrinkage cracks however, then why are they in very random formations and not perpendicular to any span direction? Also no cracks on the top surface of the slab.
Sorry for posting the questions, but I have asked every engineering around me and for six month and I haven't got an answer that can make me lay this down.
More info:
The entire slab is fairly large at 37m by 25m resting over retaining walls along most of its edge.
Slab thicknesses vary, from 350mm to 150mm.
I have also asked the contractor to consider epoxy injecting the cracks in the benefit of the longevity of the structure as I believe waterproofing has the potential to deteriorate with time and fail eventually. Again contractor refused this proposal.

 

[hokie66]

Slabs bearing on walls around the perimeter. Sounds like restraint shrinkage cracking, made worse at the soffit of the slab as it was cast in water on the forms.

 

[bones206]

Another vote for shrinkage cracks.

If the cracks are propagating towards plumbing penetrations, that is a sign of shrinkage restraint cracks IMO. It may look like a random pattern, but the slab is just relieving the shrinkage stress along the paths of least resistance. Once one crack forms, the slab finds the next path of least resistance, which may be in a completely different direction depending on the geometry of the restraints and of the slab. Usually penetrations are along the paths of least resistance because they act as perforations.

Looks like the rebar is doing it’s job keeping the cracks right, but I agree with dik that preventing water intrusion from the topside is a priority to maintain the durability of the slab over time. If you don’t correct that first, then crack injection is a waste of time. Without a topside waterproofing barrier you will be chasing leaks until the end of time. Although I am a little confused how the rainwater is getting onto the slab in the first place…

 

[Teguci]

Probably not the problem, but make sure your concrete spec checks and excludes ASR.

 

[Matt_Struc]

Thanks everyone for your invaluable insights. I am starting to lean toward shrinkage restraint cracks aggravated by bad and wet weather conditions.
Still a bit doubtful about it since there different slab types with different slab restraints, thicknesses and spans. Yet all those slabs exhibited similar patterns of cracking.

 

[jayrod12]

I'd be forcing testing of the in-situ concrete strength. Even though the actual tests from the original pour may come back meeting spec, as you indicated it was pouring rain the day they cast the slab. Did they fully clean and dry out the formwork? Did they protect the area from rain infiltration during the pouring? If the answer to either of those is "I don't know", then you have no idea if the in-situ concrete matches the tested concrete. You may have a concrete at the top and bottom of your slab that has a higher W/C ratio than the mix design. Resulting in increased shrinkage and potentially less strength.

Don't let the contractor bully you into not requiring a couple of cores. If he's flat out refusing, perhaps he's got something to hide. I can't count the number of times I've seen a concrete test get completed, and then the contractor stick a hose into the concrete truck to make the concrete more "workable".

 

[MIStructE_IRE]

I would have guessed shrinkage… until you said that the slab still leaks through when it rains! This suggests that the cracks are right through the slab? If so, there’s a more serious issue here with regard to shear capacity etc.

I’d insist on cores/cylinder testing. I’d also take a few cores through the crack to see if you can determine how deep the cracks are.

 

[dik]

Just as an aside... compressive strength, other than for shear is not likely useful. As is not affected for low amounts of rfg. I've encountered problems with cores more often than not...

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

-Dik

 

[hokie66]

Agree with dik that compressive strength means little here.

A correction of one misconception...restraint shrinkage cracks are almost always full depth.

 

[dik]

bang on...

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

-Dik