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Re: Long cracks in top slab

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Hasan,
 
Thank you.
 
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?
 
For this project I am responsible for the design job, not for supervision of construction work.
I didn't register the temperature. I knew this situation just four days ago.
On my design drawing I emphasize the entire flat slabs must be covered by wet gunny bags. But the contractor didn't do that. They sprinkled water on the flat slab. I guess sometimes the slab was wet, sometimes the slab was dry.
 
2- Do the cracks go all the way through the basement walls as well?
 
No. Two days ago I went to the construction site. I noticed that there were no significant cracks on the 50cm B1 floor slab and basement exterior wall. But I don't know whether there were cracks on the 150cm foundation slab since it was covered by another concrete layer for increasing the weight of the structure.  
 
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.
 
The water level is 4 feet below ground according to the boring test report. For conservative reason I assume the water level is exactly on the ground for anti-uplift check. 
 
At the bottom of foundation slab:
 
The buoyant force is 9.6 ton/m^2 since the excavation depth is 9.6m.
I use safety factor 1.2 for anti-uplift check.
So I must provide total structural weight of 9.6 ton/m^2  x 1.2 = 11.52 ton/m^2 at the bottom of foundation slab.
The original pressure at the depth of 9.6m is 9.6m x 2.0 ton/m^3 = 19.2 tom/m^2.
11.52 ton/m^2 < 19.2 ton/m^2
The weight of the underground concrete structure is less than the soil being removed during excavation.
The soil is very soft. The underground structure is a little bit like a boat or submarine.
 
Calvin
 
 
----- Original Message -----
Sent: Monday, January 08, 2001 6:55 PM
Subject: Re: Long cracks in top slab

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