From: Gil Brock <gil(--nospam--at)raptsoftware.com>
Date: Tue, 09 Jan 2001 09:13:47 +1000
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:
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).
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
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
Is this cause correct, or other cause?
How to retrofit the top slab?
Any thoughts are greatly appreciated.
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