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Re: cracking of concrete in post-tensioned slabs

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The ACI code may limit the tensile stress to a particular value but this is
meaningless in a slab that has already cracked. For instance, not many
designers account for slab movements and restraint effects in calculating
the stresses that they compare to the allowable stresses, so a calculated
expected tension stress could be absolutely meaningless. The normal
absolute maximum crack width would be approx .01 inch (about .3mm) for an
internal slab. This could be reduced by up to a factor of 3 for exposed slabs.

The biggest problem you will have is that unbonded tendons provide no crack
control once a crack has occurred. The way the ACI code minimum
reinforcement rules work for unbonded slabs (especially 2 way slabs), it is
very possible to have large areas of slab in tension and cracked and there
will be no unprestressed reinforcement or bonded reinforcement to limit the
crack width. So what would normally be a .1mm crack develops into a very
large and unsightly crack which could lead to bad corrosion problems and
also possibly reduced flexural or shear strength and substantially
increased deflections due to the lack of some form of bonded reinforcement
to limit the crack width.

You must first assess the structure and determine what has caused the
cracking. Once you have done that, you will need to assess the strength of
the slab accounting for normal loads plus any extra effects such as
restraint if there is such a problem. Only then can you decide if
everything is ok.

At 19:57 21/10/98 -0700, you wrote:
>
>
>>Date: Wed, 21 Oct 1998 06:04:04 -0700
>>From: Chris Lillback <dr-dirt(--nospam--at)home.com>
>>To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
>>Subject: cracking of concrete in post-tensioned slabs
>>Reply-To:seaint(--nospam--at)seaint.org
>>
>>Question: What is the expected or allowable amount of cracking for 
>>	a slab before it becomes a structural problem?
>>From a structural engineering viewpoint not aesthetics.
>>
>
>European codes limit the crack width. Here the code limits the tensile 
>stress to 12SQRTf'c as a max. 
>
>Jose Diez PE
>
>
>>
>
>
>______________________________________________________
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> 
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
email:	gil(--nospam--at)rapt.pcdc.com.au