PT - single broken strand (of 7)
PT - single broken strand (of 7)
(OP)
I have a two-way PT slab project currently under construction. The [genius] contractors decided to core a hole through 4 tendons in a 16-tendon band.
No visible damage to the slab.
After further investigation, it turns out that only 2 of the 4 were cut all the way through. The other 2 tendons each have 1 broken strand (of the 7 strands). After some reading, I believe the friction in the tendon along its length will transmit the tension from its 7 strands to the 6 remaining strands where the 1 strand is cut.
I intend to thread 2 new tendons for the 2 that are cut though. I can get away with only 14 tendons in that band.
My question is should I leave the other 2 intact or have them cut as well? Since the tension from 7 strands is now being held in 6, that's a 17% increase in tension in each strand. That's under the ultimate strength of the strand but it doesn't feel great and I don't want to risk a future break. I also don't know how much I really trust the remaining strands. It's not inconceivable that they were damaged in some way as well.
No visible damage to the slab.
After further investigation, it turns out that only 2 of the 4 were cut all the way through. The other 2 tendons each have 1 broken strand (of the 7 strands). After some reading, I believe the friction in the tendon along its length will transmit the tension from its 7 strands to the 6 remaining strands where the 1 strand is cut.
I intend to thread 2 new tendons for the 2 that are cut though. I can get away with only 14 tendons in that band.
My question is should I leave the other 2 intact or have them cut as well? Since the tension from 7 strands is now being held in 6, that's a 17% increase in tension in each strand. That's under the ultimate strength of the strand but it doesn't feel great and I don't want to risk a future break. I also don't know how much I really trust the remaining strands. It's not inconceivable that they were damaged in some way as well.






RE: PT - single broken strand (of 7)
I think you mean 'wire' - seven wires (6 outer and one central [and slightly larger diameter] inner/'king' wire) make up a 7-wire 'strand'.
Be very careful in visually examining the number of cut or nicked wires - it can be difficult to accurately determine how many wires are indeed damaged. Do not rely on the contractor to give you this info - best to go to site and check it out for yourself. If the depth to the cut strands is somewhat deep and/or the core hole small it can be real tough to determine the actual number of cut/nicked/damaged wires.
If it was me, and I needed 14 of 16 tendons to make the design calc out, I would inform the contractor to repair ALL damaged tendons. The additional cost to remove and replace 2 vs 4 tendons is very minimal. The building owner paid for 16 tendons - the contractor should give them such.
Are the subject tendons single-end stressed, or double-end stressed, or at a construction joint?
Also, the repair should also address the repair and reinstatement of the sheath at the core location - PTI has some recommended MINIMUM's for sheath repair. My preference is to use a HDPE injection welder (like a MIG welder for PE plastic) and splice a segment of new sheath and seam weld the new to old. If the exposure conditions are INTERIOR you may get away with PTI-approved duct tape, or similar.
PTI sheath repair FAQ: Link
Good luck!