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Re: Difference between Initial Prestressing force and Jacking for ce

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John, Scott, Nick etc,

A comment was made last week on this topic about stage stressing and that it was the contractors responsibility to decide whether to use it based on economical grounds. There are situations where it is not an economics question but a engineering decision.

 In any member(bonded or unbonded PT) where there are areas with no bonded reinforcement (eg flat plates with low stresses in the bottom at service with unbonded tendons), it is necessary to partially stress while the concrete is still very young to stop possible early shrinkage cracking. Typiucally this is done the morning after the concrete pour and about 25% of the total final jacking force is applied. For bonded tendons this can be 1 strand out of 4 to full load if the dead anchor type is not a bond type anchor (onion anchors, light plate dead end anchors etc rely on bond) or all strands to 25% load (more work and cost) for bond type anchors. For unbonded tendons it would have to be 25% load on all strands as the concrete strength would not be adequate to allow stressing to full load on a portion of the tendons

True economics might control the final answer. Rather than stage stressing, .001 Ac bonded reinforcement could be added in these areas to provide crack control.

The effects of the stressing sequence on losses calculations should be allowed for by the designer in his design calculations. I know this is a foreign concept to many ACI PT designers but many designers around the world have been doing it for many years.
 
Also remember that the banded/distributed design logic requires that the member be uncracked at service. This is far easier to achieve if early shrinkage cracking can be avoided.


At 10:27 AM 15/03/2003 +0200, you wrote:
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
To: seaint(--nospam--at)seaint.org
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? 

 

Regards  Gil Brock
Prestressed Concrete Design Consultants Pty. Ltd. (ABN 99003351504)
5 Cameron Street Beenleigh Qld 4207 Australia
Ph +61 7 3807 8022              Fax +61 7 3807 8422
email:          gil(--nospam--at)raptsoftware.com
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