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Re: Time Constraints on Stressing Post-Tensioned Concrete Slabs

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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. Remember that the strength used should be a site air cured culinder, not a laboratory/water/fog cured cylinder.

A minimum time would depend on development of tensile strength with time as a bursting failure is tensile strength and bond strength related. As long as these strengths are developing at the same rate as compressive strength there is really no minimum time.

If there is not a minimum area of continuous bonded reinforcement of .1% for slabs or .15% for beams, stressing should be done as soon as possible after the concrete is poured with a minimum of 25% (1000psi) on the day after the pour and the remainder at 3000psi. This is the best arrangement of stressing for shrinkage crack control, even if there is some reinforcement.

If the minimum area of bonded reinforcement is provided then there is really no limit on the maximum time as this reinforcement will provide control of shrinkage cracking.

At 09:06 28/07/99 -0700, you wrote:
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?

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)