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RE: Long cracks in top slab

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It is typical in concrete slabs that are long and narrow to have pour strips provided that break the slab into separate pieces that are more square than rectangular (preferably less than 1:1.5).  The slab reinforcing is not continuous through the pour strip to allow the separate segments to shrink without undue distress.  Per PTI, at least one pour strip should be provided for buildings of this length (whether the slab is post-tensioned or not).  We usually use two to break the slab up into two segments about 100' in length and then one segment that is the balance.  The pour strip is cast at least 28 days after the last adjacent slab pour was made.  Based on shrinkage/shortening analysis, you would expect about 1/4-1/3 of the total long term shortening to occur over the first 28 days.  It is your decision and the client's schedule that drives the actual length of time the pour strips remain open (the longer the better).  I like to leave them open as long as possible to minimize cracking, with 28 days as a minimum (56 days preferred and longer if I can get it). 
 
For a typical parking structure, you can expect total shortening something on the order of 3/4" per 100 feet of slab in each direction.  For your building this would work out to about 2.75" in the long direction (one half at each end) and 1.25" in the short direction.  The actual shortening is dependant on thickness, shape etc... and can be calculated per ACI 209 methods.  If the slab was poured as one monolithic pour I would expect to see the cracking that you are seeing.  Cracking can be exacerbated by poor curing techniques, sudden large changes in temperature, pour quality concrete (frequently the case) as well as locations of construction joints/pour strips, shearwalls etc.
 
Good luck
 
Nick
-----Original Message-----
From: Calvin Chang [mailto:ccpe(--nospam--at)ms38.hinet.net]
Sent: Monday, January 08, 2001 1:21 AM
To: seaint(--nospam--at)seaint.org
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