Code or Standard or Reference for cutting of PT tendons 1

[AlbertStructural]

Hello Everybody,
I am looking for a code or reference document or any technical approval from any renowned body regarding procedure for cutting of PT tendons. There are many situations in which due to certain changes in building layout (mostly related to MEP / HVAC services) we need to do coring in PT slabs which inevitably involves cutting of some of PT tendons. Is there any document citing the exact procedure of doing so. I myself do have a method statement to do the same but that is based purely on experience and doesn't come from a standard / code etc.
Any info is appreciated.
Regards

 

[Ingenuity]

UNBONDED on BONDED PT tendons?

For UNBONDED the PTI in the USA has some references/papers on this subject.

For BONDED the PTIA from Australia has a reference-type document. I think the UK PT association has a document too.

Let me know what type of tendons you are dealing with and I shall try and provide you with some references.

I am not aware of codes that directly address this. It is usually noted on the structural drawings with some details, and the "means and methods" are left to the contractor.

 

[AlbertStructural]

Hi Ingenuity,
Thanks for quick reply. Our system is BONDED type with cementitious grouting. We are based in middle east region so I am also not sure which standards govern. However any reference would be highly appreciable.

 

[Trenno]

Great references for truncations found HERE from PTIA.

Good references for anything bonded PT really...

 

[Ingenuity]

AlbertStructural,

Since you are in the Middle East region, my guess is that British-type standards and practices are applicable.

The UK-based PTA is worth looking at: PTA Link

The PTA has a guidance document entitled "Post-Formed Holes Through Post-Tensioned Slabs" that is attached to this post.

And as Trenno stated, the Australia-based PTIA has several download documents on the subject.

There is also this presentation entitled Truncation of Post-Tensioned Tendons that you can download here: PTIA download Link

Structural consultants have there own particular details on how they terminate grouted tendons, however most take the following form:

Some photographs of a typical tendon 'epoxy anchor' prep work (prior to epoxy placement and strand cutting):

Also, it is a good idea to use dry course aggregate as both a void-filler and heat-sink when dealing with depths of epoxy resin that are more than 50mm. When using aggregates, use an epoxy with a viscosity of approx 500 cps.

 

[rapt]

The main thing to realise from a design perspective is that if you do not provide full anchorage at the end of the strand, the strand is assumed to have 0 development at its future free end, and the force profile in the strand is then determined by the normal rules for pre-tensioned strand ends. There is a transmission length that is about 700-900mm long in which the stress is assumed to not increase under load, where the stress varies from 0 at the free end to the working stress at the time of cutting at the interior end. And cannot increase above these values under load.

And then there is a development length which could be 1.5 - 2 m long. This starts at the end of the transmission length where the stress does not increase under load to the other end of the development length where the tendon stress can develop fully under load.

The main point is there will be a zone of 2m or more where there is reduced stress in the tendons and the concrete.

 

[Ingenuity]

rapt makes important points re transmission and development lengths. Engineers often detail 'epoxy anchor' length significantly less than the lengths described above. The typical range of truncated epoxy bond length is 500 mm to 1200 mm.

 

[Lomarandil]

Rapt, Ingenuity, can you recommend any reading to better understand the difference between transmission and development lengths?

Thanks!

 

[Ingenuity]

Lomarandil,

Here are two technical papers that discuss the differences and background testing to the relevant lengths.

Note that both papers are from the USA where the term used above ("transmission length" is terminology used in Australia and the UK) is termed "transfer length".

PCI paper Link


ACI paper Link

 

[rapt]

ACI318 and Eurocode both cover it. Clause 12.9 in ACI.

 

[Lomarandil]

Thanks guys! All this time I'd just been calling the sum development length. Good to understand that a little better.

 

[AlbertStructural]

Thank you all guys. I am really enlightened about the PT repair works now and will surely put all this to practical use.
One query however still remains regarding the epoxy strengthening of BONDED systems. Above pictures and references mostly refer to UNBONDED systems but here in middle east now almost everywhere BONDED system is used due to inherent safety aspects esp with reference to unintended severance of tendons. For this purpose we use epoxy injection in the duct to further make sure that there is a strong bonding between the duct, grout and strands. Any codes / guidance in this regard which specifically talks about BONDED systems.

 

[Ingenuity]

AlbertStructural,

The references that Trenno and I gave, and the photos I posted are all BONDED systems.

UNBONDED systems use strands within greased sheaths and the above references and photos are therefore NOT relevant.

The photos above show the galvanized PT duct (flat slab duct in this instance) and grout so this is indeed a BONDED system.

So I am not sure that I follow your last post/reply.

 

[Retrograde]

Would Sikadur 42 be an appropriate epoxy to use for the termination?

 

[Ingenuity]

Seems to have perfect properties for the application, and the data sheet does state that it is suitable for horizontal surfaces up to 12" grouting depth, with low exothermic properties. However, this is a 3 component product and the 3rd component is dry aggregate so probably not the most cost effective product to use - paying for packaging and freight of aggregate that may be locally available for less cost.

Sikadur 35 (a low viscosity resin) so with user-supplied dry aggregate is probably a less expensive option.

Depending on your service temperature you may be able to use Sikadur 32, a gel-type resin with a viscosity of near 10x that of Sikadur 35, again with selected aggregate/s.

See Link for product use matrix, as follows: