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Re: Difference between Initial Prestressing force and Jacking for ce

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List,
I think that we would all agree that the issue of"age of concrete at time of stressing"
can not be dealt with without careful consideration of all the parameters involved
e.g development of strength,shrinkage and creep,need to reuse the forms or remove
the scaffolding e.t.c.Regarding creep and shrinkage effects(which are not limited to
prestressing force losses)in particular,the earlier the stressing the worse they get.
To this extent the statement"the earlier the better" could be misleading.
regards
John Sigalas
----- Original Message -----
From: nblackburn
Sent: Friday, March 14, 2003 10:19 AM
Subject: RE: Difference between Initial Prestressing force and Jacking for ce

Typically the tendons are stressed on the order of two to three days after pouring, the earlier the better.  Requests for over three days should be denied.  Concrete strength should be on the order of 2500-3000psi before stressing.  SureLock anchors have been successfully tested down to as low as 2000psi but this is not recommended. 
 
Nick Blackburn
 
-----Original Message-----
From: ken kirkland [mailto:kenmail234(--nospam--at)yahoo.com]
Sent: Friday, March 14, 2003 8:55 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Difference between Initial Prestressing force and Jacking for ce

Does anyone know WHEN the prestressing force can be applied by the contractor after the concrete was placed?