Bonded PT Tendon
Bonded PT Tendon
(OP)
Hello All
I have a simple question, that has been posed to me by a contractor. If a exisitng PT tendon has been partially cut do you have to provide a coupler to make sure the tendon is not compromised and the tension adauately transferred? Since it is only a single tendon and is still embedded in the concrete the forces should be transfered to the deck and should not be an issue but I wanted to see if there are other experts out there who would do other suggestions.
I have a simple question, that has been posed to me by a contractor. If a exisitng PT tendon has been partially cut do you have to provide a coupler to make sure the tendon is not compromised and the tension adauately transferred? Since it is only a single tendon and is still embedded in the concrete the forces should be transfered to the deck and should not be an issue but I wanted to see if there are other experts out there who would do other suggestions.






RE: Bonded PT Tendon
RE: Bonded PT Tendon
RE: Bonded PT Tendon
RE: Bonded PT Tendon
I would check the design to see if it still works without the tendon at that point, basing the check on a width of slab that is logical for a one way span of the span length that you have. Say a width of about .3 of the longitudinal span length and using the other tendons and reinforcement within that width.
Are you sure that it is bonded PT? Bonded tendons at 18" in a 6.5" slab sounds like a lot of PT. Are they single strand tendons?
RE: Bonded PT Tendon
RE: Bonded PT Tendon
But single strand bonded is very unusual. A very expensive way to do it. We used to have a single strand bonded type at Freyssinet in Australia in the 1970's but is was hardly ever used as it was very expensive.
Are you sure they are bonded?
RE: Bonded PT Tendon
I am thinking along the same lines of RAPT - probably UNbonded (greased and sheathed) monostrand tendons, in which case the partially severed strand can be fully reinstated via coupling and re-stressing.
RE: Bonded PT Tendon
RE: Bonded PT Tendon
It should be easy to determine if the strand has been partially severed via careful percussion chipping at the damaged location and visual examination.
It is possible to detension and splice a new segment of strand, couple and restress, assuming access if okay.
RE: Bonded PT Tendon
RE: Bonded PT Tendon
RE: Bonded PT Tendon
RE: Bonded PT Tendon
You say it is partly cut. What percentage of the diameter has been cut away? Has the cut part of the strand moved back into the duct at all?
RE: Bonded PT Tendon
RE: Bonded PT Tendon
So if half of the area is missing, the total load at service is now being carried by half of the area. Assuming the strand is at 50-60% of UTS after all losses (it will be normal relaxation strand so fairly high losses and it is over 30 years old) then the stress in the remaining part of the strand will be somewhere between 100 and 120% of UTS, so it should be close to fracture if not fractured.
Either that or the stress was not in the strand in the first place. Ingenuity is more into the repair side than me and is used to looking at old unbonded buildings that are not well (cannot call them sick!) and in need of repair! Maybe he can let us know the likelihood of there being much force in the strand after all of this time.
Cutting and coupling an unbonded strand will not do anything for you. The initial force in the strand will be zero, and without bond, the strand could only develop a small force under deflection. So it would be a waste of time. To be effective, it would have to be restressed after coupling!
RE: Bonded PT Tendon
In my experience, the manner in which a partially cut monostrand tendon 'takes-up' the total remaining prestressing force depends on whether the strand has corrosion damage, the grease condition and the sheath type.
For example, if a 7-wire strand has no corrosion damage and the grease has not dried, when say 3 of the 7 wires are cut those three cut wires are free to recoil (strain is released to those cut wires), and the remaining uncut 4 wires tend NOT to 'take-up' the force allocation of the 3 cut wires, so in effect the capacity of the partially cut tendon is 4/7th (not quite, as the central king wire is of larer diameter). Sort of like as if the 7 wires are not stranded, but parallel wires.
For a strand that has corrosion damage/byproduct and the grease has dried out, and of older type sheaths (Heat sealed or kraft paper) when a few wires are cut the friction/binding of the corrosion/dried grease tends not to enable the cut wire to recoil, so there is no appreciable strain relief of the cut wires, and therefore the pre-existing prestress force is taken up on the remaining uncut wires and the uncut wires are then overloaded and failure is eminent.
Every strand tendon that I have repaired is always re-stressed - 99% of the time via hydraulic jacks/rams, but SURELOCK do have a 'Grabbit' turnbuckle type splice for strands, but they are a pain to apply suitable prestress force.
As for repairs in tight spaces, seldom are splice chucks placed at the location of the cut, unless it coincides with approx mid depth of slab to accommodate PT splicing hardware. If access is available to exterior anchorage pockets it is possible to remove and replace the full strand (if double live end) or remove and replace a strand segment and splice at a convenient interior location.
Or do what HOKIE66 suggested, and run a calc assuming this strand provides no structural contribution and see its effect.
For UNBONDED slabs built in the 1970's, many were not constructed with bonded mild steel rebar and the permissible stresses for flat plate banded-uniform placement patterns were/are questionable. Crack control, serviceability, and ultimate flexural capacity of these structures need to be carefully examined.