crack in the parapet slab

[lolobau]

Cracks in the parapet slab

Please have a look on the attached photos. What can be the reason for this crack which appeared already one day after the casting. The parapet slab is supported with a permanent shutter board and has dimensions of approx 0,5m by 6m and is slightly reinforced with Y10 @ 250mm

regards

lolobau

 

[lolobau]

other photo

 

[JAE]

Looks like possibly it is surface cracking due to a very wet (high water content) mix. I see cracking actually parallel to the wall so that isn't constraint cracking. The cracks such as these would not be too deep I would guess but you would want to verify.

 

[hokie66]

Assuming this is the top of a parapet wall (not sure what is meant by parapet slab), I agree with JAE. These are probably plastic shrinkage cracks, which could have been closed while they were still plastic. With such light reinforcement, I would expect drying shrinkage cracking to occur as well due to restraint by the slab below.

 

[msquared48]

Frequently caused by contractors and/or concrete truck drivers adding water at the site to get a better flow rate.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[lolobau]

I also think that this will be just srinkage cracks but what wonders me is that the cracks appear in quite regular sections every 500mm approx. They are also orientated in the same direction, mostly

see attached sketch

or have I done a mistake that I cast such an long and narrow element? shouldn´t it be rather divided with joints? but you don´t want joints in the balustrade slab

lolobau

http://files.engineering.com/getfil...f-811d-462a-b452-72c962b84102&file=cracks.jpg

 

[lolobau]

did you see the add image?

could it be that it was to narrow and to long? so thats why the cracks appeared?

lolobau

 

[Ron]

Agree these are drying shrinkage cracks. Orientation is consistent with bottom restraint. Likely occurred first as plastic shrinkage cracks and lengthened with additional shrinkage.

What were the weather conditions at time of placement and first 24 hours afterward? Was it windy?

 

[lolobau]

To Ron:

yes it was very hot, around 30degree celsius.
You think with the right curing and covering it wouldn´t appear?
how would you cure it? Just keep it wet? and cover with plastic?

but why is the crack orientation just in the one direction? how can this be explained?

lolobau

 

[hokie66]

lolobau,
Cracks occur either before or after hardening, and we don't know which is the case for your parapet capping. If before hardening, they are plastic shrinkage or plastic settlement cracks, and could have been repaired during finishing. If they occurred after hardening, they are drying shrinkage cracks. Drying shrinkage cracks occur not because the concrete shrinks, but because the concrete element is restrained from shrinking, thus placing it in tension.

In your case, considering the weather condition and the many cracks that appeared, I would think these are plastic cracks. Drying shrinkage cracks don't usually occur that close together.

 

[rapt]

Plastic shrinkage cracks will normally only be in the surface concrete, within 25mm or so of the surface. Thye are normally caused by over working of drying concrete and/or bar curing.

Drying Shrinkage cracks will normally be full depth of the concrete and the spacing is normally regular and related to the shape of the concrete. They are caused by restraint to shrinkage as Hokie explained.
Parapet walls (and any thin concrete member poured attached to a longitidinal concrete member) are highly prone to drying shrinkage cracking as the parapet is normally poured against already cured and hardend concrete which has already experienced a large degree of its own shrinkage. As well, the walls tennd to be relatively thin so experience much higher shrinkage than the concrete they are attached to. These cracks are normally regularly spaced and fairly closely spaced at half to 1 times the wall height. Much higher reinforcement ratios are required in thees walls than the minimum required for normal slabs and walls, normally about 3-4 times as much to provide good crack control.

Your Parapet slab would not appear to have the same problem but it is hard to tell without the full details of connections.

 

[lolobau]

to Hokia and Rapt

hey guys thanks a lot for the very detailed explanation. I really appreciate it and it teached me a lot. Thanks


lolobau