cracks in concrete wall: what is the reson for it? 2

[lolobau]

Hi

we spoka about cracks in plaster already. The answers were very clear for the introduced crack patern

now what is the reason when you have the same patern of crack in a reinforced concrete wall?
see attachment

this can#t be shrinkage because it is reinforced Y12 every 200mm each face horizontal and vertical

 

[lolobau]

one more photo

 

[Ron]

Has nothing to do with reinforcement. Has everything to do with poor mix control, poor placement and poor curing.

 

[BAretired]

I agree with Ron. Looks like too much water in the mix.

BA

 

[lolobau]

so actually the same story like in my other query regarding the random cracks in the plaster

But how wrong can you mix the concrete to get it so bad? Then the mix was totaly toltaly out of any propotions but the colour still looks ok.
so you are saying definately to much water?

I thought maybe I shouldn´t have fixed the wall panel (7.0m) on the sides to the columns so that the pannel could still move or shrinke a bit

 

[CELinOttawa]

Colour is next to meaningless, and fixing the edge of the panel or not would not prevent this type of cracking. See the ACI guide "Making a condition survey of Concrete in Service". What you've got there is commonly referred to as Alligator cracking and is due to too high a water cement ratio, or (much less frequently) poorly graded aggregate in the mix.

 

[CELinOttawa]

See ACI 201-1, latest version I have is 201-1R-92. Also useful, and actually far more so albeit much harder to get your hands on is PWGSC BIM - Bridge Inspection Manual.

There are a great deal of other manuals which puport to help identify cracks in service, some with detailed commentary as to possible causes, but I've never found them to be anywhere near as good as the freely available BIM (free and available if you're Canadian at least).

 

[lolobau]

@Cel. thank you for the tips i was looking now in the internet for the ACI document but the only thing I found is 20 pages which seams to be a summary of it.
I was also looking for the Aligator cracks but it seams like they use this word only for asphalt and roads but nor really by concrete walls

 

[lolobau]

I was reading now in the internet a bit about crocodile cracks and some of the reports are even saying that the reason for it is to much cement in the mix. Can this be? So not really to much water but cement? I didn´t know that to much cement coudl also cause such cracks

 

[Ron]

Be careful how much you rely on some of the stuff you find on the internet. The information you find in these forums gets vetted by peer review and cumulative agreement or dissension. Not so in other locations. Some of it is utter bull$hit.

No, this is not an issue of too much cement. It is an issue of poor mix control which, likely, is a high water-cement ratio as CEL noted. It could also relate to poor gradation, also pointed out by CEL. You noted variable mix control but state that the color looked ok. Good and bad concrete can both be some variation of a gray color. Means nothing.

If you are designing reinforced concrete I would suggest that you learn a bit more about concrete technology. You need to know definitively when you have bad concrete.

As another observation, your columns look a bit small for the surrounding structure. Just a casual observation.

 

[darthsoilsguy2]

perhaps overconsolidation segregated the aggregate away and a left a layer of paste with a greater percentage of both cement and water (relative to the original mix design) stuck to the side of the forms in those locations. it appears the cracking is more prominent at the top in the photo. it is easy to vibrate to segregation a very fluid mix whether by water or high-range water reducers (Super-P). looks like a spandrel beam for a parking garage which may already be a pretty cement rich mix for strength reasons to start off. if the job had concrete inspections check admixtures used, slumps, and speak with the inspector about placement methods and consolidation.

 

[hokie66]

I see two types of cracks in your pictures, and they are both shrinkage cracks of different types. The random cracking near the top of the upstand has been discussed appropriately above, but an additional issue may have been lack of adequate consolidation. In the second picture, the vertical cracks are restraint shrinkage cracking. The slab was cast first, then the upstands, and the restraint of the slab caused tension in the newly cast upstand. In your first post, your statement to the effect that "it can't be shrinkage" shows lack of understanding of concrete. Reinforcement controls cracking, it doesn't prevent cracking.

 

[CELinOttawa]

"Reinforcement controls cracking, it doesn't prevent cracking.".

Well said!

 

[KootK]

I've found the CIP series by the National Ready Mix Concrete Association to be handy (and free). They're good, low tech summaries of common issues. The one most relevant to your situation would appear to be Link.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[hokie66]

I was surprised that in the paper about crazing which KootK linked, last revised in 2009, there was a reference to calcium chloride being commonly used.

 

[CELinOttawa]

Sadly it is hard to stop the ready-mix plants from adding Salt. They've been arguing for years that their mixes are proprietary and that they should not have to reveal the ingredients, only meet specified physical performance. Aside from the possible long term issues, salt is a fantastic Admix. Higher, earlier strength, and more workability. Fantastic for a batch plant and the placer, nowhere near so for the owner!

Before someone spouts the industry's propaganda, NO it is not okay to add salt because eventually most concrete is cold weather climates is salt saturated from deicing. That happens later, takes time to permeate, and linseed as well as other treatments can do a great deal to slow such penetration. Salt in the mix is just asking for long term durability issues.

 

[KootK]

It's been a few years since I first read that article and I'd forgotten about the Calcium Chloride plug. For what it's worth, it was not my intention to promote its use. I'll confess that I'm not all that well versed on the issue. Every firm that I've worked with has rejected the use of Calcium Chloride out of hand by way of drawings notes and/or specifications. Not a ringing endorsement...

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[CELinOttawa]

And every firm you've ever worked for has likely had salt added as an admix. My grandfather and great uncle used to taste the concrete to see if it was present; With the fly ash and other exotic chemical admixes (not to mention the caustic nature of the cement itself), I doubt I'm going to be trying that on for size any time soon...

I wish there were a simple site test. I've been looking, but to no avail. I heard that you could do something with pool testing kits, but I don't buy it.

 

[hokie66]

Just curious, what does calcium chloride taste like? Like sodium chloride or similar? "Salt" in the lay vernacular normally means sodium chloride, and I have never heard of that used in concrete.

 

[CELinOttawa]

Quite similar to sodium chloride, I am told. No personal experience! Lol...