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Re: Time Constraints on Stressing Post-Tensioned Concrete Slabs

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-----Original Message-----
From: Gil Brock <gil(--nospam--at)rapt.pcdc.com.au>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: Wednesday, July 28, 1999 9:17 PM
Subject: Re: Time Constraints on Stressing Post-Tensioned Concrete Slabs

The 3000psi figure for full stressing of a tendon is based on the capacity of the anchors in bearing/bursting. If you wish to use a lesser strength, you should only do it for anchorages tested and certified to work at the lower strengths. Some unbonded anchorages are rated as low as 2300psi.
By the PTI _expression_, a slight modification of the _expression_ that used to be in ACI 318,  the minimum required strength is calculable and is similar to ACI criteria for bearing stress in that it is a function of the ratio of bearing area to concentric concrete area. In that context "tested and certified" is not germain to individually anchored tendons common to building work. Where it is desired to achieve early stressing such as at 1200-1500 psi most anchor castings can be augmented with steel bearing plates. It is a simple matter to verify the capacity of the system via first principles.