Bonded PT Tendon

[NimeshEngr]

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.

 

[hokie66]

That depends on the location of the tendon damage. An assessment is impossible without knowing more about your structure.

 

[NimeshEngr]

Ok The the tendon is partially cut due to a coring sample in the slab. The tendon is a 1/2" -7 wire tendon. about 3/16" is cut based on field measurements. The location of the cut is about 15 ft from the anchorage point on an existing slab over 30 years old. It is on the 11th story of a 12 story bldg and the tendon are spaced 18" oc one way through the whole floor slab. My thought is since the slab is still bonded, I would recommend a coupler to enclose the cut tendon and then close it back up. Since ther is redudency in the flooring system and this one tendon every 18" on a 6.5" thk slab. Does this make senese?

 

[hokie66]

Yes, I think so. If the strand is 15' from an anchorage, I would think it is required for bending capacity.

 

[rapt]

Just recoupling the tendon at that point will not give its full capacity. The force in the tendon will be zero at the coupler and increase to full service force over a distance of 2 to 3m either side of the cut depending on the development rulers for PT strand.
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?

 

[NimeshEngr]

Yes they are single strand tendon..1/2" - 7 wire and the plans have them going 18"OC..I thought it was a lot and the loss of one tendon due to the amount should not be significant but will that cause the stress now to go to that area since it is a weaker point.

 

[rapt]

If they are single strand tendons, then it is not too much.

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?

 

[Ingenuity]

What country is the project located?

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.

 

[NimeshEngr]

The Project is in Florida, United States and the building was built in the 1070's

 

[Ingenuity]

Florida USA and 1970's construction is classic UNbonded PT.

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.

 

[NimeshEngr]

NO since the location is located in a closet area in a Kitchen pantry. The Tendon has a plastic coating and is completely encased in concrete completely except the cored area whihc has been partially severed.

 

[azcats]

Call me crazy, but the plastic coating means unbonded doesn't it?

 

[slickdeals]

Agree with the others that it might be unbonded PT. Do you see a duct in the cored sample or just a plastic sheathing tightly wrapped around the tendon?

 

[rapt]

OK, so it is unbonded!

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?

 

[hokie66]

Like pulling teeth, isn't it? But he did say that it is a 1/2" strand, and 3/16" has been cut. That sounds like at least half of the wires are compromised. I would want to analyse the slab for ultimate capacity without that strand and evaluate the results.

 

[rapt]

Sorry, missed that one.

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!

 

[Ingenuity]

I have repaired/strengthened a lot of 'structurally sick' PT buildings and purposely severed hundreds of UNBONDED tendons in a controlled manner which can vary from heating exposed strand, oxy-burn the anchorage wedges, or my preferred method of 5" angle grinder cutting to exposed strand.

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.