IS IT IMPORTANT TO USE STRUCTURAL JOINT

[structuralll]

DEAR STRUCTURAL
I have a flat slab without beams 10m*90m , rectangular , dose i have to use a structural joint in that

 

[youngstructural]

Flat slab on grade without grade beams, or is this a flat slab reinforced concrete structure?

You can have a large flat slab like this in either case without structural joints, but it takes compensation for temperature and shrinkage as well as general concrete cracking through other means. Shrinkage compensating admixtures, post-tensioning, proper concrete mix design, etc.

Curing will not reduce the total shrinkage cracking, but in fact may delay the onset of the total plastic shrinkage component. Often that leads to greater cracking once components are "locked in" on site.

Post more details; There are lots of people around to help!

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[structuralll]

yes its a reinforced concrete over columns and the cracks happen ,it was a real cracks , do you have an idea how to deal with that slab

 

[hokie66]

From your second post, I assume the slab is built and has cracked. It is impossible to know from your limited description whether the cracking is significant. If you can describe the structure in terms of thicknesses, size and spacing of columns, any restraining walls, reinforcing details, etc. perhaps we can assist.

 

[structuralll]

thers a deep cracking 1 mm it shown in slab 300 mm thickness with columns every 6 m on edges

 

[hokie66]

Do the cracks go across the 10 metre direction? How much reinforcement exists? Are there restraining elements near the ends?

 

[structuralll]

dear , it works along 10 meter , but in fact theres no reinforcement in top x or top y , the reinforcement exixt in bottom x,y only becouse it works ìn posative only , and the code her accept with no reinforcement

 

[youngstructural]

Sounds like you don't have reinforcing to restrain the counterflexure which occurs accross / adjacent to the columns... I would have expected cracking if I saw the drawings prior to construction.

This is a very old and well known problem.... If you prop a flat slab, it will behave as if there were beams along the shortest connecting points between columns. The cracks won't be as big, or deep, or as well located, but they will occur.

Whether or not they are significant depends on too many factors to try and help you with my limited experience. Sorry, and good luck, but I'm just an observer from here on in.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[hokie66]

With no top reinforcement, you would expect flexural cracking in the top of the slab, generally across the slab at the column lines. These cracks would not extend through the slab. Any cracks which extend completely through the slab would probably be direct tension cracks caused by restraint. If this type crack is 1 mm wide, that would indicate that an inadequate amount of shrinkage control reinforcement is present. It would be interesting for us to know your location.

 

[structuralll]

Thanks for your nice answer youngstructural and you have a real nice answer, and thanks to hokie ,you also born in 1966 , it was a very sharp answer and i think you give a real suggestion ,if there is any answers fOR treatment of cracking in flat slab ,i mean that happen according to temperature , we appreciate your suggestion

 

[hokie66]

structuralll,

Thanks for the compliment, but 1966 is the year I graduated from University. You need to determine first if the cracks are structurally significant before you consider any remedial action. Without further description of the structure, I am afraid that I can't be of further assistance.

 

[apsix]

Photos and drawings would help

 

[structuralll]

Dear i ask about the cracks . they replay that it hapen after casting in few weeks where the jacks still bneth the slab and during the sumer the cracks become bigger till it reach 1 mm , ( its unloaded slab )

 

[hokie66]

If the cracks occurred while still continuously supported, they would be direct tension drying shrinkage cracks. But if the formwork was removed, the repropping could have been insufficient to prevent flexural cracking.

I would expect the cracks to become wider in cooler weather, and have no explanation for why they would widen in hot weather.

 

[structuralll]

what is your opinion for injecting these cracks with epoxy , or we must waiting till real winter

 

[hokie66]

I have no opinion about filling the cracks without further knowledge of the structure and the reasons you have for crack repair.

 

[rapt]

structuralll,

Maybe you should listen to what others are saying/asking. You have to determine if the cracking is structural or restraint induced. Then you have to decide if the structure is adequate. Just epoxying the cracks will not help it if it is not structurally adequate.

Is the slab still supported on formwork/falsework?

Is there any top reinforcement at all, in either direction?

Is it reinforced or prestressed?

Are there any edge beams? Is the slab acting in one way or 2 way action? If it is 2way action, there has to be top reinforcement to make it structurally adequate. If it is not prestressed it is too thin to span 10m simply supported!

No-one on this list and no responsible engineer can help you without a lot more information, no matter how nicely you think they have responded.

Also, a 300mm slab is too thick to reinforce in one face only for shrinkage and temperature.

 

[structuralll]

Dear ,i make a recalculation to the structure by using finite element method , i find the reinforcement is adequate according to loads but normally when i design a flat slap i put a reinforcement in top and buttom in two direction , but this slab in some parts reinforced in bottom only , there is a big gap in reinforcement for the top ,it reach to 5 square meters , it not usual to me but its accepted according to the stresses in the plate , on the other hand and according to what i find there is no temperture or shrinkage reinforcement , the cracks happen when the jacks still lifting the slab , of course the cracks must be bigger,, , but i thought the real problem for that design coming from the missing structural joint in the long direction , and they dont use the minimum reinforcement , but on the other hand according to my limit experiance the building will resist the load that it designed for , we return back to the real q what is the limitation for the structural joints for flat slab , how can we minimize cracking , what is the treatement .
and i read what the friends answer carefully