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RE: Long cracks in top slab

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Calvin,

In a structure with concrete walls and concrete slabs, the walls are cast
and cured for several days, weeks or months  prior to forming and placing
the concrete in the elevated slab.  When the fresh concrete is placed in the
slab, the slab concrete will shrink and be restrained by the walls.  This
will result in cracks in the slab.  You can lessen the cracks in the slab by
detailing for shrinkage, prescribing proper curing practices, and proper mix
design details.  It also helps to require in the specifications special
inspection to enforce specified practices and details.

In post-tensioned concrete structures with walls, it is common to put a
"pour strip" around the perimeter to allow the concrete slab to shrink and
creep for a prescribed period of time prior to placing the concrete in the
pour strip and locking the slab into the walls.

Rebar details:
It helps to increase the amount of reinforcing steel for crack control
similar to that in ACI 350.  Another thing is to offset the splices in
alternate rebar.  The cracks tend to align along the end of the splices.

Mix design:
Require low heat of hydration cement and use flyash.  Use the maximize size
aggregate possible, and uniformly grade the aggregate.  I also like to use
polycarbonate superplasicizers like Adva.  A byproduct of the polycarbonate
superplasicizers is a decrease in the initial set without effecting the heat
of hydration or shrinkage.  This allows the finish to be applied sooner and
the curing can begin sooner.  Flyash can retard the mix causing a delay in
the curing.  I also like to specify the acceptable shrinkage of the
concrete, and the initial and final set times.  

Test pours:
I also like to see a test pour with the concrete placed in a similar manner
as the project will require (if it is pumped for the project, pump it for
the test pour with the same equipment and the same line drops).  Placing
process effects the air content which can effect set times which effects
curing which effects shrinkage which effects cracking.  This requires more
homework by the supplier, but it takes the guesswork out.

Curing:
You required a wet cure.  A wet cure is the optimal cure.  Special
inspection with curing listed in their scope will help assure that the
curing is done properly, and you don't have to be the contractor's mother.

The fix:
When the wheels fall off and the concrete cracks for whatever reason, proper
epoxy injection is about the only solution.  I would wait as long as
possible prior to injection.  Look at the shrinkage, try to quantify the
rate of shrinkage, attempt to quantify how much more shrinkage is possible
for the future.  Count on having to re-inject new cracks for the next
several years.  If you have a top slab exposed to the sun and the top slab
is cast integrally with the walls, it will crack every time the temperature
cycles.  There have been cases of the sun causing reverse camber in precast
members.  There are some remedies, but they are all bandaids.  You may have
to apply an elastomeric membrane which will keep water out of the cracks
when they form.

Regards,
Harold O. Sprague

> -----Original Message-----
> From:	Calvin Chang [SMTP:ccpe(--nospam--at)ms38.hinet.net]
> Sent:	Monday, January 08, 2001 3:21 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