Creating Opening in Existing Unbonded PT Slab 1

[FanGong]

The conventional method is to chip concrete, cut tendon, and then install new anchors, grout new anchor pockets and re-tension.

I'm wondering if there's an easier way. Anyone had success using this anchor troubleshooting device?

http://www.gti-usa.net/GTI-Trouble-Shooting-Anchor.shtml

Thanks,

 

[dik]

locate strand and avoid cutting... a downside of PT is modifying it afterwards. Use a low energy chipping hammer and be careful.

Dik

 

[Teguci]

Caution - do not chip out all of the concrete and then cut strands. Cut the strands as you uncover them. This might be obvious to us, but this mistake has been made in the field.

 

[Ingenuity]

I have used them many times, but their use for live-termination of tendons can be tricky...if you have 3 or 4 bundled tendons you have to splay the tendons to accommodate the anchorage size (so concrete chipping to splay the tendons apart), then use two jacks to take-up load before the live strand is severed within the opening, and do this is such a way that it will not shock-load the wedges and strip the teeth during the severing. If it is large opening, severing live tendons over great distances requires very careful consideration of release effects.

BUT, before you start any tendon terminations, coring, cutting or concrete chipping, you need to make sure you are no where close to existing anchorage zones. Typically stay 36"+ away from ANY anchorages. Disturbing the concrete in this area will result in very brittle and violent concrete rupture, with instantaneous release of the prestress force and hence significant 'strand whipping' forces - big safety issue to personnel.

Do you have a sketch of what you are proposing, as each situation is usually unique?

 

[dik]

Ingenuity:

Is there any type of thermite welding process can can be used to terminate the strands; I've not used that product or had to terminate existing unbonded strand. I've always avoided terminating the strand.

Dik

 

[FanGong]

Thank you Ingenuity.

Sorry I was light on describing the conventional method, which absolutely involves careful scanning to locate the tendons, reinforcing and anchorage zones.

I currently don't have this issue. However, going into CA for some new construction projects, I'd like to have a few more options for the contractors.

Can you describe more in detail about the "live-termination" procedures? Please see attached sketch on how I intend to use the device. To me, that's not really "live" termination. The wedge does need to move to have proper seating after severing tendon, which can lead to some lost tendon effective force. However, the beauty of this device is that it doesn't require re-tensioning and reviewing and verifying tendon elongation, with minimal tension lost. Lots of potential time/money saving.

Let me know your thoughts.
Thanks again,

 

[Ingenuity]

Is there any type of thermite welding process can can be used to terminate the strands;

No. Do not weld prestressing strand - by arc weld, nor exothermic welding, nor any method that introduces excessive heat.

To quote Kent Preston in his PCI Journal article from 1990 entitled 'Handling Prestressed Concrete Strand':

"When a wire is subjected to excessive heat, the fibers revert to crystals which (since the cooling is not controlled as it is in heat treating) are identical to those in the original hot-rolled rod with an ultimate strength of about 140,000 psi (965 MPa)."​

Can you describe more in detail about the "live-termination" procedures?

I tagged your sketch, as follows:

If L/2 is large, you will have a lot of elastic strain energy that will be released during strand severing. Even if L/2 is only 2 or 3 feet, if the prestress is released suddenly (say by cutting at L/2 with an abrasive cut-off grinder), there will be a shock loading imparted into the wedges of the new anchorages at A and B due to the strand trying to shorten to its unstressed length - such a shock-loading can strip the teeth off the new anchorage wedges. Sometimes oxy-acetylene (OA) heating is used to relax the strand, so as the temp increases the elastic modulus reduces sufficiently that the strand force is gradually relaxed to zero. This is what I term 'live-termination', as compared to your 'conventional technique'.

I prefer abrasive-cutting over OA because OA relaxing is slow and with greased and sheathed strand it often 'smokes' excessively. I usually use two saddle rams, one each at 'A' and 'B' and after installing the new anchor wedges, stoke each ram to say 4", then simultaneously stress each ram to say 15-16 kips (50% of Pj), then de-stress at L/2.

