Cutting bonded post tensioning tendon 1

[asixth]

Hi

I am providing a work method statement for a peno in a bonded PT slab where I need to cut thru a tendon for a 5' peno. It is bonded and I am guessing that once the tendon is severed that it will become de-stressed for a transmission length back towards the tendon anchorage. I have specified the tendon to be cut at either end of the 5' peno prior to cutting out the peno. Has anyone performed this before and is there safety issues that I should diligently notify the subcontractor carrying out the procedure prior to works commencing?

Thanks

 

[LonnieP]

Just make sure the Subcontractor doing the cutting is experienced in this kind of thing. And YES! It's very dangerous.

LonnieP

 

[Ingenuity]

Not as dangerous for grouted tendons, assuming the grout is good. Severed unbonded tendons can be a real 'dog' with specific safety protocols.

I have intentionally severed many PT tendons - as compared to those accidentally cut that a diamond core rig 'found'. Both UNBONDED and BONDED (grouted) tendons.

Given your situation of bonded (grouted) tendons, I assuming it is a flat galv ducted PT grouted system with maybe 4 or 5 strands (commonly called monstrand slab system), as compared to a multi strand tendon used in say a beam using a circular galv duct.

Assuming so, and further assuming this is to the tendons near the slab top surface, it is common practice to form 'epoxy anchors' as follows:

1) locate the applicable tendons using GPR or equal.
2) chipping the applicable concrete cover to PT tendons of an area of dimensions of approx 450mm long x 400mm wide, at the proposed cut location/s.
3) visual verify of the condition of the grout (maybe strand too) within the exposed duct.
4) chip and remove grout and duct from all strands within the chipped area, for depth of 25mm below the strands.
5) separate strands by wedging each stand and clean all exposed strands.
6) place 2-component epoxy within the chipped area, extend the mix using pre-placed aggregate or equal, if required, depending on depth of patch.
7) let epoxy cure
8) cut strands
9) coat ends of stand with epoxy for corrosion protection
10) repeat for each tendon group at perimeter of proposed opening/penetration.

The above should be performed by experienced and knowledgable speciality subcontractors. In most markets, PT subcontractors are able to do this full scope of work. Be careful of coring and cutting subs who may think they can do it - sure they can cut the strands cost effectively, but IMO it is satisfactory completion of the other 8 or 9 steps that give you the confidence of completed work.

Having stated the above, for step 3 where the quality of the grout is NOT in question, I have deleted steps 4 through 7, and the EoR was able to live with a transmission length for PT force development adjacent to the opening.

 

[dik]

Ingenuity pretty much summed it up and provided a great summary... Prior to cutting just ensure that the remaining system is capable of transferring all loads, both shear and flexure. As a caution, the development length for grouted PT strand can be several feet.

Dik

 

[msquared48]

Set up targets at each end of the cable. Offer a reward to the contractor for hitting the center.

Mike McCann
MMC Engineering

 

[Ingenuity]

Mike,

You think the grouted/bonded tendons will be projectiles and exit the slab/beam edge? I very much doubt it. Possible, but not probable.

 

[asixth]

Thanks Ingenuity, I also sought advice from some PT contractors who also recommended a procedure similar to as what you have described.

Regards

 

[rowingengineer]

I have never been so careful, I just have the contractor locate the duct, check ti is grouted and then cut away.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[rowingengineer]

and a tech note from the PTIA

http://www.ptia.org.au/Downloads.aspx

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[asixth]

RE

That's what I was planning on doing originally, just locating each end of the tendon and core thru it and epoxy coat the exposed strands when the peno goes it. Then I had a PT contractor provide a procedure similar to ingenuity above so I thought it would be best to follow the advice of the specialist.