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Re: Long cracks in top slab

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Calvin,

You have not mentioned how much reinforcement was provided in the slab. and whether it was placed throughout both faces or only in tension regions.

Also, you have not indicated the shrinkage properties of the concrete used or the country and climate in which it was poured. The details sound like a Singapore design to me but I could be wrong.

Harold's comments regarding connection to monolithic walls would indicate one contributing cause but the overall dimensions of the slab plus reinforcement content and details, concrete properties and other factors also come into it.

RC slabs of this size and type should never be designed without at least one permanent expansion joint and special reinforcing detailing as well.

The further problem you will have is that only in the order of 25-50% of your shrinkage has occurred. Any repair done now will likely not solve the long term problem and further cracking or opening of cracks will occur. Also, at 3mm, it is likely that the reinforcement has yielded at the cracks which I would not think is good for the long term viability of the slab.

At 05:21  8/01/01 +0800, you wrote:
Dear all,

I designed an underground parking structure. The structure is currently under construction. It's two storey below ground. I use RC flat plate system to increase dead weight for anti-uplift purpose. The soil is very soft with SPT number N=1~6 within the depth of 70 feet. The water level is 4 feet below ground.

The plan dimension of the underground structure is 166' x 373' (50m x 112m) without any expansion joint.
The excavation depth is 32' (9.6m).
Fc'=4000 psi.
The thickness of foundation slab is 5' (150cm).
The thickness of B1 floor slab is 1'- 8" (50cm).
The thickness of top slab is 2'- 4" (70cm).
The thickness of exterior wall is 2'- 4" (70cm).
The column spacings are 30' (9m) in both directions.
There will be 1'- 8" (50cm) backfill over the entire top slab of the underground structure.

Two weeks after the top slab concrete was placed, there were about five significant cracks in the top slab with crack width=1mm~3mm. All the five cracks are in 50m direction with length almost equal to 50m, and in the middle thirds region in 112m direction. There are no significant crack in 112m direction. The cracks are "through type", i.e., with crack depth through the entire 70cm cross section.

I am studying the cause of cracks and retrofit procedures.

Because the slabs are very thick, I emphasize on my design drawings to follow the special requirement of mass concrete, i.e., to control the temperature of concrete below 25 degree Celsius, etc. But the contractor just used ordinary concrete. I think this is a possible cause. The column spacings are equal in both directions, I assumed that the top slab stress induced by dead load and uniform live load are equal in both directions, too. The reinforcements provided in both directions are equal, too. But the five cracks are all in 50m direction. The only one explanation I guess is the top slab is not square, but rectangular with length/width ratio more than 2. When the top slab contracted, it seems to have two center of shrinkage if we consider 50m x 112m slab as two 50m x 56m slabs poured together.

Is this cause correct, or other cause?
How to retrofit the top slab?

Any thoughts are greatly appreciated.

Calvin



Regards  Gil Brock
Prestressed Concrete Design Consultants Pty. Ltd.
5 Cameron Street Beenleigh Qld 4207 Australia
Ph +61 7 3807 8022              Fax +61 7 3807 8422
email:  gil(--nospam--at)raptsoftware.com
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