Crack survey

[concreteworld]

Dear All

Upon deshuttering formwork on road barrier concrete casting the horizontal or longitudinal cracks are developing on-site as seen in the attached photo. Its new jersey crack as commonly know.

Anyone possibly ascertain the reasons and preventive measures?

Thanks in advance..

 

[bridgebuster]

I assume this is reinforced concrete. My guess is that during the placement the concrete wasn't brought up to top of the formwork; then, additional concrete was added to bring it flush after the underlying concrete had set up.

 

[Ron]

Cracks occurred while the lower concrete was in a plastic state and the upper concrete was drying, hardening and expanding longitudinally relative to the mass below. The crack occurs at the transition between the two. The top is unrestrained and the lower part is protected from drying.

Do a petrographic examination through the crack and you'll likely see that the coarse aggregate is not fractured in the crack...the cement paste separated from the coarse aggregate as the top dried out.

 

[concreteworld]

Thanks for valuable notes. Are these cracks not due to formwork imperfection ? How can we possibly avoid such cracks from occurring other than to protect the freshly placed concrete from dry wind, sun etc ?

These cracks are occurring on daily basis

 

[hokie66]

That looks like a plastic settlement crack. Lack of consolidation would be my guess. Not only do you need to improve vibration, but probably need to revibrate after some period of time. The lower concrete has settled under its own weight, leaving the top part behind. Something may be restricting the top part from moving down, but I wouldn't venture a guess as to what that may be.

 

[Ron]

hokie66...I agree. I think the top part is being held up by "bridging" caused by drying and initial strength gain.

 

[hokie66]

Could be some bars in the top are hanging it up as well. I wonder if the verticals have hooks at the top, or even if they are made into a "U" shape. Horizontal bars can do it as well, especially if the cage is welded. Perhaps sarabia05 can enlighten us further. I don't think it is a formwork issue.

 

[TXStructural]

Really wet mix, perhaps? High shrinkage, formwork flexibility or displacement, poor consolidation and curing practices, formwork and/or soil extracting moisture from the uncured paste, settlement under the concrete while in the semi-set state. A properly reinforced barrier should not do this if the concreting practices are sound.

 

[hokie66]

Sarabia05,

A followup on this would be appreciated. We would like to know if your problem is solved. Many miles of New Jersey barrier have been constructed all over the world without this type of cracking, so it must be something in your process that is deficient. Settlement of the plastic concrete below top reinforcement, concrete that is too soupy, lack of consolidation, etc. have been suggested. Probably all of the above, but rather than guess, we would like to know your conclusions.