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Re: Long cracks in top slab

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Greetings,
 
A couple of questions regarding your situation.
 
1- Did you register a large drop in atmospheric temperature since the day the top slab was poured? And was the top slab adequately protected during the curing phase?
2- Do the cracks go all the way through the basement walls as well?
 
If so, and since they are in the 50m direction and with the middle third region of the 112 m, most probably they are temperature cracks
 
Just out of curiosity, with the SPT's so low coupled with the thick concrete sections you are using, how did you manage to keep the soil stresses below the allowable net bearing capacity? I am surprised you didn't need piles.
 
Regards,
Hasan Hindawi
 
----- Original Message -----
Sent: Monday, January 08, 2001 11:21 AM
Subject: Long cracks in top slab

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