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Re: Time Constraints on Stressing Post-Tensioned Concrete Slabs

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-----Original Message-----
From: Jeff Creagan <jeffc(--nospam--at)kpff.com>
To: SEAOC Listserver <seaint(--nospam--at)seaint.org>
Cc: John Hochwalt <johnh(--nospam--at)kpff.com>
Date: Wednesday, July 28, 1999 9:10 AM
Subject: Time Constraints on Stressing Post-Tensioned Concrete Slabs

Industry standard for when P/T slabs may be stressed is based on concrete strength reaching 3000 psi (or 3500).  Considering high early strength concrete, is there a guideline for the minimum amount of time before the tendons may be stressed?  Also, to prevent temporary shrinkage cracking in P/T slabs, is there a guideline for the maximum time a slab should remain unstressed?
I specify 2500 psi whenever practicable and 72 hours maximum.  My experience with 24 hour cycles (and 1500 f'ci) is from more than 25 years ago when creep/relax/shrink losses were not calculated  and there may large increments of same which are not well defined in early stressing. Howver, the economies of this extreme short cycle could bear conservatism in loss calculation. In the ensuing 25 years tendons have become a commodity and capabilty of tendon suppliers to calculate bearing stress, etc,  is decidedly lacking. I would consider 48 hours to be more desirable in hot/dry conditions, particularly in view of standard curing which consists only of a spray-on membrane, but the simplest way to attain strength seems to be more cement, which is antihetical to shrinkage reduction.