Sudden de-stressing of a free-length of strand is dangerous. The six outer wires of the 'stranded' strand will uncoil - very quickly and in a dangerous manner if not contained.

A recent live-termination that I was involved, with 5 bundled/grouped tendons, the distance between points A and B was about 30 feet - so we chipped a few feet at A and at B and left about 22 feet of concrete remaining between A and B (to 'confine' a severed tendon) and also stripped back the sheath and de-greased the strand at the opening side of A and B and formed a temporary anchorage via an epoxy-bond anchor. We cut the strand behind the ram near A, with an elastic shortening length was only about 3 feet, and then cut the strand behind the ram at B (also with an elastic shortening length of about 3 feet), resulting in less shock loading to the rams, and the cured epoxy 'restrained' the others end of the strand, than if we had to deal with 20+ feet of free-release length.

The following photo shows the 5 tendons, with RHS that remains, and the LHS to be demoed. The timber forms on the LHS is where the temporary epoxy-bond plug was used to 'control' sudden release over 20+ feet.

In this instance we did not have access to the dead and live-end anchorgaes, and no shoring was able to the installed either, so we had to do 'live-terminations'.

 

[dik]

Thanks... didn't think it was a good idea to heat strand...

Dik

 

[FanGong]

Hi Ingenuity,

Thanks for the detailed explanation.

The intent is by stressing the ram on each side to about 15-16 kips, we pretty much end up without any pre-tension in the zone between point A and B right? Then cutting the strand at L/2 would not result in any sort of energy release. My concern is by loading an extra 15 kips into the existing tendon would push it to its ultimate capacity, assuming 27 kip effective stress after stressing considering losses. The ultimate capacity is about 41 kips. By putting an extra 15 kips onto the 27 kips, we essentially yielded the tendon?

At step 2, my intent is to install the trouble shooting device and grout the anchor pocket right after. Then, chip out concrete locally at L/2 to de-tension. Sounds like I should still try to ease the energy release between point A and B. I'm thinking by locally chipping out concrete at L/2 and torch to cut would be the easiest way to go?

Thanks again for your help,

Fan

 

[Ingenuity]

Fan:

The intent on stressing the ram at A and B is to partially pre-load the ram/jack wedges, not induce additional load into the strand. Since the strand is probably currently stressed to about 25 kips (assuming 1/2" dia 7-wire strand), as soon as the jack is placed on an already stressed strand and 'stressed' the gauge pressure will go from zero to about 4,500 psi (depending on the effective ram area of the jack - in the US most twin-ram jacks P_j gauge pressure is about 5,300 psi) WITHOUT (or a very minor) elongation of the tendon. If this is done at both A and B (with 2 rams), when the tendon is severed at L/2, the jack wedges are partially loaded so the shock load induced to the jack is less.

Keep in mind that severing 'open' tendons between A and B is dangerous!

At step 2, my intent is to install the trouble shooting device and grout the anchor pocket right after. Then, chip out concrete locally at L/2 to de-tension. Sounds like I should still try to ease the energy release between point A and B. I'm thinking by locally chipping out concrete at L/2 and torch to cut would be the easiest way to go?

After you install the new trouble-shooting anchors at A and B, you will need to set the new anchor wedges - manual/hand setting the wedges will not work as when your sever the strand at L/2 the strand will likely slip past the new anchor wedges, or strip the teeth, or partially engage one wedge and possibly crack the other.

Also, keep in mind that single tendons in isolation are somewhat rare - most often they are in pairs or groups of 3's, or 4's - so you have to splay non-single tendons apart to install the trouble shooting anchors.

There is a Concrete International article from the late 80's by Fischli and Marti from VSL entitled: 'Cutting Openings into Post-Tensioned Floor Slabs' that is worth reading. They developed a double-acting hydraulic ram that de-stresses tendons hydraulically - but it is a beast and custom equipment, and not an economic nor practical way of creating openings in my opinion.

 

[FanGong]

Hi Ingenuity,

Understood. So, the rams are to pre-load the wedge, not the strand. Pre-seating the wedge to reduce impact loading before severing tendon.

