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RE: Cracks in an existing composite floor deck

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It is common practice to NOT put in control joints in slabs on metal deck.
It is also common experience to have shrinkage cracking.  The cracks are
most pronounced in composite beam / girder systems with composite decks.  I
generally use rebar over girders and stagger the ends of the bar.  Rebar is
placed on chairs, and the rebar can then support the mesh.  

There is no reason to repair the cracks unless you are going to place a
flooring that could reflect the cracks.  In that case, just inject them with
epoxy ($6.00 per lineal foot), and be done with it.  The cracks are NOT
active if they are indeed shrinkage cracks.  Bear in mind that that the beam
rotation and resulting cracks are significantly less when the concrete has
age (shrinkage is time dependent) and the strength to work as a composite.  

I had this same question and asked Walt Schultz, the director of R&D for
Vulcraft in Norfolk, NE.  He said that the mesh is strictly for shrinkage,
and that shrinkage cracks in the slab do not reduce the load carrying
capacity of the composite slab.  

You will note that the SDI shrinkage steel is less than the ACI shrinkage
steel.  I will often bump the steel to meet ACI requirements.  It is not
that costly.  Although with steel price rises, I might have to revisit that.


You can also mitigate concrete shrinkage to a degree by adjusting your slab
concrete mix.  

Regards,
Harold O. Sprague

-----Original Message-----
From: Stephen L. Fisher [mailto:slf(--nospam--at)idgcorp.net] 
Sent: Thursday, April 01, 2004 2:11 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Cracks in an existing composite floor deck

My firm is evaluating an existing composite concrete floor deck (Verco W3
deck with 3 1/4" lightweight concrete topping, total 6 1/4") to receive new
equipment loading.  The structure (ie deck, beams, girders, etc) was already
evaluated and determined to be adequate for the proposed new loading.

The contractor has removed the existing flooring, cleaned the flooring
adhesive and lightly bead blasted the concrete topping.  With the concrete
topping exposed there are numerous cracks evident and the question was
raised by the client whether or not the floor was structurally adequate.

I have gone out to inspect the floor and noticed the following:

	1.	The cracks occur, almost without fail, parallel to the
girders and
beams.  This makes sense because the deck is in negative bending with the
top surface of the concrete topping in tension.  If the WWF was not place
properly this could very easily occur.  Even with the WWF placed properly
this cracking might have occured.

	2.	The cracks are between 1/16" and 1/8" wide and appear to be
on the order
of 5/16" deep.  There are a few locations where the cracks are as wide as
1/4" and as deep as 1/2", but those incidents are less frequent.

	3.	There was no evidence of sawcut or cast in control joints
anywhere in
the concrete topping slab.

	4.	The area of the building involved in this remodel is
approximately 80
feet by 90 feet.  The columns are spaced at 40 feet on center in one
direction and 30 feet on center in the other direction.

My current conclusions are as follows:

	1.	Since the majority of the cracks are narrow and shallow they
likely
occured as a resulting combination of shrinkage cracking,  no prepared
control joints, and possibly poorly placed reinforcing which lead to tension
cracks over the supporting beams.

	2.	These cracks should not affect the performance of the
composite floor
system and therefore the new equipment can still be located as planned.

	3.	Any attempt to fill the cracks would mearly be cosmetic
because they
would open right back up in areas of negative bending when new loading (from
the equipment) is placed.

	4.	Some of the cracks may open up a little more as a result of
the
originally expected live load being partially replaced with real dead load.

My questions to the List are the following:

	1.	Does anybody have any other thoughts or concerns about the
situation I
described?

	2.	Has anybody seen this type of cracking and had any repair
methods worth
pursuing?

Sincerely,

Stephen Fisher,SE
Sacramento, CA
 

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