Post-tension tensioning irregularities
Post-tension tensioning irregularities
(OP)
I am working on a project and doing the elongation reports for a large apartment complex subdivision. All the building's tendons are tensioning fine at 5500 psi lengths from 14 to 100+ feet but when the garages are tensioned specifically the 23 foot long tendons they are needing nearly 6000 psi to get to the specified elongation. The long tendons on the garages (76 foot) are tensoning fine at 5500 psi on the same garage. Why are these short tendons only on the garages needing so much more pull? all the concrete is same mix same day, same jack pulling tendons.





RE: Post-tension tensioning irregularities
RE: Post-tension tensioning irregularities
RE: Post-tension tensioning irregularities
RE: Post-tension tensioning irregularities
RE: Post-tension tensioning irregularities
If the calculated extensions were calculated correctly, the different amount of angle change and friction should have been allowed for in the estimated extensions.
hardhatpoke,
Stressing should be done to load/pressure with checks on extension after the stressing is finished to make sure all is ok. It should not be done to an extension. This is a recipe for over stressing and breaking strands. The pressure in the gauge (if it is calibrated correctly) is telling you the force in the strand at the stressing end. The maximum allowed value of this force is an important limit and it is normally the defined stressing force, so overstressing to achieve a calculated extension results in higher forces in the strand than allowed and the possibility that the strand is past yield during the stressing operation. If there were a problem in the duct, such as a slurry blockage, you might only be stressing half the length of the strand and you would end up snapping the strand! This is BAD practice.
We often find that measured extensions on short tendons are high. Never have been able to come up with a definitive reason. If yours are low, you might need to look at the calculated values. Are thhese bonded or unbonded tendons?
RE: Post-tension tensioning irregularities
RE: Post-tension tensioning irregularities
RE: Post-tension tensioning irregularities
RE: Post-tension tensioning irregularities
I agree with RAPT that stressing should be done to force with elongations used as a check. In this case the check shows a problem which should be investigated.
RE: Post-tension tensioning irregularities
RE: Post-tension tensioning irregularities
RE: Post-tension tensioning irregularities
RE: Post-tension tensioning irregularities
RE: Post-tension tensioning irregularities
A very good reason for stressing to pressure and NOT stressing to extension. Those strands are now overstressed by nearly 10%!
What are you going to do to solve that problem?
RE: Post-tension tensioning irregularities
With a shorter length tendon, 1/8" is a larger percentage of the expected elongation. Sometimes, it may be easier to replace these with conventional rebar.