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Old 05-28-2009, 11:30 PM   #1
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slab cracks


the contractor did not do curing to the 150mx5m slab. There appeared zig zag cracks.

Are these shrinkage cracks

What is the remdy

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Old 05-29-2009, 12:53 AM   #2
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slab cracks


Not enough information provided!!! What do you mean that they didn't "do curing"? Curing is a chemical process in concrete, not something that the contractor does. There are curing compounds such as soybean oil that are sometimes sprayed to prevent evaporation of the water from the upper surface of the concrete, but whether or not they're needed depends on a multitude of factors.

Understand that all concrete cracks to some degree. Whether or not the cracks are visible depends on a number of factors. Mix design, time from batching to placement, finishing techniques, temperature on the day of the pour, wind, whether or not spray cure was used (not necessarily a requirement), slump of the mix (water content) and reinforcement or lack thereof all play a major part.

Tiny cracks in the surface are often indicative of premature evaporation of the surface moisture, or over-finishing and bringing too much slurry to the surface. But it could be a lot of other things.

Remedy? There isn't one. You get what you get with concrete, especially with concrete that isn't properly placed and finished.

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Old 05-29-2009, 02:48 AM   #3
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Thanks for reply
This is in Dubai where Temp is high. At the time of curing may be 35 degree C. crack width is 1.2mm. M30 concrete with normal designed reerbars are used There were four days holidays when curing was completed. No oil is used Curing I mean no water was used. Now when water is spread water leaks from the slab directly down. circuler samples were taken after 39 days at the location of cracks. And cracks were through and through (200mm thick slab). I must add same size 150mx5mM slabs (2 no) with same specifications in next block are without problems.
Third party checker commented that expension joint was to be introduced. Slab is discontinous from both sides.

I hope i hve given complete picture. any thing more pl ask

thanks and regards
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Old 05-29-2009, 07:15 AM   #4
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slab cracks


'mite's correct in all he posts but, in my experience, the most common cause of the crks you describe is the lack of properly installed/placed/located/depth of control joints,,, the most common method of installing them is by diamond sawing,,, the current spec is t/3 ( thickness multiplied by .333 OR 1" deep in a 3" slab - 2" deep in a 6" slab - 3" deep, 9" - etc )

as water hydrates from conc during the curing process, extreme tension stress is introduced to the conc as all conc likes to be square in relation to its thickness,,, UNLESS the control joints are done correctly, the conc will relieve the tension itself thereby forming randon cracking,,, this random cracking will probably new act as ' working joints ' similar to contraction jnts.

steel reinforcing primarily adds flexural strength but not compressive,,, hi temp placing & curing, wind, sun, etc adds more influence on plastic conc which must be considered when timing the cutting of control/contraction jnts.
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Old 05-29-2009, 09:46 AM   #5
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Dear yesitsconcrete

I do not agree with you as 100 % same slab is casted at same location, at sime time, by same contractor. And there are no cracks. Then why in this case?

And please suggest remedy if any ?
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Old 05-29-2009, 10:50 AM   #6
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Yesitsconcrete mentioned something that is very valid and in all my rambling I failed to mention it even though I was thinking about it! I couldn't agree with him more!

Unfortunately, as I stated, there is no "remedy". You're on a commercial project as a structural engineer/project manager according to your post in the introductions subforum...I'd suggest that your company take action against the subcontractor and/or reject the work.
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Old 05-29-2009, 03:38 PM   #7
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slab cracks


Quote:
Originally Posted by sandeep View Post
Dear yesitsconcrete

I do not agree with you as 100 % same slab is casted at same location, at sime time, by same contractor. And there are no cracks. Then why in this case?

And please suggest remedy if any ?
The second part of what Yesitsconcrete said was that there are many variables that also affect cracking. I believe this is why sometimes one slab will crack and a seemingly identical slab will not crack. I would suggest that you look carefully at the differences between the contractors methodology between the slab that cracked and the one that didn't. Was there a temperature difference at the time of the pours? Was there a difference in the amount of water added? Pouring concrete in a very hot environment like you are in can tempt a contractor to use a lot of water in the mix. My limited experience as a do-it-yourselfer suggests that pours with more water are more inclined to crack.

You really won't know for sure unless you do a detailed analysis of future pours. Track every bit of data: time of day, temperature, wind speed, base compaction, water content, slump, mix, additives, reinforcement, curing methodology, etc, etc. Then monitor the results for cracks. Something might jump out at you right away or it may take many pours to determine what is causing the difference.

I have heard many folks say that all concrete cracks but I have also seen many slabs that have not cracked. And I also know that some contractors frequently have cracks and others rarely have cracks. As KC said, you could kick it back to the contractor. If he has to replace it chances are he'll figure out how to prevent future cracks much faster than if it costs him nothing. ....Or he might just decline future projects....
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Old 05-29-2009, 05:31 PM   #8
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slab cracks


Surely this is an attempt at humor, albeit not very funny. This is a DIY site primarily for residential scale construction, the post is about a large scale commercial project. Anyone running a commercial project who solicits information from a DIY site like this is either wasting their time, our time, or is a child looking for some distraction. I suggest censoring this thread, probably the poster as well.
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Old 05-29-2009, 07:39 PM   #9
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I've got my eye on it Daniel, thanks. The post is informative about concrete regardless of the OP's scope of work, and as long as it stays that way there's no need to censor it. The OP is in Dubai, UAE and is new to our site. I've offered guidance to him on another thread suggesting that this site probably isn't the best place for him to find information, given the scope of his commercial projects.
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Old 05-29-2009, 08:32 PM   #10
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I really hope this goes well.

Not too many 35 story buildings going up in Dubai right now that are at the 9th level. Here's one of them. This photo was taken this week. Not exactly DIY, but interesting.
Attached Thumbnails
slab cracks-dubai-marina.jpg  

Last edited by Aggie67; 05-29-2009 at 08:40 PM.
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Old 05-30-2009, 06:07 AM   #11
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slab cracks


that only illustrates ignorance as no 2 slabs are EVER the same,,, wind, temp, grade type, moisture barrier, grade moisture, mix design, sun, clouds, sun, who pee'd in the corner, who stepp'd on the mesh, who didn't vibrate,,, the only constant, from 44 floors up in nyc to flat-on-your-*** dubai where i've also plac'd conc, is experience,,, there are some concrete common denominators but the main ingredient is experience.

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