Thank you so much for patiently describing your method.

May I ask where are you currently practicing structural engineering?
I'm a practicing structural engineer in Oakland, California, United States. This is definitely the method I would like to implement to my future projects. However, since it's not documented by PTI, I will definitely need to have good references to justify this method. I'm wondering if you have used this method in the United States?

Thank you very much,

Fan

 

[Ingenuity]

Understood. So, the rams are to pre-load the wedge, not the strand. Pre-seating the wedge to reduce impact loading before severing tendon.

Correct. I sometimes take the jacks (simultaneously at A and B) up until I see the ram 'stroke' increases marginally (say 1/8", so the strand is elongating slightly at A and B) so that I get a feel for the approx load in the existing strand and ensure that the jack wedges are therefore 'pre-loaded'.

Thank you so much for patiently describing your method.

You are welcome.

May I ask where are you currently practicing structural engineering?

The 50th state - Hawaii.

I'm a practicing structural engineer in Oakland, California, United States. This is definitely the method I would like to implement to my future projects.

Since you are in Northern California specialist companies like Schwager-Davis [Link] [italic][EDIT: I am not associated with Schwager-Davis in any way, however, I am familiar with their work and general capabilities] [/italic] are probably the best to contact to gain local knowledge, know-how and capabilities of this type of work. Personally I would avoid the typical rebar/PT installer contractor who does buildings - they typically know little about concrete, don't like to chip with it, and all too often give advise that can be dangerous (recent safety accident here about 2 months ago where a concrete laborer asked PT installer if it was okay to chip 14" away from a live PT anchorage - installer said 'yes', laborer suffered a concussion and 6 stitches to the face when the concrete blew-out in dramatic fashion).

Be careful in your drawings/specifications that you don't get into deep 'means and methods' descriptions in your repairs/retrofits etc.

However, since it's not documented by PTI, I will definitely need to have good references to justify this method.

PTI is pretty silent on openings. The 5th edition of the PTI Manual on page 375 has this photo of a pair of tendons being heated to de-stress the strands - NOTE the large bolted plates to reduce the withdrawal of the strands.

[italic][EDIT: See attached file of a PTI Journal article from 2006 entitled REPAIRS, MODIFICATIONS, AND STRENGTHENING WITH POST-TENSIONING that gives a good overview of related subject matter.][/italic]

If you are a 'younger' engineer with more design-based PT experience that field-based experience, I feel it is very advantageous to go out on site with a PT repair contractor for the day (or a week) and see how they do their stuff - some very resourceful and experienced field guys can make your project very successful - and the opposite it true, too - and you will learn a lot too, and very quickly.

I'm wondering if you have used this method in the United States?

The project I posted the photo above was in Hawaii in late 2015. There was a contractor and engineer on the project (a retrofit of a late 70's building), and the contractor's sub for rebar and PT had no idea how to do the PT 'live-terminations' (or PT generally, IMO), and the EOR and I had worked on past projects together for many years so I was engaged by the owner/contractor/engineer team to provide a methodology along with sketches, step-by-step procedures, materials etc. All came together well - no safety concerns, time efficient, but the sub tried to 'ream' the GC and owner on cost - trying to cost-out his lack of experience with PT as a reason to inflate his costs - submitted a change order of $50K for 5 tendons! - one of the reasons why such work needs to be carefully documented and coordinated.

 

[Ingenuity]

==> The file did not upload in the previous post, so I am trying again:

See attached file of a PTI Journal article from 2006 entitled REPAIRS, MODIFICATIONS, AND STRENGTHENING WITH POST-TENSIONING that gives a good overview of related subject matter.

And weblink here Link to article if the file upload did not work.

 

[hokie66]

This thread is a great promo for BONDED PT.

 

[Ingenuity]

This thread is a great promo for BONDED PT.

I concur, provided the grouting is done correctly.

 

[rapt]

But then Ingenuity wouldn't be getting rich fixing up unbonded stuff-ups!